EP4359777A2 - Thermal fluid sensor - Google Patents

Thermal fluid sensor

Info

Publication number
EP4359777A2
EP4359777A2 EP22753607.5A EP22753607A EP4359777A2 EP 4359777 A2 EP4359777 A2 EP 4359777A2 EP 22753607 A EP22753607 A EP 22753607A EP 4359777 A2 EP4359777 A2 EP 4359777A2
Authority
EP
European Patent Office
Prior art keywords
temperature sensing
sensing element
fluid
heating element
sensor
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP22753607.5A
Other languages
German (de)
French (fr)
Inventor
Florin Udrea
Syed Zeeshan Ali
Ethan Gardner
Jonathan Hardie
Jon CALLAN
Sean DIXON
Daniel Popa
Claudio Falco
Julian William Gardner
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Flusso Ltd
Original Assignee
Flusso Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US17/354,645 external-priority patent/US20220120701A1/en
Application filed by Flusso Ltd filed Critical Flusso Ltd
Publication of EP4359777A2 publication Critical patent/EP4359777A2/en
Pending legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N25/00—Investigating or analyzing materials by the use of thermal means
    • G01N25/18—Investigating or analyzing materials by the use of thermal means by investigating thermal conductivity
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
    • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
    • G01F1/68—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using thermal effects
    • G01F1/684—Structural arrangements; Mounting of elements, e.g. in relation to fluid flow
    • G01F1/6845—Micromachined devices
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
    • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
    • G01F1/68—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using thermal effects
    • G01F1/684—Structural arrangements; Mounting of elements, e.g. in relation to fluid flow
    • G01F1/6847—Structural arrangements; Mounting of elements, e.g. in relation to fluid flow where sensing or heating elements are not disturbing the fluid flow, e.g. elements mounted outside the flow duct
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N25/00—Investigating or analyzing materials by the use of thermal means
    • G01N25/005—Investigating or analyzing materials by the use of thermal means by investigating specific heat

Definitions

  • the present disclosure relates to a micro-machined sensor, particularly but not exclusively, the disclosure relates to a fluid sensor for sensing concentration of a fluid or concentration of components of a fluid based on thermal conductivity of the fluid.
  • Gas sensors can be based on many different principles and technologies.
  • One such principle is using thermal conductivity to determine the composition of gases.
  • the senor is able to determine composition of a fluid and concentration of different components within the fluid, in a zero flow environment
  • thermal isolation of the heated element which reduces power dissipation, increases sensitivity and provides a fast, dynamic response of the sensor
  • a suitable dielectric material used for the dielectric membrane improves mechanical robustness of the membrane
  • a suitable dielectric material with low thermal conductivity
  • the dielectric membrane with low thermal mass
  • CMOS Complementary Metal Oxide Semiconductor
  • MEMS Micro-Electro-Mechanical Systems
  • the presently disclosed fluid sensor is able to measure the composition of the fluid based on the different thermal conductivity of each of the components of the fluid.
  • a first temperature sensing element spatially separated from the heating element, wherein the first temperature sensing element is located outside of the first dielectric membrane and over the semiconductor substrate, or wherein the first temperature sensing element is located on or within the first dielectric membrane and wherein the fluid sensor comprises at least one recessed region within the first dielectric membrane configured to thermally isolate the heating element from the first temperature sensing element, wherein the heating element is further configured to operate as a second temperature sensing element, and wherein the separation between the second temperature sensing element and the first temperature sensing element introduces a temperature difference between the heating element and the first temperature sensing element, such that a differential signal between the first temperature sensing element and the second temperature sensing element is indicative of the concentration or composition of the fluid based on a thermal conductivity of the fluid.
  • the first temperature sensing element is spatially separated from the heating element, so that there is a temperature difference between the heating element and the first temperature sensing element.
  • the heat generated by the heater diffuses into the dielectric membrane, above and below the dielectric membrane, and into the fluid surrounding the heating element.
  • the amount of heat lost to the fluid surrounding the heating element will depend on the thermal conductivity of the fluid. Therefore, a temperature profile of the heating element will depend on the thermal conductivity of the fluid within the sensor. Dependent on the thermal conductivity of the fluid, the heating element will use a different amount of power to heat to a given temperature.
  • the first temperature sensing element is outside the membrane, or within the dielectric membrane and thermally isolated from the heating element. Therefore, the temperature of the first temperature sensing element will remain at ambient or room temperature or at a significantly colder temperature than that of the heating element. As the temperature of the heating element is dependent on the heat conducted through the fluid within the sensor and thus the thermal conductivity of the fluid, the differential signal is also dependent on the thermal conductivity of the fluid. Different target fluids within the sensor have different thermal conductivities, and therefore the temperature of the second
  • 11649661-1 4 temperature sensing element (or the heating element) can be used to determine the concentration or composition of the fluid within the sensor.
  • the differential signal is indicative of a composition or concentration of the fluid, and the sensor may be further configured to determine the composition or concentration of the fluid based on the differential signal or the temperature of the first temperature sensing element.
  • the change in power required or the temperature change due to heat loss to the fluid is generally small compared to the measured ambient temperature. Therefore, by measuring the differential signal the measured ambient temperature can effectively be cancelled out to improve measurement of the change in power required or the temperature change due to heat loss to the fluid. This can be done using a Wheatstone bridge, or schemes based on differential/instrumentation amplifiers.
  • the heating element is the same as the second temperature sensing element, i.e. the heating element operates as a resistive temperature detector.
  • the heating element can be driven in a constant temperature, constant voltage/current or constant resistance mode, and instead of measuring the differential resistance between the first and second resistive temperature sensing elements, the differential voltage, current or power can be measured.
  • the thermal conductivity of the fluid around the sensor changes, the amount of voltage, current and/or power required to keep the heater at the same resistance or temperature will change, and thus the differential voltage/current/power between the first and second temperature sensing elements will change.
  • the heating element may be configured to operate as a sensing element by, for example, sensing the change in the resistance due to the change in temperature, as it is the case of resistive temperature detectors.
  • the heating element may operate simultaneously as both a heating element and a sensing element.
  • the heating element can be considered as electrically equivalent to a resistor.
  • the electrical conductivity of most heaters materials (Tungsten, Titanium, Platinum, Aluminium, polysilicon, monocrystalline silicon) varies with temperature. This variation is mostly linear and is characterised by the TCR (Temperature coefficient of resistance).
  • the TCR can be positive or negative, but most metals have a positive and stable TCR, meaning that their resistance increases when the temperature is increased.
  • the advantage of this embodiment is simplicity and reduced number of additional elements on the membrane.
  • the first temperature sensing element By providing the first temperature sensing element on the substrate or on the same membrane and thermally isolated (i.e. not on a separate membrane), the first temperature sensing does not need to be separately heated. Therefore, the power consumption of the device is reduced.
  • a fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising: a semiconductor substrate comprising a first etched portion; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first etched portion of the semiconductor substrate; a heating element located within the first dielectric membrane; a first temperature sensing element spatially separated from the heating element, wherein the first temperature sensing element is located outside of the first dielectric membrane and over the semiconductor substrate, or wherein the first temperature sensing element is located on or within the first dielectric membrane and wherein the fluid sensor comprises at least one recessed region within the first dielectric membrane configured to thermally isolate the heating element from the first temperature sensing element ; and a second temperature sensing element located on or within the first dielectric membrane, wherein the second temperature sensing element is substantially identical in shape and size to the first temperature sensing element, and wherein the first temperature sensing element is located a first distance away from the heating element, and wherein the
  • 11649661-1 6 temperature sensing element is indicative of the concentration or composition of the fluid based on a thermal conductivity of the fluid.
  • the fluid sensor may comprise a semiconductor substrate made of a semiconductor material such as silicon, silicon carbide or Gallium Nitride, and comprising an etched portion.
  • the fluid sensor may also comprise a dielectric region comprising of oxides and/or nitrides such as silicon dioxide and silicon nitride, where the portion of the dielectric region adjacent to the etched portion is referred to as a dielectric membrane.
  • the dielectric membrane may have embedded structures made of semiconductor material or metal structures.
  • the semiconductor substrate may be any semiconductor such as silicon, silicon on insulator (SOI), Silicon Carbide, Gallium Nitride or Diamond.
  • silicon is advantageous, as it guarantees sensor manufacturability in high volume, low cost and high reproducibility.
  • the use of a silicon substrate could also enable on-chip circuitry for sensor performance enhancement and system integration facilitation. Such on-chip circuitry could be implemented by using analogue or digital or mixed-signal blocks placed outside the dielectric membrane.
  • the dielectric membrane or multiple dielectric membranes may be formed by back- etching using Deep Reactive Ion Etching (DRIE) of the substrate, which results in vertical sidewalls and thus enabling a reduction in sensor size and costs.
  • DRIE Deep Reactive Ion Etching
  • the back- etching can also be done by using anisotropic etching such as KOH (Potassium Hydroxide) or TMAH (TetraMethyl Ammonium Hydroxide) which results in sloping sidewalls.
  • anisotropic etching such as KOH (Potassium Hydroxide) or TMAH (TetraMethyl Ammonium Hydroxide) which results in sloping sidewalls.
  • the dielectric layers within the membrane which could be formed by oxidation or oxide deposition could be used as an etch stop during the DRIE or wet etching processes.
  • the membrane can also be formed by a front-side etch (using most commonly wet etch techniques) or a combination of a front-side and back-side etch to result in a suspended membrane structure, supported only by two or more beams.
  • the membrane may be circular, rectangular, or rectangular shaped with rounded corners to reduce the stresses in the corners, but other shapes are possible as well.
  • the semiconductor substrate may be silicon and the dielectric membrane may be formed mainly of oxide and nitride materials, or oxinitride (a pre-formed combination of oxide and nitride) and where the heater element may be made of a metal such as
  • the heater may have a shape of a meander, spiral or a hotwire.
  • the dielectric region may comprise a dielectric layer or a plurality of layers including at least one dielectric layer.
  • the dielectric region may comprise layers of more than one material, such as silicon dioxide, silicon nitride, or aluminium oxide.
  • the heating element may be fully embedded or partially embedded within the dielectric membrane.
  • the membrane may also comprise one or more layers of spin on glass, and a passivation layer over the one or more dielectric layers.
  • materials with low thermal conductivity e.g. dielectrics
  • Temperature sensing elements or heaters made of materials such as monocrystalline or polycrystalline semiconductors or metals could be suspended or embedded in the dielectric membrane.
  • the dielectric membrane may also have other structures made of metal or other conductive or other materials with higher mechanical strength. These structures can be embedded within the membrane, or may be above or below the membrane, to engineer the thermo-mechanical properties (e.g. stiffness, temperature profile distribution, etc.) of the membrane and/or the fluid dynamic interaction between the fluid and the membrane. More generally, these structures can be also outside the membrane and/or bridging between inside and outside the membrane.
  • a dielectric membrane region may be located immediately adjacent or above (or below if a flip-chip technology is used) to the etched portion of the substrate.
  • the dielectric membrane region corresponds to the area of the dielectric region directly above or below the etched cavity portion of the substrate.
  • Each dielectric membrane region may be over a single etched portion of the semiconductor substrate.
  • the membrane maybe a “closed membrane”, supported by the substrate along its entire perimeter, or can be a bridge type structure - supported by a number of dielectric beams.
  • the fluid sensor may be configured to sense or measure a fluid (this may be a gas but could also be a liquid), and the gas may be made of air and the components of interest could be any of CO 2 , methane or hydrogen or other gases in dry air or humid air.
  • the component of interest can be any fluid that has a different thermal conductivity than that of air.
  • the disclosed sensor could be applicable to a variety of gases and liquids, but we make specific reference to Carbon dioxide (CO 2 ), methane and hydrogen as these specific gases have thermal conductivity properties which are significantly different from those of air.
  • CO 2 Carbon dioxide
  • methane and hydrogen these specific gases have thermal conductivity properties which are significantly different from those of air.
  • the sensor may be a thermal conductivity fluid sensor incorporated in a MEMS structure comprising a heating element and at least one other sensing element (such as a temperature sensing element) that may be able to detect separately the fluid flow properties, such as velocity, volume flow rate, mass flow rate.
  • the temperature sensing element may be able to also detect the composition the fluid based on the difference in thermal conductivity, specific heat capacity, dynamic viscosity, density (and other thermo-mechanical properties, hereafter simply referred to as thermal properties) of different components of the fluid.
  • the heat generated by the heater diffuses into the dielectric membrane, above and below the dielectric membrane, and into the fluid surrounding the heating element.
  • the amount of heat lost to the fluid surrounding the heating element will depend on the thermal conductivity of the fluid. Therefore, a temperature profile of the second temperature sensing element will depend on the thermal conductivity of the fluid within the sensor.
  • the heating element will use a different amount of power to heat the second temperature sensing element to a given temperature.
  • the differential signal is also dependent on the thermal conductivity of the fluid. Different target fluids within the sensor have different thermal conductivities, and therefore the differential signal can be used to determine the concentration or composition of the fluid within the sensor.
  • the differential signal is
  • the senor may be further configured to determine the composition or concentration of the fluid based on the differential signal.
  • the first temperature sensing element may be located a first distance away from the heating element, and the second temperature sensing element may be located a second distance away from the heating element, and wherein the first distance may be greater than the second distance.
  • the second temperature sensing element may be located closer to the heating element than the first temperature sensing element.
  • the second temperature sensing may be located such that the second temperature sensing element has the same temperature as the heating element during operation of the sensor.
  • the differential signal may be measured as a temperature difference, voltage difference, current difference, power difference, or resistor difference.
  • the difference in the resistance of, current through, or voltage across the two resistive temperature detectors can be measured and this gives an indication of the composition of the fluid and the concentration of its one or more components. If the composition of the fluid (or concentration of a component of the fluid) around the sensor changes, its thermal conductivity also changes and this will change the thermal losses and the temperature of the heater - in turn changing the resistance of the second resistive temperature detector, without changing (or changing insignificantly) the resistance of the first temperature resistive temperature detector. The change in resistance could be measured directly, or could be measured as a voltage change, current change or power change.
  • the first temperature sensing element and the second temperature sensing element may be both located on or within the first dielectric membrane, and the fluid sensor may comprise at least one recessed region within the first dielectric membrane configured to thermally isolate the heating element and the second temperature sensing element from the first temperature sensing element.
  • the second temperature sensing element may be located in a same layer of the dielectric region as the heating element and the second temperature sensing element may laterally surround the heating element.
  • the second temperature sensing element may be located below or above the heating element.
  • the second temperature sensing element may be located directly above or below the heating element, so that the second temperature sensing element is not laterally spaced from the heating element.
  • Having the second temperature sensing element in a same layer or below or above the heating element has the advantage that the temperature of the second temperature sensing element is substantially the same of that of the heater. This increases the differential signal between the first temperature sensing element and the second temperature sensing element, therefore improving sensitivity of the sensor.
  • the second temperature sensor element can be either laterally spaced but close to the heating element, and can be made of the same material layer as the heating element.
  • the second temperature sensing element can be made of a different material layer than the heater and can be vertically spaced from the heater, either above or below the heater.
  • the two temperature resistive detectors can be identical in size, shape and resistance.
  • the first temperature sensing element may be configured to have a higher resistance at room temperature than a resistance of the second temperature sensing
  • first temperature sensing element and the second temperature sensing element may be configured to have substantially the same resistance at an operating temperature of the sensor without a fluid present.
  • the semiconductor substrate may comprise an additional etched portion, and the dielectric layer may comprise an additional dielectric membrane located over the additional etched portion of the semiconductor substrate.
  • the sensor further may comprise an additional heating element located within the additional dielectric membrane and an additional first temperature sensing element and an additional second temperature sensing element.
  • the heating element may be a resistive heating element. At least one of the first temperature sensing element and the second temperature sensing may be resistive temperature sensing elements, also known as resistive temperature detectors (RTDs).
  • RTDs resistive temperature detectors
  • the resistive temperature detector elements may comprise metal (Tungsten, Al, Copper, Platinum, Gold, Titanium) or semiconductor material (Silicon, Polysilicon, Silicon Carbide, Gallium Nitride, Aluminium Gallium Nitride, or Gallium Arsenide or a two dimensional electron gas)
  • resistive temperature detectors may have a high, reproducible and stable TCR (Temperature Coefficient of Resistance).
  • TCR Temporal Coefficient of Resistance
  • resistive temperature detectors are linear in temperature (i.e. their resistance varies linearly with the temperature).
  • the sensing elements may be temperature sensitive and may be any of resistive temperature detectors, diodes, transistors or thermopiles, or an array in series or parallel or a combination of those.
  • Such sensors can be implemented in bulk COMOS, SOI (Silicon on Insulator) CMOS technology.
  • SOI membranes can be made by using the buried oxide as an etch stop.
  • SOI diodes, transistors and thermopiles can be made by using the thin silicon layer above the buried oxide which can be doped n or p-type.
  • sensing element One type of sensing element may be used or a combination of different types of sensing elements may be used.
  • thermopile comprises one or more thermocouples connected in series.
  • Each thermocouple may comprise two dissimilar materials which form a junction at a first region of the membrane, while the other ends of the materials form a junction at a second region of the membrane or in the heat sink region (substrate outside the membrane area), where they are connected electrically to the adjacent thermocouple or to pads for external readout.
  • the thermocouple materials may comprise a metal such as aluminium, tungsten, titanium or combination of those or any other metal available in the process.
  • the thermocouple materials may comprise thermocouples based on n-type and p-type silicon or polysilicon or combinations of metals and semiconductors. The position of each junction of a thermocouple and the number and the shape of the thermocouples may be any required to adequately map the temperature profile distribution over the membrane to achieve a specific performance.
  • the sensitivity and selectivity to the flow composition may be enhanced by using extra sensing elements, symmetrical or asymmetrical recessed regions, and/or an additional heater.
  • the first temperature sensing element may be located above the semiconductor substrate.
  • the first temperature sensing element may be directly above the semiconductor substrate, so that the first temperature sensing element is completely above a substrate portion of the substrate and is not above the etched region of the substrate and is not located within the dielectric membrane. This increases thermal isolation between the first temperature sensing element and the components within the dielectric membrane, therefore improve the sensitivity of the device.
  • the first temperature sensing element may be located within the dielectric region, but preferably outside the dielectric membrane area or at an edge of the membrane area.
  • the first temperature sensor could also be placed at the edge of the membrane region (in order for example to reduce the chip area).
  • the fluid sensor may further comprise circuitry configured to determine the concentration or composition of the fluid based on the temperature of the first temperature sensing element or the differential signal.
  • control circuitry that measures the differential signal between the first and second temperature sensor elements and uses it to determine the concentration of a fluid or particular fluid components based on different thermal conductivities.
  • a control and measurement unit/circuitry that drives the heater in constant current, constant voltage or constant power mode may be provided.
  • the driving could be preferably in pulse mode, but continuous mode or AC mode are also possible.
  • the circuitry may be located on a same chip as the fluid sensor.
  • Analogue/digital circuitry may be integrated on-chip.
  • Circuitry may comprise IPTAT, VPTAT, amplifiers, analogue to digital converters, memories, RF communication circuits, timing blocks, filters or any other mean to drive the heating element, read out from the temperature sensing elements or electronically manipulate the sensor signals.
  • IPTAT IPTAT
  • VPTAT voltage regulator
  • amplifiers analogue to digital converters
  • memories RF communication circuits
  • timing blocks filters or any other mean to drive the heating element, read out from the temperature sensing elements or electronically manipulate the sensor signals.
  • a heating element driven in constant temperature mode results in enhanced performance and having on-chip means to implement this driving method would result in a significant advancement of the state-of-the-art flow sensors.
  • the driving method known a 3w may be implemented via on-chip means, or any other driving method, such as constant temperature difference and time of flight, needed to achieve specific performance (e.g.
  • this disclosure also covers the off-chip implementation of such circuital blocks when applied to a fluid sensor.
  • Such off-chip implementation may be done in an ASIC or by discrete components, or a mix of the two.
  • the circuitry may include one or more alternating current (AC) sources and/or lock-in amplifier measurements to reduce noise.
  • the one or more AC sources may be used in conjunction with fast Fourier transform (FFT)-based techniques.
  • FFT fast Fourier transform
  • the use of lock-in amplifiers facilitate the measurements of very small AC signals, for example AC signals of a few nanovolts or less. Accurate measurements of the AC signals can be made even when noise sources are higher than the signal of interest. Because these techniques only measure AC signals at or near the test frequency, the effects of noise, including thermoelectric voltages (both DC and AC), may be reduced.
  • the driving circuit may be implemented though any suitable means, such as via on-chip means, provided externally in an ASIC, a Field-programmable gate array
  • FPGA micro-controller/micro-processor forms, by using discrete components, or any combination of the above means.
  • the circuitry may comprise reversible direct current (DC) sources.
  • the reversible DC sources may be used in conjunction with techniques based on voltage measurements with currents of opposite polarity to cancel out thermoelectric noise (i.e., electromotive forces (EMFs)) that is generated when different parts of a circuit are at different temperatures, and/or when conductors made of dissimilar materials are joined together.
  • Thermoelectric noise such as EMFs may be reduced or, in implementations, cancelled using consecutive voltage measurements made at alternating test current polarities.
  • the fluid sensor may be operated using a two voltage measurement method or a three voltage measurement method (also referred to as a ‘delta technique’), wherein two or three voltages are applied to any heating element(s) and/or the temperature sensing element(s).
  • the two voltage measurement technique may reduce or cancel a thermoelectric voltage offset term from the measurement results.
  • the three voltage measurement method may either reduce or cancel a thermoelectric voltage offset, and may additionally remove the thermoelectric voltage change (drifting) term from the measurement results, thus greatly improving the measurement noise immunity (e.g. the signal to noise ratio) when compared to many other techniques.
  • the driving circuit may be implemented within the same chip (i.e. monolithic integration) or may be provided externally (such as in an off-chip implementation).
  • the driving and reading circuits may be implemented with any suitable means, such as an ASIC, FPGA, by using discrete components, or any combination of the above.
  • Output signals may be computed using any suitable processor and/or controller, such as a micro-controller or micro processor.
  • this technique makes use of currents of equal (or approximately equal) magnitude and opposite polarities.
  • Opposite polarity currents can be used to more accurately measure the resistance and/or any change in the resistance of the heating element(s) (such as a heater) itself or any temperature sensing element(s) (e.g. a resistive temperature detector or thermal sensor) adjacent or otherwise close to the heater.
  • the heating element(s) such as a heater
  • any temperature sensing element(s) e.g. a resistive temperature detector or thermal sensor
  • the opposite polarity currents are provided by opposite polarity current sources.
  • One or more of the opposite polarity current sources may be e.g. precision current sources.
  • the opposite polarity currents may be provided by the same current source. This may be achieved by swapping the terminals of the (two-terminal) element to be measured.
  • This element to be measured may be a heating element, such as a heating element used as a thermal sensor, or a sensing element, such as a resistive temperature detector, that is near or adjacent to the heating element.
  • the swapping of the terminals may be facilitated using switching elements (e.g. a network of switching elements), such as switching elements comprising switching transistors operatively connected to the terminals of the resistor.
  • the opposite polarity current can be provided by using the same current source but changing the direction of the current (e.g. by biasing the thermal sensor in the opposite way), such that the current flows from the second terminal to the first terminal.
  • the switching elements may be transistors which are configure such that their gate/control terminal determine the direction of the current flow.
  • the switching elements may be monolithically integrated, or may be provided externally.
  • the timing of the opposite polarity currents can be adjusted.
  • the on-time, off-time and/or delay between the pulses of the opposite polarity currents may be adapted as desired.
  • the magnitude of the opposite polarity currents can also be adjusted, as desired.
  • the above operations of the flow sensor may be applied in multiple steps, e.g. by running the heater at several (different) temperature levels (given by different levels of power levels), to aid in the selectivity of different components of the fluid.
  • the circuitry may comprise one or more of: a constant current or constant resistor drive circuit, a constant or alternating current source, a Wheatstone bridge, an amplifier, an Analog to Digital convertor, a Digital to Analog Convertor, or
  • Differential signals can be obtained by using a combination of current sources and differential amplifiers, bridge type circuits or other types of subtraction circuits or instrumentation amplifiers.
  • the first temperature sensing element and the second temperature sensing may be located on two sides of a bridge circuit (also referred to as an instrumentation bridge, and can be a Wheatstone bridge), and the sensor may be configured such that an output of the bridge circuit may be a function of the thermal conductivity of the fluid around the sensor.
  • the output of the bridge circuit may therefore also be a function of the concentration of particular fluid components with different thermal conductivities.
  • the first resistive temperature detector and second temperature detector may be placed together with other components on the sides of an instrumentation bridge, such as a Wheatstone bridge, and the differential output of the bridge could be a function of the thermal conductivity of the fluid around the sensor and the concentration of particular fluid components with different thermal conductivities.
  • an instrumentation bridge such as a Wheatstone bridge
  • Such differential signals can be further amplified by using amplifiers, either located on the same chip, to maintain low noise, or placed within the same package, module or system.
  • the fluid sensor may comprise at least one recessed region within the first dielectric membrane and between the heating element and the first temperature sensing element.
  • the recessed region may be located between the first temperature sensing element and the second temperature sensing element - therefore there is a greater recessed volume between the heating element and the first temperature sensing element than between the heating element and the second temperature sensing element, such that the recessed region introduces a temperature difference between the first temperature sensing element and the second temperature sensing element due to differences in heat conduction through the dielectric membrane.
  • the recessed regions or discontinuities in the dielectric membrane provide an interruption (or partial interruption) in the thermal conduction path through the solid of the dielectric membrane. This in turn will mean that the heat path will occur more through the fluid above the recess (via conduction and convention) or through the cavity space formed as a result of the recess (mainly through fluid conduction). In both cases (heat above the cavity space or within the cavity space), the heat dissipation will depend on the thermal conductivity of the fluid. This increases the sensitivity of the differential signal to the thermal conductivity of the fluid.
  • the at least one recessed region may comprise one or more discontinuous regions where the thickness of the dielectric membrane is discontinuous or varies from an average or most common dielectric membrane thickness.
  • the at least one recessed region may be located between the heating element and an edge of the dielectric membrane.
  • An edge of the dielectric membrane may refer to a perimeter edge of the dielectric membrane, in other words, the area where the dielectric membrane meets or joins the semiconductor substrate.
  • the area of the dielectric region above the semiconductor substrate may refer to the area of the dielectric region outside the dielectric membrane.
  • the recessed region may be located between the heating element and the edge of the dielectric membrane spaced from the heating element.
  • the recessed regions maybe defined such that there is one recessed region between the heating element and the edge of the membrane, one recessed region between the first temperature detector element and the edge of the membrane, and no recessed region between the heater and the first temperature detector element.
  • the recessed regions may be holes (perforations) through the dielectric membrane. This would be advantageous, as the thermal conduction path through the solid of the dielectric membrane will be impeded and this will mean that the thermal conduction will occur through the holes (mainly via conduction) or above the holes (via both conduction and convection), thus facilitating the measurement of the composition of the fluid based on the different thermal conductivity of each of the components of the fluid.
  • the at least one hole also disrupts the thermal conduction path through the solid dielectric membrane, forcing more heat to dissipate via convection and conduction through the environment.
  • the presence of the at least one hole also helps to reduce the power consumption of the device (for the same heater temperature), because of the reduction in the heat conduction losses (through the solid membrane).
  • the presence of the at least one hole allows for a lower thermal mass of the membrane thus reducing the time needed for the heater to heat up and cool down.
  • the at least one hole or recessed region may be used to enhance the sensitivity/selectivity to any fluid or component of the fluid (e.g. air with a concentration of CO 2 ) with a thermal conductivity that is different to that of a reference fluid or another component of the fluid (e.g. air).
  • any fluid or component of the fluid e.g. air with a concentration of CO 2
  • a thermal conductivity that is different to that of a reference fluid or another component of the fluid (e.g. air).
  • An arrangement and specific design of different holes and different sensing elements is provided to enhance the sensitivity to any fluid or component of the fluid (e.g. air with a concentration of CO 2 ) with a thermal conductivity that is different to that of a reference fluid or another component of the fluid (e.g. air).
  • a fluid or component of the fluid e.g. air with a concentration of CO 2
  • a thermal conductivity that is different to that of a reference fluid or another component of the fluid (e.g. air).
  • the arrangement of different holes or slots may be placed symmetrically around the heating element and the second temperature sensing element.
  • the at least one recessed region may comprise one or more holes.
  • the holes may refer to apertures, perforations or slots extending through an entire height or depth or thickness of the dielectric membrane. This forms a fluid flow path and provides fluid connection between area above and area below membrane.
  • the at least one of the one or more holes may comprise an elongate slot extending towards opposite edges of the dielectric membrane.
  • the elongate slot may not extend completely to the edges of the dielectric membrane or completely isolate the dielectric membrane either side of the elongate slot.
  • the elongate slot increases thermal isolation across a width of the dielectric membrane of the device.
  • the elongate slot may
  • 11649661-1 19 be extending in a same direction as one or more heating elements and/or sensing elements.
  • the elongate slots may be, for example, rectangular, square, or semicircle.
  • the one or more holes may comprise an array of perforations.
  • the perforations may comprise individual holes significantly smaller than a width of the dielectric membrane of the device.
  • the array of perforations may can extend substantially across a width of the device.
  • the at least one recessed region may comprise a partial recess within the dielectric membrane.
  • the partial recess or trench may extend from a top surface of the dielectric membrane or may extend from a bottom surface of the dielectric membrane.
  • the partial recess may extend partially through a height or depth or thickness of the dielectric membrane.
  • the at least one perforation may be in the form of a trench formed from the top or the bottom surface but not penetrating the other surface.
  • the discontinuities may be referred to as a gap in the membrane from the top surface to the bottom surface.
  • a discontinuity could also refer to a trench or partial hole created from either the top or the bottom surface (if an upside-down membrane is used) without penetrating the other surface.
  • the advantage of such partial holes is that they could impact less the mechanical strength of the membrane and in some cases they may be easier to be manufactured.
  • such partial holes could be used to hermetically seal the bottom side of the membrane or allow no fluid penetration below the membrane.
  • the at least one recessed region may have a meander shape.
  • the discontinuity may have a non-standard shape such as a concertina or corrugated shape formed of a series of regular sinuous curves, bends, or meanders.
  • the etched region of the semiconductor substrate may have sloped sidewalls.
  • the etched region of the semiconductor substrate may not extend through the entire depth of the semiconductor substrate.
  • the semiconductor substrate may comprise an additional etched portion, and the dielectric layer may comprise an additional dielectric membrane located over the additional etched portion of the semiconductor substrate.
  • the sensor may further
  • 11649661-1 20 comprise an additional heating element located within the additional dielectric membrane, and an additional first temperature sensing element.
  • the additional heating element and the additional first temperature sensing element may operate similar to the heating element and first temperature sensing element. This increases sensitivity of the device.
  • the heating element and the additional heating element may be connected in series.
  • the additional first temperature sensing element and the first temperature sensing element may be connected in series.
  • the sensor may comprise an additional second temperature sensing element connected in series to the second temperature sensing element.
  • the heating elements may be connected in series and operated substantially at the same temperature.
  • the heating elements may be connected in series and the second temperature sensing elements may also be connected in series.
  • the first temperature sensing elements may also be connected in series.
  • a differential signal between the series combination of the first resistive temperature detectors and the second resistive temperature detector is obtained and used to determine the concentration of a fluid or particular fluid components based on different thermal conductivities. This allows the sensitivity of the sensor to be increased (by scaling up with the number of membranes, heating elements, and temperature sensing element) and also lowers the minimum resolution of the concentration of a particular gas component that can be sensed based on its difference in thermal conductivity compared to the rest of the fluid.
  • the heating element and the additional heating element may be configured to operate at different temperatures.
  • Each sensing element in combination with a corresponding first and second temperature sensing elements may operate independently and preferably at different temperatures to improve selectivity to different gases.
  • the heating element may be driven at more than one temperature, to increase the selectivity of the device. Gas thermal conductivity varies with temperature, and this variation is different dependent on the gas.
  • the heater can be driven at a temperature where the thermal conductivity of air and carbon dioxide are identical, and then used to detect another gas (e.g. hydrogen or methane). In this scenario, there will be known unwanted response from present carbon dioxide and thus the selectivity of the device is improved.
  • the heater can also be run at the temperature that provides the optimum sensitivity for the gas that is being measured.
  • the fluid sensor may comprise an array of multiple dielectric membranes located over multiple etched portions of the semiconductor substrate, each membrane having: a heating element located within the dielectric membrane; a second resistive temperature detector element located in the proximity of the heating element and within the dielectric membrane.
  • the fluid sensor may comprise a first resistive temperature detector element located outside of the membrane and used as a reference.
  • a differential signal may be measured between the at least one first resistive temperature detector element and the at least one second resistive temperature detector such that the differential signal is a function of the thermal conductivity of the fluid around the sensor and the concentration of particular fluid components with different thermal conductivities.
  • the array may contain one or several first resistive temperature detectors outside the dielectric membrane.
  • the fluid sensor may further comprise a covering located on a surface of the sensor, where the covering may comprise a hole configured to allow fluid travel from an outer surface of the covering to a fluid channel above the dielectric membrane.
  • the fluid sensor may further comprise a further temperature sensing element located outside the membrane region.
  • the further temperature sensing element may be thermally isolated from the heating element.
  • An additional or further temperature sensor may be placed outside the dielectric membrane as a reference temperature sensing element to measure the ambient temperature or the temperature of the fluid, and the signal from the further temperature
  • 11649661-1 22 sensor may be used for temperature compensation for a more accurate calculation of the concentration of one or more specific components of the fluid.
  • the reference temperature sensing element could be used as part of a combination sensor (or a sensor fusion system) to read multiple physical properties of the environment (fluid composition and concentration of different components, fluid temperature or ambient temperature, or fluid velocity of fluid flow rate).
  • a separate temperature sensor could be integrated on-chip as an extra resistive temperature detector, a diode or a transistor.
  • An ambient temperature sensor could also be provided as part of the ASIC as a VPTAT or IPTAT sensor based on bandgap reference.
  • the temperature compensation can be done by using both the temperature reading from the additional temperature sensing element/elements and the differential reading between the first and second resistive temperature sensors. This can be implemented by either a formula (within an algorithm) to adjust the final reading, or using a look up table and interpolation to determine the final reading.
  • the fluid sensor may further comprise an additional first temperature sensing element outside the membrane region and an additional second temperature sensing element located on or within the dielectric membrane region.
  • the fluid sensor may further comprise a pair of temperature sensing elements located on the dielectric membrane, wherein a first temperature sensing element of the pair of temperature sensing elements may be located on a first side of the heating element and a second temperature sensing element of the pair of temperature sensing elements may be located on a second side of the heating element.
  • the device is able to simultaneously sense properties of the fluid flow such as speed, mass, volume, shear stress as well as the composition of the flow (e.g., whether the fluid, in this case, the gas, has a certain CO 2 or hydrogen or methane percentage/ppm within air).
  • the fluid sensor may comprise a first pair of sensing elements and a second pair of sensing elements, and a differential signal between the first pair of further sensing
  • 11649661-1 23 elements may be configured to measure a property of a composition of the flow (such as different components of the fluid and their concentrations based on their different thermal conductivities), and a differential signal between the second pair of sensing elements may be configured to measure a flow property (such as flow rate, flow direction, velocity or flow mass or flow volume rates).
  • a property of a composition of the flow such as different components of the fluid and their concentrations based on their different thermal conductivities
  • a differential signal between the second pair of sensing elements may be configured to measure a flow property (such as flow rate, flow direction, velocity or flow mass or flow volume rates).
  • the flow could be measured by employing the pair of temperature sensing elements displaced on either side of the heating element within the same dielectric membrane, and optionally used as a differential pair.
  • the differential pair may be formed of one upstream sensing element and one downstream sensing element.
  • Holes or discontinuities may be placed so that they affect less the differential signal between the pair of temperature sensing elements that measure the properties of the flow but they affect significantly more the differential signal between the sensing elements that measure the composition of the flow.
  • a fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising: a semiconductor substrate comprising a first etched portion and a second etched portion, wherein the first etched portion and the second etched portion are substantially identical in size and shape; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first etched portion of the semiconductor substrate and a second dielectric membrane located over the second etched portion of the semiconductor substrate; a single active heating element, wherein the active heating element is located only within the first dielectric membrane; a first temperature sensing element located within the second dielectric membrane; and a second temperature sensing element located on or within the first dielectric membrane, wherein the second temperature sensing element is substantially identical in shape and size to the first temperature sensing element, and wherein the separation between the second temperature sensing element and the first temperature sensing element introduces a temperature difference between the second temperature sensing element and the first temperature sensing element, such that a differential
  • the first temperature sensing element may be placed on a second dielectric membrane wherein the second dielectric membrane does not comprise an active heating element.
  • the two membranes may be located side by side, laterally spaced from each other, and may be identical in size and shape.
  • the first temperature sensing element and the second temperature sensing element may be placed in a similar or identical position inside each of their respective dielectric membranes. Providing the temperature sensing elements in identical membranes improves matching characteristics.
  • the sensor comprises a single active heating element, wherein the active heating element is located only within the first dielectric membrane. Therefore, the sensor may comprise only one active heating element, such that there is no active heating element or electrically connected or powered heating element in the second dielectric membrane.
  • the second temperature sensing element may be a separate temperature sensing element, or the heating element may be configured to operate as the second temperature sensing element.
  • the sensor may further comprise an auxiliary structure located within the second dielectric membrane, and the auxiliary structure may be electrically isolated.
  • the auxiliary structure may be configured such that the first dielectric membrane and the second dielectric membrane have the same mechanical and thermal stress properties.
  • an auxiliary structure (also referred to as a dummy layer) (not connected electrically) may be located on or within the second dielectric membrane, such that the two temperature sensing elements have similar or identical structures in their proximity (i.e. neighbouring structures) and the two dielectric membranes with their respective embedded structures have substantially the same mechanical and thermal mass properties.
  • the auxiliary structure may be electrically isolated, in other words the dummy structure in the second dielectric membrane may be not connected to any electrical signal.
  • a sensor assembly comprising the fluid sensor as described above and an application specific integrated circuit (ASIC) coupled to the sensor.
  • ASIC application specific integrated circuit
  • the control circuitry can be located on the same chip as the sensor (monolithically integrated), or can have an application specific integrated circuit (ASIC) coupled to the sensor.
  • ASIC application specific integrated circuit
  • the ASIC can be on a separate chip, but within the same package, as a hybrid, co-packaged or using system in package (SIP) solutions.
  • SIP system in package
  • the ASIC could be placed outside the package, on a PCB (Printed Circuit Board) or within the same case/box.
  • the ASIC may be located underneath the sensor, for example using a die stack technique. Alternatively, the ASIC may be located side by side with the sensor or elsewhere.
  • the ASIC may be connected to the sensor using wire bonding and pads, or using through-silicon-vias (TSV) extending through the semiconductor substrate.
  • TSV through-silicon-vias
  • the sensor and the ASIC can be located on the surface of a common PCB or embedded in a PCB.
  • An ASIC may be provided within the same system or the same package or on-chip to provide electronic circuitry to drive, read-out signals and process signals from the sensor.
  • the ASIC may be placed in a stack die configuration under the sensor and the sensor and ASIC are placed within a manifold or an open package, to allow contact to the fluid.
  • a sensor assembly comprising a sensor housing; and a fluid sensor as described above located within the flow sensor housing.
  • the fluid sensor housing may comprise an inlet and an outlet, and a fluid flow path for directing a fluid flow through the sensor.
  • the sensor may be packaged within a packaging house or manifold with an inlet, outlet and a channel to provide more accurate measurements of the flow or the composition of the fluid.
  • a sensor assembly comprising the fluid sensor as described above, wherein the fluid sensor may be packaged on a printed circuit board in a flip-chip configuration.
  • the device may be packaged in a metal TO type package, in a ceramic, metal or plastic SMD (surface mount device) package.
  • the device may also be packaged directly on a PCB, or with a flip-chip method.
  • the device may also be embedded in a substrate, such as a customised version of one of the previously mentioned package, a rigid PCB, a semi-rigid PCB, flexible PCB, or any other substrate, in order to have the device surface flush with the substrate surface.
  • the package can also be a chip or wafer level package, formed for example by wafer-bonding.
  • the package maybe designed such that there is a surface very close to the membrane, on one side or both sides of the membrane, for example in a flip-chip scenario, such that the surface is less than 50um from the membrane. This increases the power loss through the fluid and improves the sensitivity of the sensor.
  • a method of measuring a concentration or composition of a fluid using a fluid sensor as described above comprising: applying an electrical bias to the heating element; and monitoring the electrical bias applied to the heating element and using the value of the electrical bias applied to the heating element and the differential signal to determine the concentration or composition of the fluid based on thermal conductivity of the fluid.
  • Applying an electrical bias to the heating element may comprise applying an electrical bias such that the differential signal between the first temperature sensing element and the second temperature sensing element may be minimised.
  • Minimised may refer to reducing the differential signal to zero or substantially zero.
  • the electrical power, current, or voltage applied to the heating element may be adjusted to bring to zero or substantially zero the differential signal between the first and second temperature detector elements (by varying the heating element power, current, or voltage could be such that the resistances of the two temperature detectors or the voltages across the temperature detectors are equal). This may be done during the calibration of the sensor or during the operation of the sensor. This could be set as
  • 11649661-1 27 calibrated point, giving a zero differential signal.
  • this could be set during the operation and the heater power/current/voltage could be measured as an indication of the fluid compositions or the concentration of its components.
  • the change in the electrical power, voltage or current through the heater may be monitored to measure one or more concentrations of specific components of the fluid based on their different thermal conductivities.
  • the first and second temperature sensing elements may be connected to a differential amplifier, a Wheatstone bridge, a lock-in amplifier, or a current reversal-based method type circuit such that the differential signal may be used to measure one or more concentrations of specific components of the fluid based on their different thermal conductivities.
  • the measurement of the differential signal can be performed in a number of ways.
  • a first way is to apply a constant current to both the first and second temperature sensing elements (temperature resistive detectors) and measure the voltage difference between them using a differential amplifier.
  • Further methods include the use of a Wheatstone bridge or other type of bridges, or current reversal-based techniques. For all these methods, a calibration can be done initially to set a zero-point value. This can either set a differential voltage value when the target fluid (or component of the target fluid) is not present, or modify the current to one of the resistors to ensure the differential voltage is at zero when the target fluid is not present.
  • the calibration can be done initially to set a zero-point value of the differential signal when the component of the fluid (e.g. CO2) is known (e.g. 400 ppm of CO2 in air) by using an external precision C02 device (e.g. NDIR sensor).
  • the component of the fluid e.g. CO2
  • an external precision C02 device e.g. NDIR sensor
  • the method may comprise driving the heating element in pulse mode or AC mode to modulate the temperature of the heating element to vary the differential signal; and using the differential signal to selectively differentiate between different fluid components and/or determine the concentration of the different components.
  • this may comprise heating the sensor to a first temperature where the thermal conductivity of air and the thermal conductivity of the target gas (for example carbon dioxide) are the same. This then facilitates the determination of the effect of other gases in the air (such as water vapour).
  • the sensor may then operate at a second (different) temperature, and the effect of other gases may be accounted for (i.e. reduced or cancelled) using e.g. a
  • the 11649661-1 28 look up table or a formula, so that only the effect of the target gas is determined.
  • the formula and/or look up table may be predetermined for the target gas.
  • the device may therefore comprise circuitry ora control system facilitating temperature modulation of the sensor using e.g. the heating element(s) or other suitable heater.
  • the thermal conductivity fluid sensor can be used in a mode where, instead of a constant DC temperature, the temperature is varied.
  • Thermal conductivity sensors generally work at a constant (DC) operating temperature and measure the heat loss to the surrounding medium.
  • This DC method is typically most effective when there is a single known gas (such as hydrogen) and the signal can be directly related to the concentration of the known gas.
  • the gas type is unknown then the concentration of the target gas cannot be determined using this method.
  • the gas concentration cannot be determined. That means it is not possible to determine the concentration of the target gas in a mixture (e.g. H 2 and He) or where the target gas is an unknown gas.
  • the thermal time constant associated with the transition depends not only upon the thermal conductivity l of the gas, but also on the specific heat capacity c of the gas (and the density of the gas p).
  • V is the volume of a body and A is surface area of the body.
  • the differences in density and heat capacity can be used to distinguish between different gases, in addition to the differences in the thermal conductivity.
  • the temperature of the heating element may be modulated by varying the current, voltage or power to different levels and/or with different electrical pulses such as to vary the differential signal between the first and second resistive temperature detectors in
  • 11649661-1 29 order to selectively differentiate between different fluid components and/or to provide information regarding the concentration of such components.
  • the temperature of the heater may be modulated and the voltage difference between the first and second temperature sensing elements at different temperatures may be assessed against reference values, and the difference between the two may be indicative of the flow composition.
  • the heating element temperature may be modulated by applying different power levels to increase sensitivity and selectivity to different fluid components based on their thermal conductivity variation with temperature. For example, the difference between the thermal conductivities of CO 2 and the air is higher at room temperature than at high temperatures. The opposite is true for Methane, so the difference between the thermal conductivities of methane and the air is lower at room temperature than at high temperatures. Hydrogen has also a different variation of the thermal conductivity with temperature than that of CO 2 or air. By running the heater at different temperature levels (i.e. modulating the temperature of the heater), it is entirely possible to differentiate between the contributions of different concentrations of fluid components in the fluid. In this way, for example, Hydrogen and CO 2 contributions can be decoupled and their concentration values can be found.
  • the heater also referred to as the heating element
  • the heater may be operated in a pulse mode (e.g. driven with a square wave, sinusoidal wave, Pulse Width Modulated wave (PWM), Pulse Density Modulation, etc.) or continuous mode.
  • the pulse mode has, among others, the advantage of reduced power consumption, reduced electromigration for enhanced device reliability/lifetime, and improved fluid properties sensing capabilities. Pulses could be used in different polarities to further reduce the impact of electromigration on the heating element.
  • the heater can be driven using PWM, and the off time of the PWM can be used to measure heater resistance, and/or differential signal. This measurement can be done in a very short time, faster than the thermal time constant of the membrane to avoid self-heating.
  • Selectively differentiating between different fluid components and/or determining the concentration of the different components may comprise using a neural network.
  • the sensor or a fluid sensing system may further comprise a controller or a processing system comprising a neural network.
  • the neural network may be trained using data from different known gases or mixture of gases at different temperatures. The use of a trained neural network to identify known gases or a mixture of gases can improve accuracy, sensitivity and selectivity of the fluid sensor.
  • the neural network may be trained to recognise the composition of a gas mixture based on the differential signal between the first and second temperature sensing elements.
  • the neural network could be trained using supervised learning based on a set of data of sensor output values for known gas mixtures at a set of heating element temperatures,.
  • the inputs to the neural network could be the sensor output values at a predetermined set of temperatures.
  • the neural network may be configured to process each differential signal from the first and second temperature sensing elements in order to determine the components of the gas mixture and the concentrations of each component in the gas mixture.
  • the outputs from the neural network could be the fraction of each gas in the mixture.
  • Synthetic training data could be generated to enhance the training by providing, for example, many more combinations of gases than would be practically realisable in a real laboratory.
  • a support-vector machine could be trained to discriminate between different gases.
  • the method may comprise: applying a modulated function to the heating element, the first temperature sensing element, or the second temperature sensing element; measuring the modulation, the time delay, or the phase shift of the differential signal between the first temperature sensing element and the second temperature sensing element; and determining a concentration or composition of the fluid using the measured modulation, time delay or phase shift.
  • a transient, modulated, or pulsed signal may be applied to either the heater element or the first or second temperature sensing elements, and the signals from the first or second temperature detectors will consequently be transient, and their time shape, time delay, or phase shift depends on both the thermal conductivity and the thermal diffusivity of the
  • the heaters or the first or second resistive temperature detectors can be biased with a transient signal (e.g. AC, square wave, pulsed, step).
  • a transient signal e.g. AC, square wave, pulsed, step.
  • the thermal diffusivity can be determined using the measured values from the first and second temperature sensing elements. In this way, more information can be extracted from the environment.
  • a step change in input current can be applied to the heater and the time constant for the temperature rise in the heater can be measured. This time constant can give information about the thermal conductivity and diffusivity of the environment. Both can be used to identify gas concentration.
  • a sinusoidal wave can be applied to the heater.
  • the change in amplitude and change in phase shift can provide information on thermal conductivity and thermal diffusivity, thus providing information on the gas concentration.
  • the heating element(s) may be provided with a DC bias point onto which a small AC signal (such as e.g. an AC, square wave, pulsed or step signal) can be superimposed.
  • a small AC signal such as e.g. an AC, square wave, pulsed or step signal
  • the thermal diffusivity, conductivity and/or thermal capacity of the target fluid can be determined using the measured values from the temperature sensing elements of the first and second dielectric material.
  • the changes in the amplitude, phase shift and/or changes in frequency of the measured values can provide information on thermal conductivity and/or thermal diffusivity, thus providing information on the gas concentration, or facilitating the selection between different components of the gas.
  • any of the resistive temperature detectors may be driven in short pulses of power, voltage or current.
  • the temperature sensing elements may be driven in a pulse mode (e.g. driven with a square wave, sinusoidal wave, Pulse Width Modulated wave, Pulse Density Modulation, etc.) or continuous mode.
  • the pulse mode has, among others, the advantage of reduced self-heating of the temperature sensing elements, which minimises the noise and increases the sensitivity or the signal
  • any other method of driving the sensor that can provide information on the environment that is being measured may be used.
  • a fluid sensor for sensing a concentration or composition of a fluid
  • the sensor comprising: a semiconductor substrate comprising a first etched portion and a second etched portion; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first etched portion of the semiconductor substrate and a second dielectric membrane located over the second etched portion of the semiconductor substrate; two or more active heating elements, wherein a first active heating element is within the first dielectric membrane and a second active heating element is located within the second dielectric membrane; a first heat diffuser located over the first heating element, wherein an edge of the first heat diffuser extends beyond an edge of the first heating element; a second heat diffuser located over the second heating element, wherein an edge of the second heat diffuser is approximately aligned with an edge of the second heating element; and one or more conductive elements located on or over one or both of the first and second dielectric membranes, the conductive elements arranged such that a conductive heat loss through the second dielectric membrane is
  • a differential signal between the first heating element and the second heating element is related to a thermal conductivity of the fluid or air mixture.
  • this differential signal is indicative of the concentration or composition of the fluid or air mixture being measured, and may be used to determine a concentration or composition of the fluid.
  • the conductive elements and heat diffusers may be configured such that the total heat loss from the first dielectric membrane is approximately or substantially equal to the total heat loss from the second dielectric membrane.
  • the additional heat loss to air from the first active heating element due to the larger area of the first heat diffuser compared to the second heat diffuser
  • the additional conductive heat loss from the conductive elements located on the second dielectric membrane such that the total heat loss from the two membranes is substantially the same.
  • the conductive elements may comprise a material with a higher thermal conductivity than the thermal conductivity of the first and second dielectric membranes. Alternately, the conductive elements may comprise holes or slots within the first and/or second dielectric membranes. In some implementations, the conductive elements may comprise materials with a lower thermal conductivity that the thermal conductivity of the first and/or second dielectric membranes, to thereby reduce the conductive heat loss of one or both of the first and second dielectric membranes.
  • the first and second etched portions may be substantially identical, i.e. such that they have approximately the same dimensions and/or shape.
  • Each heating element of the two or more active heating elements may be substantially identical in e.g. shape and/or size. Alternatively, one or more of the heating elements may be different to the other heating elements.
  • the first and second heat diffusers may be any element suitable for spreading heat, such as heat spreading plates. Some or all of the edges of the first heat diffuser may extend beyond corresponding edges of the first heating element, such that a size of the first heat diffuser is greater than the size of the first heating element. Similarly, some or all of the edges of the second heat diffuser may approximately align with corresponding edges of the second heating element, such that the second heat diffuser and second heating element are approximately or substantially the same size. As a result, the heat loss to air from the first heating element may be greater than the heat loss to air from the second heating element.
  • the first and second heat diffusers may comprise a material with higher thermal conductivity that the thermal conductivity of the first and second dielectric membranes.
  • a fluid sensing system comprising a fluid sensor as described above; and a controller configured to perform a method as described above.
  • the fluid sensing system may include a hardware or software interface wherein an algorithm is implemented to facilitate to selectively differentiate between different fluid components and/or to provide information regarding the concentration of such components.
  • a software algorithm configured to perform any of the methods as described above could be implemented to differentiate between these components and increase sensitivity related to each of the components of the fluids.
  • the software algorithm could be implemented in a local microprocessor.
  • Calibrated data could be stored in a memory device or integrated circuit.
  • the software could be incorporated within an ASIC and driving of the sensor and processing of the signal could be done within an ASIC.
  • Processing of the signal could also be done remotely in a sensor hub, or on an external server accessed using the Internet (for example, the cloud).
  • Sampling and averaging of the data, as well as ways to remove outliers from the data could also be used as part of an algorithm and could be implemented in hardware using different electronic components such as micro-controllers, memories or could be done using an ASIC.
  • Readings from the sensor may be averaged in several ways, for example using a moving mean average or a moving median average.
  • a moving mean average is useful for removing random noise from the signal.
  • a moving median average is useful for removing outliers.
  • a method of manufacturing a fluid sensor as described above comprising: forming a first dielectric membrane located over a first etched portion of a semiconductor substrate semiconductor substrate comprising a first etched portion; forming a heating element
  • 11649661-1 35 located within the first dielectric membrane; forming a first temperature sensing element spatially separated from the heating element, wherein the first temperature sensing element is located outside of the first dielectric membrane and over the semiconductor substrate, or wherein the first temperature sensing element is located on or within the first dielectric membrane and wherein the fluid sensor comprises at least one recessed region within the first dielectric membrane configured to thermally isolate the heating element from the first temperature sensing element.
  • Figure 1 shows a top view of a thermal conductivity fluid sensor with one circular resistor within a membrane, and one circular resistor outside the membrane, and circuitry to control & measure the sensor;
  • Figure 2 shows a top view of a thermal conductivity fluid sensor with two circular resistors within the membrane
  • Figure 3 shows the cross-section of a thermal conductivity fluid sensor shown in Figure
  • Figure 4 shows the top view of a thermal conductivity fluid sensor with recessed regions formed of slots within the membrane
  • Figure 5 shows the cross-section of a thermal conductivity fluid sensor with slots in the membrane, as shown in Figure 4;
  • Figure 6 shows the top view of a thermal conductivity fluid sensor with recessed regions formed of arrays of circular holes
  • Figure 7 shows the top view of a thermal conductivity sensor fluid with one wire resistive temperature detector within a membrane, and one wire resistor outside the membrane;
  • Figure 8 shows the top view of a thermal conductivity fluid sensor with two wire resistive temperature detectors within a membrane
  • Figure 9(a) shows the top view of a thermal conductivity fluid sensor with two wire resistive temperature detectors within a membrane and recessed regions shaped as slots;
  • Figure 9(b) shows an alternative thermal conductivity fluid sensor with a greater number of recessed regions within the dielectric membrane
  • Figure 9(c) shows an alternative thermal conductivity fluid sensor having a resistive wire having a meander shape within the same layer as the heater
  • Figure 9(d) shows an alternative thermal conductivity fluid sensor having a wire having a meander shape and a connecting element connecting two portions of the wire;
  • Figure 10(a) shows the cross-section of the device in Figure 9(a);
  • Figure 10(b) shows the cross-section of the device in Figure 9(c);
  • Figure 11 shows the top view of a thermal conductivity fluid sensor with two wire resistive temperature detectors within a membrane, and recessed regions comprising arrays of circular holes;
  • Figure 12(a) shows the top view of a thermal conductivity fluid sensor with the circuitry on the same chip
  • Figure 12(b) shows the top view of an alternative thermal conductivity fluid sensor, where the first temperature sensing element is placed on a second dielectric membrane;
  • Figure 12(c) shows the top view of an alternative thermal conductivity fluid sensor, where the first temperature sensing element is placed on a second dielectric membrane, having a dummy element;
  • Figure 13 shows the cross-section of a thermal conductivity fluid sensor with sloping sidewalls of the etched semiconductor substrate
  • Figures 14(a), 14(b), and 14(c) show cross-sections of alternative thermal conductivity fluid sensors where the etched portion of the substrate does not extend through the entire thickness of the substrate;
  • Figure 15 shows the top view of a thermal conductivity fluid sensor where the second resistive temperature detector element (shown as a resistive wire) is also on the membrane;
  • Figures 16(a) and 16(b) shows two alternative thermal conductivity fluid sensors comprising of an array of membranes and resistive temperature detectors
  • Figure 17 shows a thermal conductivity fluid sensor packaged such that there is a very thin channel above the membrane
  • Figure 18 shows a thermal conductivity fluid sensor packaged in a flip-chip configuration
  • Figure 19 shows the top view of a thermal conductivity fluid sensor with identical meander shaped resistive temperature detectors on and off the membrane;
  • Figure 20 shows a plot of gas thermal conductivity function with respect to the temperature for various gases
  • Figure 21a shows a circuit diagram for measuring the thermal conductivity fluid sensor comprising a Wheatstone bridge
  • Figure 21b shows an alternative circuit diagram for measuring the thermal conductivity fluid sensor comprising a Wheatstone bridge
  • Figure 21c shows a circuit diagram for measuring the thermal conductivity fluid sensor with a Wheatstone bridge, where the thermal conductivity sensor comprises an array of identical membranes;
  • Figure 22 shows a circuit diagram using constant current sources for both the resistive temperature detectors and the heating element
  • Figure 23 shows a circuit diagram where the differential current between the two resistive temperature sensors is measured
  • Figure 24 shows a circuit diagram comprising a Wheatstone bridge where each arm of the bridge can have a different voltage applied to keep the bridge balanced;
  • Figure 25 shows a circuit diagram with a Wheatstone bridge and a balancing resistor in the branch with the reference resistive temperature detector
  • Figure 26 shows a circuit diagram with a Wheatstone bridge, with the reference resistive temperature detector in series with a transistor
  • Figure 27 shows a flow chart giving a method that can be used to electronically balance the Wheatstone bridge
  • Figure 28 shows a top view of the thermal conductivity sensor, with an additional on-chip temperature sensor to determine the ambient temperature, or the die temperature;
  • Figure 29(a) shows a circuit diagram where a single resistor is used both as the heater and the first resistive temperature sensor element of the fluid sensor;
  • Figure 29(b) shows the current through the heater of the sensor of Figure 30(a);
  • Figure 30 shows a top view of the thermal conductivity fluid sensor where there are two resistive temperature detectors within the membrane region and two resistive temperature detectors outside the membrane region;
  • Figure 31 shows a circuit diagram of the thermal conductivity fluid sensor for the configuration where there are two resistive temperature detectors within the membrane region, and two outside the membrane region;
  • Figure 32 shows a top view of a thermal conductivity fluid sensor having a further pair of sensing elements configured to operate as a flow sensor
  • Figure 33 shows the cross-section of a fluid sensor assembly having a thermal conductivity fluid sensor contained in a package
  • Figure 34 shows the cross-section of an alternative fluid sensor assembly having a thermal conductivity fluid sensor contained in a package
  • Figure 35 shows the cross-section of an alternative fluid sensor assembly having a thermal conductivity fluid sensor contained in a package
  • Figure 36 shows a circuit diagram of a thermal conductivity fluid sensor where the heater is controlled via a feedback loop from the differential amplifier;
  • Figure 37 shows the top view and cross-section of a thermal conductivity fluid sensor comprising two recessed regions designed with different thermal properties
  • Figure 38 shows an alternate circuit diagram for a thermal conductivity fluid sensor
  • Figure 39 shows the top view of an alternate thermal conductivity fluid sensor.
  • Figure 40 shows a graph with thermal conductivities of air and different gases at different temperatures.
  • Figure 41 shows the top view and cross-section of a thermal conductivity fluid sensor comprising two sealed cavities, with one containing holes.
  • Figure 42 shows an alternate circuit diagram for a thermal conductivity fluid sensor.
  • Figure 42a shows the current through the heater for the circuit in Figure 42.
  • Figure 43 shows an alternate circuit diagram for a thermal conductivity fluid sensor.
  • Figure 43a shows the current through the heater and the voltage across it, as well as the reading computation for the circuit in Figure 43.
  • Figure 44 and 45 show alternate circuit diagrams for a thermal conductivity fluid sensor based on Figure 43a.
  • Figure 46 shows a comparison between DC and current-reversal DC voltage measurements.
  • Figure 47 shows a circuit schematic for driving the sensors in who different current directions.
  • Figure 48 shows a table of thermal properties of different gases.
  • Figure 1 shows a top view of a thermal conductivity fluid sensor. It comprises a chip 1 made of a semiconductor substrate and a dielectric layer or region suspended on or over an etched portion of the semiconductor substrate, defining a region of the dielectric layer above the etched portion as a dielectric membrane 4. There is a resistor 2 embedded within the membrane, and track 7 connect it to bond pads 6. The resistor 2 is configured to operate as a heating element 2, and in this embodiment, it also acts as a resistive temperature detector element. There is another temperature detector element (also referred to as a thermal detector element) 3 outside the dielectric membrane.
  • the fluid sensor also includes circuitry 5, that uses a differential signal from the two temperature detector elements 2, 3 to determine the composition of the fluid based on its thermal conductivity.
  • the heater 2 operates at a higher temperature than the first temperature sensing element 3 even in zero flow (or when no flow is present) when the heater 3 is powered up.
  • the temperature of the first temperature sensing element 3 is dependent on the ambient temperature, and the temperature of the heating element 2
  • 11649661-1 41 can vary depending on the heat loss to the surrounding fluid - which is dependent on the thermal conductivity of the fluid.
  • the temperature differential (differential signal) between the heating element 2 and the first temperature sensing element 3 may be proportional to the concentration of a fluid.
  • the thermal conductivity of the CO2 is smaller than that of air
  • the temperature difference between the heater 2 and the first temperature sensing element 3 will be greater as the thermal conductivity of the CO2 is smaller than that of air.
  • the temperature difference between the heating resistor 2 and the first temperature sensing element 3 could be translated into a voltage difference or resistance difference, depending on the temperature sensing element employed. For diodes supplied with constant current, or for thermopiles, the voltage difference is appropriate. For Resistive Temperature Detectors (RTD), several read-out techniques could be employed such as using instrumentation bridges to measure change in the resistance or using current mirrors and sensing the voltage difference.
  • RTD Resistive Temperature Detectors
  • the membrane is shown as circular. However, it can be rectangular, rectangular with rounded corners or any other shape.
  • the resistors 2 and 3 are shown as circular, but can be any shape including ring, meander or rectangular.
  • the resistor maybe made of a CMOS metal such as aluminium, tungsten, titanium or copper, or a non-CMOS metal such as gold or platinum, or from polysilicon or single crystal silicon.
  • Figures 2 shows a top view of an alternative thermal conductivity fluid sensor
  • Figure 3 illustrates a cross-section of the sensor of Figure 2.
  • the thermal conductivity fluid sensor of Figures 2 and 3 has two circular resistors 2, 8 within the membrane region 4.
  • One of the resistors is configured to operate as a heater element 2, and the other resistor within the dielectric membrane is configured to operate as temperature detector element 8.
  • the two resistors 2, 8 are made of different layers within the dielectric layer 10 and can be in close proximity to each other so that they are at substantially the same temperature.
  • the substrate 11 is a
  • the second temperature sensing element 8 operates at a higher temperature than the first temperature sensing element 3 even in zero flow (or when no flow is present) when the heater 2 is powered up.
  • Figures 4 shows a top view of an alternative thermal conductivity fluid sensor
  • Figure 5 illustrates a cross section of the sensor of Figure 4.
  • the thermal conductivity fluid sensor of Figures 4 and 5 has a circular resistive heater 2 acting as both a heater element and a temperature detector element, and has two recessed regions within the membrane, which are shown as two slots 12. The slots are circular around the heater.
  • the recessed regions minimise the thermal path through the solid dielectric membrane, forcing more heat to dissipate via convection and conduction through the environment (mostly above the membrane via conduction and convection), but partly also via heat conduction through the space formed by the slots or below the membrane. In this way a larger proportion of the heat loss of the heating element is to the surrounding fluid. So when there is a change in the thermal conductivity of the fluid the change in temperatures of the heating element and the second temperature sensing element are increased - thus the recessed regions increase the sensitivity of the device.
  • the presence of the slots also helps to reduce the power consumption of the device (for the same heater temperature), because of the reduction in the total heat losses. Furthermore, the slots help to reduce the thermal response time (increase the speed at which the heater heats up when supplied with an electrical power pulse) due to the decrease in the thermal mass of the membrane.
  • Figure 6 shows a top view of a thermal conductivity fluid sensor with a circular resistive heater 2 acting as both a heater element and a temperature detector element, and several recessed regions around the resistor in the shape of small circular holes 13.
  • the circular holes 13 increase the proportion of power loss to the fluid and there improve sensitivity, reduce thermal response time and power consumption.
  • Figure 7 shows a top view of a thermal conductivity sensor with a wire shaped resistor 2 within the membrane region, and one wire shaped resistor outside the membrane 3, with the resistor within the membrane 2 acting as both a heater element and as a first temperature detector element.
  • the membrane 4 is in case is a rectangular membrane with rounded corners - but can also be of any other shape. This sensor operates similarly to the sensor of Figure 1.
  • Figure 8 shows a top view of a thermal conductivity sensor with two wire resistors within the membrane region, with one operating as a heater element 2, and the other operating as a temperature detector element 8.
  • Figures 9 (a) to 9(d) each show a top view of an alternative thermal conductivity fluid sensor with recessed region 12 within the dielectric membrane. Recessed regions reduce the thermal losses from the heater, and increase the percentage of power loss to the fluid, thus improving the device sensitivity.
  • Figure 9a shows sensor where the heater element 2 is a wire resistor, and a second thermal detector element 8 is also a wire resistor.
  • Figure 10a shows the cross-section of the device in figure 9a.
  • Figure 9b shows a sensor where there are four recessed regions 12 on the membrane, two on either side of the heater and second temperature detector element. By increasing the number of recessed regions, the sensitivity of the device is increased.
  • the second thermal detector element 8 has a meander shape and is designed such that it is located on both sides of the heater 2. In this configuration, the second thermal detector element 8 is located in a different layer of the dielectric layer than the heater 2. The shape of the first temperature sensing element 3 is also the same as the second temperature sensing element 8.
  • Figure 10b shows the cross-section of the device in figure 9c.
  • the first thermal detector element 8 is in two parts, and split either side of the heater.
  • Figure 9d shows a sensor in which the second thermal detector element 8 also has a meander shape and is located on both sides of the heater 2.
  • the second temperature detecting element 8 is formed of two portions, with a first portion of the second temperature detecting element 8 located on a first side of the heating element 2, and a second portion of second temperature detecting element 8 located on a second, opposite side of the heating element.
  • a connection between the two portions of the second temperature detecting element is located outside the dielectric membrane 4, and is formed of a connecting element 20. This allows the first thermal detector element 8 to be made within the same layer of the dielectric layer as the heater element 2, with only the connecting element 20 located within a different layer of the dielectric region, and used to bridge the two portions of the second thermal detector element 8.
  • the shape of the first temperature sensing element 3 is also the same as the second temperature sensing element 8.
  • Figure 11 shows a top view of a thermal conductivity fluid sensor with a recessed region 13 including arrays of circular holes within the dielectric membrane region 4.
  • Figure 12(a) shows a top view of a thermal conductivity fluid sensor with circuitry 5 located on the same chip as the heating element 2, and the first and second temperature sensing elements 3, 8.
  • the circuitry 5 is used to control and drive the heater 2, and also measure the differential signal between the first temperature detector element 3 and the second temperature detector element 8. It may comprise a constant current or constant resistor drive circuit, a constant current source, a Wheatstone bridge, an amplifier, an Analog to Digital convertor, a Digital to Analog Convertor and/or a microcontroller.
  • Figure 12(b) shows a top view of a further fluid sensor where the first temperature sensing element 3 is on a second membrane, 4a, separate and identical in dimensions with the first membrane 4.
  • the second dielectric membrane 4a has no active heating element. Common mode effects such as extra temperature rise due to self-heating when the two temperature sensing elements are biased can be removed. The effect of pressure and/or residual stress/strain in the membranes can also be cancelled out.
  • thermal mass associated with the two temperature sensing elements can be identical (or very similar) and thus dynamic mismatching effects can be minimised.
  • Figure 12(c) shows a similar fluid sensor to the one shown in Figure 12(b).
  • an auxiliary structure 201 including one or more further dummy layers or elements is located on or within the second dielectric membrane 4a, such that the two temperature sensing elements have similar structures in their proximity (i.e. neighbouring structures) and the two membranes with their respective embedded structures seem to be identical from a mechanical and thermal mass perspective.
  • the auxiliary structure 201 may have identical size, shape and materials as the heating element 2.
  • the dummy elements 201 in the second membrane are not connected to any electrical signal, and are therefore electrically isolated.
  • this fluid sensor is that the two temperature sensing elements 3, 8 are very well matched (they are both on identical membranes, they have similar neighbouring structures around them) in terms of their characteristics, including stress, or deformations. Moreover, the two sensing elements see similar mechanical stress profile and therefore common mode effects such as ambient pressure or vibrations can be removed.
  • Figures 13, 14(a), and 14(b) show etched regions within the semiconductor substrate 11 a thermal conductivity fluid sensor.
  • the etched region has sloping sidewalls, which can be achieved by use of KOH or TMAH etching. Such an etching method is cheaper, but requires a larger chip area.
  • Figures 14(a) and 14(b) show thermal conductivity fluid sensors where the etched region does not extend through the entire semiconductor substrate 11. This can be achieved by etching from the front side of the substrate. This process results in a membrane or bridge structure supported by a dielectric beam. This results in a sensor with lower thermal power losses, but also with lower mechanical robustness compared to the sensor of Figure 13.
  • etching is performed such that it stops at the crystal plane of the semiconductor substrate 11, resulting in an etched region having a triangular profile.
  • the etching is isotropic, resulting in an etched region having a rounded profile.
  • the etching is performed similar to Figure 14a in that it stops at the crystal planes of the semiconductor substrate 11, but the stop point of the etching process is
  • 11649661-1 46 also controlled (for example by timing) so that it does etch completely, resulting in an etched region having a trapezoid profile.
  • Figure 15 shows a top view of a thermal conductivity sensor design where both the first and second thermal detector elements 3, 8 are located on or within the same dielectric membrane 4.
  • the heater element 2 and the second temperature detector 8 element are both located between two slotted recessed regions 12.
  • the first temperature detector element 3 is thermally isolated from the first temperature detector element 8 and the heating element 2 by one of the slotted recessed regions 12. In this configuration, the heater element 2 and the second thermal detector element 8 are at substantially the same temperature during operation of the sensor, while the first thermal detector element 3 is at a different temperature, and is closer to the ambient temperature.
  • Figures 16a and 16b shows top views of two thermal conductivity fluid sensors each comprising an array of membranes.
  • FIG 16a there are three dielectric membranes 4, and the heater 2 and thermal detector elements 3, 8 from each membrane are connected in series.
  • Each of the heating elements 2 are connected in series, each of the first temperature sensing elements 3 are connected in series, and each of the second temperature sensing elements 8 are connected in series. If this system is operated in a constant current mode for the heater 2 and the thermal detector elements 3, 8, then the differential voltage signal will be higher.
  • the differential voltage signal will be multiplied by three compared to sensors having a single dielectric membrane with a single heating element and first and second temperature detecting elements). This is given as an example, but greater or fewer number of membranes can also be used within the fluid sensor.
  • Figure 16b shows an alternative thermal conductivity fluid sensor comprising 4 membranes, but the elements in each membrane are connected separately to bond pads. This allows much more flexibility in the design and use of the sensor.
  • the four heaters 2 could be driven separately, for example at different temperatures, or with different drive modes.
  • the heating elements 2 can be connected in series externally in a manner similar to figure 16a to increase the output signal.
  • Figure 17 shows a cross section of a thermal conductivity fluid sensor where there is a covering 25 forming a very thin fluid channel 26 above the dielectric membrane of the dielectric layer 10.
  • One or more holes 30 through the covering layer 25 allow fluid of various concentrations to diffuse or flow into the fluid channel region 26.
  • the thin channel 26 increases the thermal losses from the heating element 2 to the fluid (from the membrane 4 to the covering 25) as the heat transferred through the fluid only needs to travel a smaller distance from the heating element 2 to the covering 25, the amount of heat loss through the fluid is increased.
  • the heat needs to travel a greater distance to the closest solid surface (which may be the chip surface, as the heat transfer isn’t required to be in a straight line). Therefore, the covering 25 increases the sensitivity of the device.
  • the covering 25 can be a semiconductor bonded by wafer bonding. It can also be glass, or plastic.
  • Figure 18 shows a cross section of a thermal conductivity fluid sensor packaged in a flip chip method.
  • Solder balls 36 form electrical connections to a Printed Circuit Board (PCB) 35.
  • PCB Printed Circuit Board
  • This also forms a thin channel 26 between the membrane of the dielectric layer 10 and the PCB 35, allowing for an increase in sensitivity of the device to fluid concentration similar to the device in figure 18.
  • Figure 19 shows a top view of a thermal conductivity fluid sensor with the resistive temperature sensors 3, 8 having a meander shape.
  • the second resistive temperature sensor 8 is configured such that the wire element of the second resistive temperature sensor 8 loops around one of the bond pads of the heater 2 and the second resistive temperature sensor 8 has two bond pads located on either side of the other bond pad of heater 2.
  • the second temperature sensing element 8 can be made in a single layer, and preferably within the same material layer as the heater 2.
  • the first resistive temperature sensor 3 is the same shape as the second resistive temperature sensor 8, but is located outside the membrane region 4.
  • Figure 20 shows how the thermal conductivity measured by the fluid sensor varies with temperature for the gases of air, carbon dioxide, hydrogen and methane. This figure illustrates that the temperature dependence of gas thermal conductivity is different for different gas compositions. This means that heaters can be used at an optimum temperature for sensitivity of the device to different gases. In addition to this, the inset shows a detailed view of the temperature that air and carbon dioxide have the same
  • 11649661-1 48 value of thermal conductivity This can be advantageous for the device selectivity, and multiple heater temperatures can be used to help identify, or ignore, certain gases (e.g. running the device at the temperature where carbon dioxide and air are identical eliminates any response to carbon dioxide in air).
  • Figure 21a shows an example circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor.
  • This circuitry could be used in conjunction with any of the sensors described above having a heater 2, and first and second temperature sensing elements 3, 8.
  • the heater 2 is driven by a current source.
  • the first and second resistive temperatures sensors 3, 8 are located on sides of a Wheatstone bridge along with two additional resistors 40 and 41. One side of the bridge (between the first and second resistive temperatures sensors 3, 8) is connected to a reference voltage 50, while the other side 60 is grounded.
  • a differential amplifier 55 measures the differential voltage between the two legs of the Wheatstone bridge.
  • the heater 2 may be drive with a constant current. When the concentration of the target gas changes, then the temperature of the heater 2, and hence the temperature and resistance of the second resistive temperature sensor 8 will change. This will change the differential voltage between the two arms of the Wheatstone bridge and can be detected.
  • the circuit may be calibrated in a standard environment (for example, with no target gas present) to know what the nominal or calibrated differential voltage is. Deviation from this calibrated differential voltage indicates presence of the target gas.
  • the resistors 40 and 41 are chosen such amplifier 55 outputs a zero voltage at a normal of calibrated condition (for example Oppm of the target gas in air, or in case the target gas is carbon dioxide then in 400ppm of carbon dioxide in air).
  • the resistors 40 and 41 maybe trimmed during the calibration of the device. If resistors 40 & 41 are not chosen in such a way, then they may be calibrated to know what the differential voltage will be in the calibration conditions.
  • Another way to drive the fluid sensor is to control the current through the heater 2 such that the differential voltage across the Wheatstone bridge is always constant. In this case, the change in current required within the heater 2 could be measured to indicate the presence of a target gas.
  • Figure 21b shows another arrangement of the Wheatstone bridge where the resistors 3 and 40 are swapped. Besides this many other arrangements of the bridge are possible.
  • Figure 21c shows an example of circuitry for driving a thermal conductivity fluid sensor and measuring the output from the fluid sensor comprising an array of membranes and, heater and temperature sensor elements connected in series.
  • This circuitry could be used in conjunction with either of the sensors shown in Figures 17a and 17b.
  • the Wheatstone bridge configuration of Figure 22b can be used in a similar manner to that described in relation to Figure 22a. The devices connected in this way can amplify the sensitivity due to increased voltage changes.
  • Figure 22 shows an example circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor.
  • the heater 2, and the first and second resistive temperature sensors 3, 8 are each driven by their own, separate current sources 45, 46, 47.
  • the fluid sensor is first calibrated in a standard, predetermined environment, and current sources 46 and 47 are adjusted such that the output from the differential amplifier 55 is zero.
  • the current sources 46 and 47 are driven at the calibrated current levels, and the deviation of the output from zero of the differential amplifier 55 indicates the presence of the target gas.
  • Figure 23 shows alternative circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor, having a Wheatstone bridge similar to Figure 21.
  • the heater 2 is driven by a voltage source VHTR.
  • the arm of the Wheatstone bridge that has the first resistive temperature sensor 3 also has a variable resistor 44 in series with the first resistive temperature sensor 3.
  • the first and second resistive temperature sensors 3, 8 can have different resistances during heater operation, but during calibration the variable resistor 44 can be adjusted such that the output from the differential amplifier 55 is zero.
  • the variable resistor 44 can be adjusted manually or electronically.
  • Figure 24 shows alternative circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor, however each side of the bridge has a different supply voltage.
  • One side is kept constant at VREF, while the other is kept at an adjustable voltage VBAL.
  • VBAL can be adjusted so that the differential amplifier 55 gives an output of zero volts. This VBAL value can then be stored in either in firmware or
  • VBAL 11649661-1 50 software of the sensor. This VBAL value is then applied whenever the device is operated, and deviation of the differential amplifier output from zero indicates the presence and concentration of the target gas.
  • VBAL can be controlled during operation to keep the output signal at zero, and changes in the required VBAL value can be measured to indicate the presence of a gas.
  • Figure 25 shows alternative circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor comprising a Wheatstone bridge with a variable resistor 44 similar to figure 23, but the heater is driven using a current source.
  • Figure 26 shows alternative circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor.
  • the first and second resistive temperature sensors 3, 8 are both in the bottom side of the Wheatstone bridge.
  • the branch comprising the first resistive temperature sensing element 3 also has a Field Effect Transistor (FET) 65 in series with the first resistive temperature sensing element 3.
  • FET 65 is similar to the variable resistor of Figure 25, however this can be controlled electronically allowing calibration without manual intervention.
  • Figure 27 shows steps in of a method of balancing the Wheatstone bridge shown in figure 24. This method uses a fixed number of iterations. A counter is set to the maximum number of iterations. At each iteration, the counter is reduced by 1. If the value of the counter is negative then the current VBAL value is set as the balance voltage. Otherwise, the output from the differential amplifier is checked. If the output is positive, then the VBAL value is increased, otherwise it is decreased.
  • This method can be used in two ways. It can be used in calibration of the fluid sensor to determine the required balance voltage at a standard environment. Alternatively, it can be used during operation of the fluid sensor to keep the Wheatstone bridge balanced, and the VBAL value can be measured to determine presence and concentration of gas.
  • Figure 28 shows the top view of a thermal conductivity fluid sensor where there is an additional temperature sensing element 75 on the chip, and outside the dielectric membrane 4.
  • This additional temperature sensing element 75 can be used to compensate for effects of ambient temperature changes. Most effects of ambient temperature changes will be cancelled out due to the differential measurement method of the fluid sensor. However, compensating for ambient temperature changes using a temperature sensing element 75 will further improve accuracy.
  • the additional temperature sensing element 75 shown in the figure is a resistive temperature sensor. However, it can also be a diode, transistor or a standard temperature measurement circuit such as an iptat or a vptat circuit.
  • Figure 29a shows alternative circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor where a single resistor 2 is used as the heater and the second resistive temperature sensing element, similar to the sensor shown in Figure 1.
  • this sensor there is a bridge circuit having two sides. One side has the first resistive temperature sensing element 3, and an additional resistor 41. The other side comprises the heating resistor 2 and a further additional resistor 40, with resistor 40 ideally identical to resistor 41.
  • the resistor 2 When using the resistor 2 to measure sensing, the current from the current source 45 flows through resistors 2 and 40. The signal at the output of the differential amplifier 55 will be dependent on the temperature of resistor 2.
  • a switch 70 is closed, allowing a larger current to flow.
  • Figure 29b shows a PWM (Pulse Width Modulation) waveform which shows with time the current and/or voltage in the heater of Figure 29a.
  • the pulses have a high frequency such that there is little change in heater temperature during the “off” mode of the pulsed heater.
  • the resistance of the heater can be measured to determine the heater temperature, and a differential signal between the temperature of the heating element and the fist temperature sensing element can be used to determined thermal conductivity of the fluid in the sensor.
  • This method can be used in cases where the heater is also used as the second resistive temperature detector element as shown in figure 29a, by opening and closing the switch 70.
  • Figure 30 shows a top view of a thermal conductivity fluid sensor where in addition to the first and second temperature detector elements 3, 8 there are two additional temperature detector elements 3A, 8A. Temperature sensing elements 8 and 8A are both on or within
  • Figure 31 shows circuitry for driving the thermal conductivity fluid sensor and measuring the output from the thermal conductivity fluid sensor shown in figure 31.
  • the second temperature detector elements that are on the membrane 8, 8A are placed on opposite sides of the Wheatstone bridge.
  • both the first temperature detector elements outside the membrane region 3, 3A are also placed on opposite sides to each other. This configuration doubles the sensitivity of the thermal conductivity sensor.
  • Figure 32 shows the top view of a thermal conductivity fluid sensor comprising two additional resistive elements 100 either side of the heater 2 and the second temperature detector element 8.
  • This allows the device to be used as not only a thermal conductivity sensor, but also a flow sensor.
  • One resistive element of the pair of resistive elements 100 is located upstream of the heating element 2 and another resistive element of the pair of resistive elements 100 is located downstream of the heating element 2.
  • the heating element 2 extends in a direction substantially perpendicular to the direction of flow through the sensor. When the fluid passes over the top of the membrane 4, the heater 2 cools down due to heat convention losses. In the presence of the flow, the downstream sensing element sees a higher temperature than the upstream sensing element.
  • the temperature difference between the pair of resistive elements 100 increases with the flow rate (or flow velocity). In the presence of a fluid flow, there will be a difference in resistance between the two additional resistive elements 100 depending on the speed and direction of the fluid flow. Whilst shown as resistive elements, the two additional elements 100 for flow sensing can be based on other temperature detection principles such as diode based temperature detectors, or a thermopile temperature detector.
  • Figure 33 shows a cross-section of a thermal conductivity fluid sensor assembly. It comprises a package base 101 and a package lid 102. Within the package is an ASIC (Application Specific Integrated Circuit) chip 103 that is used to control and measure the thermal conductivity sensor chip. Above this ASIC chip 103 is the thermal conductivity sensor chip comprising a substrate 10 and dielectric region or layer 11. The sensor chip may include any fluid sensor as described above. Wire bonds 104 electrically connect the thermal conductivity sensor fluid chip to the ASIC chip 103, and wire bonds 105
  • a hole 108 within the package lid 102 allows the ambient air or gas to diffuse into the package and around the thermal conductivity sensor. More than one hole may be present within the package lid, and the size and shape of the hole 108 can be varied, and filters may be placed around or within the hole 108 or holes to protect against particles or liquids.
  • Figure 34 shows a cross-section of an alternative thermal conductivity fluid sensor assembly.
  • the ASIC chip 103 and the fluid sensor chip are not stacked on top of each other, but are located side by side within the package. Wire bonds 106 connect the sensor chip to the ASIC chip 103.
  • Figure 35 shows a cross-section of an alternative thermal conductivity fluid sensor assembly.
  • the lid 102 has two ports, one as an input port 106 and one as an output port 107.
  • Figure 36 shows alternative circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor.
  • a circuit block 80 to control the heater 2.
  • the output from the instrumentation amplifier 55 is part of a feedback loop into the heater control 80.
  • the heater 2 can then be controlled such that it keeps the output of the instrument amplifier 55 at zero voltage.
  • the bias or control signal required to the heater 2 is then used to determine presence and concentration of gas within the fluid sensor.
  • Figure 37 shows a top view and cross-section of an example thermal conductivity fluid sensor.
  • a first membrane 4 and a second membrane 4a. Both membranes have identical heaters 2 and 2a which can also be configured to act as temperature sensors.
  • There is an additional layer 200 on the chip which forms structures 203, 203a and 202.
  • Layer 200 is preferably made of a material with thermal conductivity higher than the dielectric membranes 4, 4a.
  • Structure 203 is located above the first membrane 4, and is a circle, or a plate shape that is larger than the size of the heater 2 within the first membrane.
  • Structure 203a is located above the second membrane 4a, and forms a circle or a plate that is the same or a similar size as the heater 2a.
  • structures 203 and 203a do not have to be circular, and may be any other shape, such as rectangles. However, in some implementations, circular structures 203, 203a may provide a more uniform heat distribution and/or enhanced mechanical stability, in comparison to other structure
  • Structure 202 is located above the chip, but is absent above the bond pads 6 and some areas of the membranes 4, 4a. As shown, an edge of structure 202 may be approximately aligned with to the edge of the first membrane area 4, but structure 202 does not extend above the first membrane area 4. However, structure 202 is does extent above the second membrane 4a area, leaving a circular portion in the middle of the second membrane 4a that is uncovered.
  • Such a construction can be configured so that the power consumption of both the heaters 2 and 2a is the same or approximately the same for a given temperature.
  • the ratio of power loss to air or other fluids as compared to power loss through the membrane(s) 4, 4a may be different for each of the heaters 2, 2a.
  • both heaters may give a different response when in the presence of a target fluid such as a target gas, and a differential signal between them (e.g. using the heaters 2, 2a as temperature sensors) can therefore be used to determine the concentration or composition of the target gas.
  • Figure 38 shows an example circuit for measuring the thermal conductivity fluid sensor shown in Figure 37.
  • a Wheatstone bridge is used with the heaters 2 and 2a connected in the circuit along with two fixed resistors 40, 41.
  • the differential voltage measured between the two arms of the bridge can be used to determine the concentration of the target gas.
  • Figure 39 shows a top view of another thermal conductivity fluid sensor.
  • the active heater 2 can also be configured to act as a temperature sensor.
  • a first temperature sensor 3 is placed between the active heater 2 and the edge of the dielectric membrane 4.
  • An elongated slot 12 may optionally be placed next to the heater on the opposite side from the first temperature sensor.
  • a second temperature sensor 8 is placed on the semiconductor chip 1 outside the dielectric membrane 4.
  • the temperature of the first temperature sensor 3 may be a fixed portion of the difference between the temperature of the active heater 2 and the second temperature sensor 8. As the ratio is fixed, a reference can be constructed in the circuitry 5 using the resistances of the heater 2 and the second temperature sensor 8 and the fixed ratio to compare with the resistance of the first temperature sensor 3.
  • 11649661-1 55 difference between the constructed reference and the signal from the first temperature sensor can be used to determine the concentration of the target gas.
  • Figure 40 shows a graph plotting the thermal conductivity of air and different gases across different temperatures.
  • the graph illustrates that if the heater is driven at around 800K, then air and carbon dioxide have the same thermal conductivities, and any deviation from normal is caused by other effects, such as humidity.
  • the heater can then be run at a lower temperature where in addition to other effects, carbon dioxide also causes a deviation in signal. Using algorithms or a look up table the deviation due to other effects can then be cancelled to determine the deviation solely due to carbon dioxide.
  • FIG 41 shows another example thermal conductivity fluid sensor.
  • the two membranes are identical, apart from the two (or more) holes 12 on membrane 4.
  • Membrane 4a has no holes.
  • Both membranes have identical heaters 2 and 2a, and sensing elements 3 and 3a.
  • the thermal conductivity fluid sensor is located on a base of a package 8, where the cavities 210 and 210a under the membranes 4 and 4a are both sealed.
  • Such a construction can be designed so that the sensing element 3a, which is exposed to the gas only on one side of the membrane 4a can be used as a reference for the sensing element 3, which is exposed to the gas on both sides of the membrane 4.
  • This design could lead to a faster and more reliable temperature compensation, since both sensing elements are directly exposed to the same environment, however only sensing element 3 is exposed to a higher gas concentration.
  • Figure 42 shows an example circuit for measuring a thermal conductivity fluid sensor such as that shown in Figure 41.
  • two AC current sources 45 and 45a are used with heaters 2 and 2a respectively. Both current sources can independently generate square wave signals, as shown in Figure 42a, with adjustable intensity and frequency.
  • the differential voltage signal, measured across heaters 2 and 2a is then processed by a lock-in amplifier 55, or a fast Fourier transform (FFT)-based digital signal processing (DSP) circuit.
  • FFT fast Fourier transform
  • DSP digital signal processing
  • Figure 43 shows another example circuit for measuring a thermal conductivity fluid sensor such as that shown in Figure 41.
  • this circuit two reversible DC current sources
  • Figure 44 shows a variation of the circuit presented in Figure 43.
  • two DC current sources 45 and 45a are used with heaters 2 and 2a respectively, while two reversible DC current sources 46 and 46a, providing a much smaller current, are used with sensing elements 3 and 3a respectively.
  • Figure 45 shows variation of the circuit presented in Figure 44.
  • the sensing element 3 and 3a are connected in series and driven by a single reversible DC current source 46.
  • Using a single current source may improve the circuit immunity to common mode noise while simplifying circuit overall.
  • Figure 46 shows a comparison between -1200 DC voltage measurements of a -60 W heater made with -8 mA test current taken approximately over 120 seconds.
  • the DC measurements fluctuate with a voltage error of up to 30%, whereas the three-point DC reversal method measurements fluctuate with less than 5% error.
  • These figures can be further significantly improved by using a smaller (e.g. less than a few mA) test current.
  • Figure 47 shows a circuit schematic for driving the sensors in two different current directions.
  • a control 301 that provides the electrical bias to the sensors 302.
  • the sensors are read by a read out circuit 303.
  • the transistors 305,306,307 and 308 control the direction of current within the sensors 302. When transistors 305 and 306 are on, and 307 and 308 are off then the current flows in one direction through the sensors. While when the transistors 305 and 306 are off, and transistors 307 and 308 are on, the current flows in the opposite direction. This method can be used to improve the accuracy of measurements using the delta method.
  • the control system 301 can be just a current
  • the sensors 302 may be e.g. a temperature sensing resistor, or may comprise more than one resistor.
  • the more than one resistors may be provided in a bridge configuration - where all the branches have active sensors, and/or some branches have fixed resistors.
  • the read out circuit 303 can have a differential amplifier, filter and/or an analog to digital circuit.
  • Figure 48 provides the thermal properties of different example gases of interest as well as those of dry and wet air (at standard temperature and pressure).
  • the values of nitrogen and oxygen are also provided to demonstrate how sensitive the values can be to the oxygen content of air.
  • Figure 48 also shows the thermal response time relative to dry air for each of these gases. For example, it can now be seen that Helium gas is 8.3x faster than dry air and 7.1x faster than wet air. It can be seen that hydrogen gas is 6.9x faster than methane. Wet air is 1.13 or 13% faster than dry air. Finally, it can be seen that C02 is 2.1x slower than dry air and 2.4x slower than wet air.
  • This AC method decouples the thermal signal of the target gas (for example C02) from that of a variable background gas (for example, other components of air such as nitrogen and/or oxygen ), and therefore provides a much more accurate way of measuring a gas concentration or composition than DC techniques.
  • a target gas for example C02
  • a variable background gas for example, other components of air such as nitrogen and/or oxygen
  • Second temperature sensing element 75 Ambient temperature sensing element 8A Additional second temperature 80 Heater control sensing element 100 Pair of temperature sensing

Abstract

We disclose herein a fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising a semiconductor substrate comprising a first etched portion, a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first etched portion of the semiconductor substrate, a heating element located within the first dielectric membrane, and a first temperature sensing element spatially separated from the heating element. The fluid sensor further comprises a second temperature sensing element within the dielectric membrane, or the heating element may be further configured to operate as a second temperature sensing element. The separation between the second temperature sensing element and the first temperature sensing element introduces a temperature difference between the second temperature sensing element and the first temperature sensing element, such that a differential signal between the first temperature sensing element and the second temperature sensing element is indicative of the concentration or composition of the fluid based on a thermal conductivity of the fluid.

Description

1
Thermal Fluid Sensor
Technical Field
The present disclosure relates to a micro-machined sensor, particularly but not exclusively, the disclosure relates to a fluid sensor for sensing concentration of a fluid or concentration of components of a fluid based on thermal conductivity of the fluid.
Backqround
There is an increasing demand for gas sensors to monitor pollutants in our environment. Gas sensors can be based on many different principles and technologies. One such principle is using thermal conductivity to determine the composition of gases.
For example, in G. De Graaf and R. F. Wolffenbuttel, "Surface-micromachined thermal conductivity detectors for gas sensing.” 2012 IEEE International Instrumentation and Measurement Technology Conference Proceedings, pp. 1861-1864, a thermal conductivity gas sensor based on silicon technology is described.
Mahdavifar et.al. in “Simulation and Fabrication of an Ultra-Low Power miniature Microbridge Thermal Conductivity Gas Sensor,” Journal of the Electrochemical Society, 161 (2014) B55, describe a device comprising a suspended thin polysilicon resistor that acts as a heater and a temperature sensor as part of a thermal conductivity sensor. The change in resistance of the polysilicon with temperature allows its use as a temperature sensor.
US10598621, US8667839B2, and US63572279B1, US86889608 and US10408802B2 describe further sensors. Kommandur et. al., “A microbridge heater for low power gas sensing based on the 3-omega technique,” Sensors and Actuators A 233 (2015) 231- 238, also describes a thermal conductivity sensor.
Many of the state-of-the-art devices use a differential signal between the main sensor and the reference. However, in all cases the reference device is a heater as well and thus doubles the power consumption of the device.
11649661-1 2
Summary
Presently available sensors have, among others, the following disadvantages:
• high power dissipation, high electrical noise, low sensitivity and slow dynamic response of the sensor;
• mechanical fragility and vibration sensitivity;
• reduced mechanical robustness of sensor supporting structures;
• complex fabrication processes;
• manufacturing processes that are not fully CMOS compatible; and
• manufacturing processes that are expensive.
The devices of the present disclosure are advantageous over the state-of-the-art devices for at least the following reasons:
• the sensor is able to determine composition of a fluid and concentration of different components within the fluid, in a zero flow environment;
• thermal isolation of the heated element which reduces power dissipation, increases sensitivity and provides a fast, dynamic response of the sensor;
• reduced mechanical fragility and vibration sensitivity of the membrane structure compared to a beam structure;
• a suitable dielectric material used for the dielectric membrane improves mechanical robustness of the membrane;
• a suitable dielectric material (with low thermal conductivity) used for the dielectric membrane (with low thermal mass) reduces power dissipation, increases sensitivity and provides a fast, dynamic response of the sensor;
• discontinuities within the membrane mitigate power dissipation, sensitivity and dynamic response issues; and
• the devices are fully CMOS (Complementary Metal Oxide Semiconductor) and/or MEMS (Micro-Electro-Mechanical Systems) compatible and therefore can be manufactured using fully CMOS and/or MEMS compatible processes.
The presently disclosed fluid sensor is able to measure the composition of the fluid based on the different thermal conductivity of each of the components of the fluid.
Aspects and preferred features are set out in the accompanying claims.
11649661-1 3
According to a first aspect of the present disclosure, there is provided a first temperature sensing element spatially separated from the heating element, wherein the first temperature sensing element is located outside of the first dielectric membrane and over the semiconductor substrate, or wherein the first temperature sensing element is located on or within the first dielectric membrane and wherein the fluid sensor comprises at least one recessed region within the first dielectric membrane configured to thermally isolate the heating element from the first temperature sensing element, wherein the heating element is further configured to operate as a second temperature sensing element, and wherein the separation between the second temperature sensing element and the first temperature sensing element introduces a temperature difference between the heating element and the first temperature sensing element, such that a differential signal between the first temperature sensing element and the second temperature sensing element is indicative of the concentration or composition of the fluid based on a thermal conductivity of the fluid.
The first temperature sensing element is spatially separated from the heating element, so that there is a temperature difference between the heating element and the first temperature sensing element. During operation of the heating element, the heat generated by the heater diffuses into the dielectric membrane, above and below the dielectric membrane, and into the fluid surrounding the heating element. The amount of heat lost to the fluid surrounding the heating element will depend on the thermal conductivity of the fluid. Therefore, a temperature profile of the heating element will depend on the thermal conductivity of the fluid within the sensor. Dependent on the thermal conductivity of the fluid, the heating element will use a different amount of power to heat to a given temperature.
The first temperature sensing element is outside the membrane, or within the dielectric membrane and thermally isolated from the heating element. Therefore, the temperature of the first temperature sensing element will remain at ambient or room temperature or at a significantly colder temperature than that of the heating element. As the temperature of the heating element is dependent on the heat conducted through the fluid within the sensor and thus the thermal conductivity of the fluid, the differential signal is also dependent on the thermal conductivity of the fluid. Different target fluids within the sensor have different thermal conductivities, and therefore the temperature of the second
11649661-1 4 temperature sensing element (or the heating element) can be used to determine the concentration or composition of the fluid within the sensor. The differential signal is indicative of a composition or concentration of the fluid, and the sensor may be further configured to determine the composition or concentration of the fluid based on the differential signal or the temperature of the first temperature sensing element.
The change in power required or the temperature change due to heat loss to the fluid is generally small compared to the measured ambient temperature. Therefore, by measuring the differential signal the measured ambient temperature can effectively be cancelled out to improve measurement of the change in power required or the temperature change due to heat loss to the fluid. This can be done using a Wheatstone bridge, or schemes based on differential/instrumentation amplifiers.
The heating element is the same as the second temperature sensing element, i.e. the heating element operates as a resistive temperature detector. The heating element can be driven in a constant temperature, constant voltage/current or constant resistance mode, and instead of measuring the differential resistance between the first and second resistive temperature sensing elements, the differential voltage, current or power can be measured. When the thermal conductivity of the fluid around the sensor changes, the amount of voltage, current and/or power required to keep the heater at the same resistance or temperature will change, and thus the differential voltage/current/power between the first and second temperature sensing elements will change.
The heating element may be configured to operate as a sensing element by, for example, sensing the change in the resistance due to the change in temperature, as it is the case of resistive temperature detectors. The heating element may operate simultaneously as both a heating element and a sensing element. The heating element can be considered as electrically equivalent to a resistor. The electrical conductivity of most heaters materials (Tungsten, Titanium, Platinum, Aluminium, polysilicon, monocrystalline silicon) varies with temperature. This variation is mostly linear and is characterised by the TCR (Temperature coefficient of resistance). The TCR can be positive or negative, but most metals have a positive and stable TCR, meaning that their resistance increases when the temperature is increased.
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The advantage of this embodiment is simplicity and reduced number of additional elements on the membrane. The larger the number of elements on the dielectric membrane, the higher the probability of impaired reliability or malfunction of the sensor.
By providing the first temperature sensing element on the substrate or on the same membrane and thermally isolated (i.e. not on a separate membrane), the first temperature sensing does not need to be separately heated. Therefore, the power consumption of the device is reduced.
In use, with no flow or static flow, this allows sensing of different components of a fluid using a differential signal between two sensing elements.
According to a further aspect of the disclosure, there is provided a fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising: a semiconductor substrate comprising a first etched portion; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first etched portion of the semiconductor substrate; a heating element located within the first dielectric membrane; a first temperature sensing element spatially separated from the heating element, wherein the first temperature sensing element is located outside of the first dielectric membrane and over the semiconductor substrate, or wherein the first temperature sensing element is located on or within the first dielectric membrane and wherein the fluid sensor comprises at least one recessed region within the first dielectric membrane configured to thermally isolate the heating element from the first temperature sensing element ; and a second temperature sensing element located on or within the first dielectric membrane, wherein the second temperature sensing element is substantially identical in shape and size to the first temperature sensing element, and wherein the first temperature sensing element is located a first distance away from the heating element, and wherein the second temperature sensing element is located a second distance away from the heating element, and wherein the first distance is greater than the second distance, and wherein the separation between the second temperature sensing element and the first temperature sensing element introduces a temperature difference between the second temperature sensing element and the first temperature sensing element, such that a differential signal between the first temperature sensing element and the second
11649661-1 6 temperature sensing element is indicative of the concentration or composition of the fluid based on a thermal conductivity of the fluid.
The fluid sensor may comprise a semiconductor substrate made of a semiconductor material such as silicon, silicon carbide or Gallium Nitride, and comprising an etched portion. The fluid sensor may also comprise a dielectric region comprising of oxides and/or nitrides such as silicon dioxide and silicon nitride, where the portion of the dielectric region adjacent to the etched portion is referred to as a dielectric membrane. The dielectric membrane may have embedded structures made of semiconductor material or metal structures.
The semiconductor substrate may be any semiconductor such as silicon, silicon on insulator (SOI), Silicon Carbide, Gallium Nitride or Diamond. In particular, the use of silicon is advantageous, as it guarantees sensor manufacturability in high volume, low cost and high reproducibility. The use of a silicon substrate could also enable on-chip circuitry for sensor performance enhancement and system integration facilitation. Such on-chip circuitry could be implemented by using analogue or digital or mixed-signal blocks placed outside the dielectric membrane.
The dielectric membrane or multiple dielectric membranes may be formed by back- etching using Deep Reactive Ion Etching (DRIE) of the substrate, which results in vertical sidewalls and thus enabling a reduction in sensor size and costs. However, the back- etching can also be done by using anisotropic etching such as KOH (Potassium Hydroxide) or TMAH (TetraMethyl Ammonium Hydroxide) which results in sloping sidewalls. The dielectric layers within the membrane which could be formed by oxidation or oxide deposition could be used as an etch stop during the DRIE or wet etching processes. The membrane can also be formed by a front-side etch (using most commonly wet etch techniques) or a combination of a front-side and back-side etch to result in a suspended membrane structure, supported only by two or more beams. The membrane may be circular, rectangular, or rectangular shaped with rounded corners to reduce the stresses in the corners, but other shapes are possible as well.
Preferably, the semiconductor substrate may be silicon and the dielectric membrane may be formed mainly of oxide and nitride materials, or oxinitride (a pre-formed combination of oxide and nitride) and where the heater element may be made of a metal such as
11649661-1 7 tungsten, titanium, copper, aluminium, gold, platinum or a combination of those or a semiconductor such as highly doped n type or p type silicon or polysilicon. The heater may have a shape of a meander, spiral or a hotwire.
The dielectric region may comprise a dielectric layer or a plurality of layers including at least one dielectric layer. The dielectric region may comprise layers of more than one material, such as silicon dioxide, silicon nitride, or aluminium oxide. The heating element may be fully embedded or partially embedded within the dielectric membrane.
The membrane may also comprise one or more layers of spin on glass, and a passivation layer over the one or more dielectric layers. The employment of materials with low thermal conductivity (e.g. dielectrics) enables a significant reduction in power dissipation as well as an increase in the temperature gradients within the membrane with direct benefits in terms of sensor performance (e.g. sensitivity, frequency response, range, etc.). Temperature sensing elements or heaters made of materials such as monocrystalline or polycrystalline semiconductors or metals could be suspended or embedded in the dielectric membrane.
The dielectric membrane may also have other structures made of metal or other conductive or other materials with higher mechanical strength. These structures can be embedded within the membrane, or may be above or below the membrane, to engineer the thermo-mechanical properties (e.g. stiffness, temperature profile distribution, etc.) of the membrane and/or the fluid dynamic interaction between the fluid and the membrane. More generally, these structures can be also outside the membrane and/or bridging between inside and outside the membrane.
Generally speaking, a dielectric membrane region may be located immediately adjacent or above (or below if a flip-chip technology is used) to the etched portion of the substrate. The dielectric membrane region corresponds to the area of the dielectric region directly above or below the etched cavity portion of the substrate. Each dielectric membrane region may be over a single etched portion of the semiconductor substrate. The membrane maybe a “closed membrane”, supported by the substrate along its entire perimeter, or can be a bridge type structure - supported by a number of dielectric beams.
11649661-1 8
The fluid sensor may be configured to sense or measure a fluid (this may be a gas but could also be a liquid), and the gas may be made of air and the components of interest could be any of CO2, methane or hydrogen or other gases in dry air or humid air. The component of interest can be any fluid that has a different thermal conductivity than that of air.
The disclosed sensor could be applicable to a variety of gases and liquids, but we make specific reference to Carbon dioxide (CO2), methane and hydrogen as these specific gases have thermal conductivity properties which are significantly different from those of air.
The sensor may be a thermal conductivity fluid sensor incorporated in a MEMS structure comprising a heating element and at least one other sensing element (such as a temperature sensing element) that may be able to detect separately the fluid flow properties, such as velocity, volume flow rate, mass flow rate. The temperature sensing element may be able to also detect the composition the fluid based on the difference in thermal conductivity, specific heat capacity, dynamic viscosity, density (and other thermo-mechanical properties, hereafter simply referred to as thermal properties) of different components of the fluid.
During operation of the heating element, the heat generated by the heater diffuses into the dielectric membrane, above and below the dielectric membrane, and into the fluid surrounding the heating element. The amount of heat lost to the fluid surrounding the heating element will depend on the thermal conductivity of the fluid. Therefore, a temperature profile of the second temperature sensing element will depend on the thermal conductivity of the fluid within the sensor. Dependent on the thermal conductivity of the fluid, the heating element will use a different amount of power to heat the second temperature sensing element to a given temperature. As the temperature of the second temperature sensing element is dependent on the heat conducted through the fluid within the sensor between the heating element and the second temperature sensing, the temperature of the second temperature sensing element is dependent on the thermal conductivity of the fluid. Therefore, the differential signal is also dependent on the thermal conductivity of the fluid. Different target fluids within the sensor have different thermal conductivities, and therefore the differential signal can be used to determine the concentration or composition of the fluid within the sensor. The differential signal is
11649661-1 9 indicative of a composition or concentration of the fluid, and the sensor may be further configured to determine the composition or concentration of the fluid based on the differential signal.
There may be a circuit to measure a differential signal between the first and second resistive temperature detector elements and use it to determine the concentration of a fluid or particular fluid components based on different thermal conductivities.
The first temperature sensing element may be located a first distance away from the heating element, and the second temperature sensing element may be located a second distance away from the heating element, and wherein the first distance may be greater than the second distance.
The second temperature sensing element may be located closer to the heating element than the first temperature sensing element. Preferably, the second temperature sensing may be located such that the second temperature sensing element has the same temperature as the heating element during operation of the sensor.
The differential signal may be measured as a temperature difference, voltage difference, current difference, power difference, or resistor difference.
The difference in the resistance of, current through, or voltage across the two resistive temperature detectors can be measured and this gives an indication of the composition of the fluid and the concentration of its one or more components. If the composition of the fluid (or concentration of a component of the fluid) around the sensor changes, its thermal conductivity also changes and this will change the thermal losses and the temperature of the heater - in turn changing the resistance of the second resistive temperature detector, without changing (or changing insignificantly) the resistance of the first temperature resistive temperature detector. The change in resistance could be measured directly, or could be measured as a voltage change, current change or power change.
Thus the difference in resistances (or voltages or currents) between the first and second temperature sensing elements allows measurement of the thermal conductivity of the surrounding fluid, and hence the composition of the surrounding fluid. Changes in
11649661-1 10 ambient temperature affect both temperature sensing elements almost equally and hence does not affect significantly the difference in resistances.
The first temperature sensing element and the second temperature sensing element may be both located on or within the first dielectric membrane, and the fluid sensor may comprise at least one recessed region within the first dielectric membrane configured to thermally isolate the heating element and the second temperature sensing element from the first temperature sensing element.
The second temperature sensing element may be located in a same layer of the dielectric region as the heating element and the second temperature sensing element may laterally surround the heating element.
Alternatively, the second temperature sensing element may be located below or above the heating element. The second temperature sensing element may be located directly above or below the heating element, so that the second temperature sensing element is not laterally spaced from the heating element.
Having the second temperature sensing element in a same layer or below or above the heating element has the advantage that the temperature of the second temperature sensing element is substantially the same of that of the heater. This increases the differential signal between the first temperature sensing element and the second temperature sensing element, therefore improving sensitivity of the sensor.
The second temperature sensor element can be either laterally spaced but close to the heating element, and can be made of the same material layer as the heating element. Alternatively, the second temperature sensing element can be made of a different material layer than the heater and can be vertically spaced from the heater, either above or below the heater. An advantage of both these configurations is that the second temperature sensing element should have substantially the same temperature as the heater element during operation.
The two temperature resistive detectors can be identical in size, shape and resistance. Alternatively, the first temperature sensing element may be configured to have a higher resistance at room temperature than a resistance of the second temperature sensing
11649661-1 11 element at room temperature, and the first temperature sensing element and the second temperature sensing element may be configured to have substantially the same resistance at an operating temperature of the sensor without a fluid present.
The semiconductor substrate may comprise an additional etched portion, and the dielectric layer may comprise an additional dielectric membrane located over the additional etched portion of the semiconductor substrate. The sensor further may comprise an additional heating element located within the additional dielectric membrane and an additional first temperature sensing element and an additional second temperature sensing element.
The heating element may be a resistive heating element. At least one of the first temperature sensing element and the second temperature sensing may be resistive temperature sensing elements, also known as resistive temperature detectors (RTDs).
The resistive temperature detector elements may comprise metal (Tungsten, Al, Copper, Platinum, Gold, Titanium) or semiconductor material (Silicon, Polysilicon, Silicon Carbide, Gallium Nitride, Aluminium Gallium Nitride, or Gallium Arsenide or a two dimensional electron gas)
Firstly, for increased sensitivity and stability, such resistive temperature detectors may have a high, reproducible and stable TCR (Temperature Coefficient of Resistance). Secondly, it is preferable that such resistive temperature detectors are linear in temperature (i.e. their resistance varies linearly with the temperature).
The sensing elements may be temperature sensitive and may be any of resistive temperature detectors, diodes, transistors or thermopiles, or an array in series or parallel or a combination of those.
Such sensors can be implemented in bulk COMOS, SOI (Silicon on Insulator) CMOS technology. SOI membranes can be made by using the buried oxide as an etch stop. SOI diodes, transistors and thermopiles can be made by using the thin silicon layer above the buried oxide which can be doped n or p-type.
One type of sensing element may be used or a combination of different types of sensing elements may be used.
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A thermopile comprises one or more thermocouples connected in series. Each thermocouple may comprise two dissimilar materials which form a junction at a first region of the membrane, while the other ends of the materials form a junction at a second region of the membrane or in the heat sink region (substrate outside the membrane area), where they are connected electrically to the adjacent thermocouple or to pads for external readout. The thermocouple materials may comprise a metal such as aluminium, tungsten, titanium or combination of those or any other metal available in the process. Alternatively, the thermocouple materials may comprise thermocouples based on n-type and p-type silicon or polysilicon or combinations of metals and semiconductors. The position of each junction of a thermocouple and the number and the shape of the thermocouples may be any required to adequately map the temperature profile distribution over the membrane to achieve a specific performance.
The sensitivity and selectivity to the flow composition may be enhanced by using extra sensing elements, symmetrical or asymmetrical recessed regions, and/or an additional heater.
The first temperature sensing element may be located above the semiconductor substrate. The first temperature sensing element may be directly above the semiconductor substrate, so that the first temperature sensing element is completely above a substrate portion of the substrate and is not above the etched region of the substrate and is not located within the dielectric membrane. This increases thermal isolation between the first temperature sensing element and the components within the dielectric membrane, therefore improve the sensitivity of the device.
The first temperature sensing element may be located within the dielectric region, but preferably outside the dielectric membrane area or at an edge of the membrane area.
Alternatively, the first temperature sensor could also be placed at the edge of the membrane region (in order for example to reduce the chip area).
The fluid sensor may further comprise circuitry configured to determine the concentration or composition of the fluid based on the temperature of the first temperature sensing element or the differential signal.
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There may be control circuitry that measures the differential signal between the first and second temperature sensor elements and uses it to determine the concentration of a fluid or particular fluid components based on different thermal conductivities.
A control and measurement unit/circuitry that drives the heater in constant current, constant voltage or constant power mode may be provided. The driving could be preferably in pulse mode, but continuous mode or AC mode are also possible.
The circuitry may be located on a same chip as the fluid sensor. Analogue/digital circuitry may be integrated on-chip. Circuitry may comprise IPTAT, VPTAT, amplifiers, analogue to digital converters, memories, RF communication circuits, timing blocks, filters or any other mean to drive the heating element, read out from the temperature sensing elements or electronically manipulate the sensor signals. For example, it is demonstrated that a heating element driven in constant temperature mode results in enhanced performance and having on-chip means to implement this driving method would result in a significant advancement of the state-of-the-art flow sensors. The driving method known a 3w may be implemented via on-chip means, or any other driving method, such as constant temperature difference and time of flight, needed to achieve specific performance (e.g. power dissipation, sensitivity, dynamic response, range, fluid property detection, etc.). In absence of on-chip circuitry, this disclosure also covers the off-chip implementation of such circuital blocks when applied to a fluid sensor. Such off-chip implementation may be done in an ASIC or by discrete components, or a mix of the two.
The circuitry may include one or more alternating current (AC) sources and/or lock-in amplifier measurements to reduce noise. The one or more AC sources may be used in conjunction with fast Fourier transform (FFT)-based techniques. In some implementations, the use of lock-in amplifiers facilitate the measurements of very small AC signals, for example AC signals of a few nanovolts or less. Accurate measurements of the AC signals can be made even when noise sources are higher than the signal of interest. Because these techniques only measure AC signals at or near the test frequency, the effects of noise, including thermoelectric voltages (both DC and AC), may be reduced. The driving circuit may be implemented though any suitable means, such as via on-chip means, provided externally in an ASIC, a Field-programmable gate array
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(FPGA), micro-controller/micro-processor forms, by using discrete components, or any combination of the above means.
The circuitry may comprise reversible direct current (DC) sources. The reversible DC sources may be used in conjunction with techniques based on voltage measurements with currents of opposite polarity to cancel out thermoelectric noise (i.e., electromotive forces (EMFs)) that is generated when different parts of a circuit are at different temperatures, and/or when conductors made of dissimilar materials are joined together. Thermoelectric noise such as EMFs may be reduced or, in implementations, cancelled using consecutive voltage measurements made at alternating test current polarities.
The fluid sensor may be operated using a two voltage measurement method or a three voltage measurement method (also referred to as a ‘delta technique’), wherein two or three voltages are applied to any heating element(s) and/or the temperature sensing element(s). The two voltage measurement technique may reduce or cancel a thermoelectric voltage offset term from the measurement results. Similarly, the three voltage measurement method may either reduce or cancel a thermoelectric voltage offset, and may additionally remove the thermoelectric voltage change (drifting) term from the measurement results, thus greatly improving the measurement noise immunity (e.g. the signal to noise ratio) when compared to many other techniques. The driving circuit may be implemented within the same chip (i.e. monolithic integration) or may be provided externally (such as in an off-chip implementation). The driving and reading circuits may be implemented with any suitable means, such as an ASIC, FPGA, by using discrete components, or any combination of the above. Output signals may be computed using any suitable processor and/or controller, such as a micro-controller or micro processor.
Generally, this technique makes use of currents of equal (or approximately equal) magnitude and opposite polarities.
Opposite polarity currents can be used to more accurately measure the resistance and/or any change in the resistance of the heating element(s) (such as a heater) itself or any temperature sensing element(s) (e.g. a resistive temperature detector or thermal sensor) adjacent or otherwise close to the heater.
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In some implementations, the opposite polarity currents are provided by opposite polarity current sources. One or more of the opposite polarity current sources may be e.g. precision current sources.
Additionally or alternatively, the opposite polarity currents may be provided by the same current source. This may be achieved by swapping the terminals of the (two-terminal) element to be measured. This element to be measured may be a heating element, such as a heating element used as a thermal sensor, ora sensing element, such as a resistive temperature detector, that is near or adjacent to the heating element. The swapping of the terminals may be facilitated using switching elements (e.g. a network of switching elements), such as switching elements comprising switching transistors operatively connected to the terminals of the resistor. For example, for a thermal sensor comprising two terminals, with the current flowing from the first terminal 1 to the second terminal, the opposite polarity current can be provided by using the same current source but changing the direction of the current (e.g. by biasing the thermal sensor in the opposite way), such that the current flows from the second terminal to the first terminal. The switching elements may be transistors which are configure such that their gate/control terminal determine the direction of the current flow. The switching elements may be monolithically integrated, or may be provided externally.
In some implementations, the timing of the opposite polarity currents can be adjusted. For example, the on-time, off-time and/or delay between the pulses of the opposite polarity currents may be adapted as desired. Additionally or alternatively, the magnitude of the opposite polarity currents can also be adjusted, as desired.
The above operations of the flow sensor may be applied in multiple steps, e.g. by running the heater at several (different) temperature levels (given by different levels of power levels), to aid in the selectivity of different components of the fluid.
The circuitry may comprise one or more of: a constant current or constant resistor drive circuit, a constant or alternating current source, a Wheatstone bridge, an amplifier, an Analog to Digital convertor, a Digital to Analog Convertor, or
11649661-1 16 a microcontroller.
Differential signals can be obtained by using a combination of current sources and differential amplifiers, bridge type circuits or other types of subtraction circuits or instrumentation amplifiers.
The first temperature sensing element and the second temperature sensing may be located on two sides of a bridge circuit (also referred to as an instrumentation bridge, and can be a Wheatstone bridge), and the sensor may be configured such that an output of the bridge circuit may be a function of the thermal conductivity of the fluid around the sensor. The output of the bridge circuit may therefore also be a function of the concentration of particular fluid components with different thermal conductivities.
The first resistive temperature detector and second temperature detector may be placed together with other components on the sides of an instrumentation bridge, such as a Wheatstone bridge, and the differential output of the bridge could be a function of the thermal conductivity of the fluid around the sensor and the concentration of particular fluid components with different thermal conductivities. Such differential signals can be further amplified by using amplifiers, either located on the same chip, to maintain low noise, or placed within the same package, module or system.
The fluid sensor may comprise at least one recessed region within the first dielectric membrane and between the heating element and the first temperature sensing element.
The recessed region may be located between the first temperature sensing element and the second temperature sensing element - therefore there is a greater recessed volume between the heating element and the first temperature sensing element than between the heating element and the second temperature sensing element, such that the recessed region introduces a temperature difference between the first temperature sensing element and the second temperature sensing element due to differences in heat conduction through the dielectric membrane.
There may be no recessed region between the heating element and the second temperature sensing element so that the second temperature element is at substantially the same temperature as the heating element during operation of the device.
11649661-1 17
The recessed regions or discontinuities in the dielectric membrane provide an interruption (or partial interruption) in the thermal conduction path through the solid of the dielectric membrane. This in turn will mean that the heat path will occur more through the fluid above the recess (via conduction and convention) or through the cavity space formed as a result of the recess (mainly through fluid conduction). In both cases (heat above the cavity space or within the cavity space), the heat dissipation will depend on the thermal conductivity of the fluid. This increases the sensitivity of the differential signal to the thermal conductivity of the fluid.
The at least one recessed region may comprise one or more discontinuous regions where the thickness of the dielectric membrane is discontinuous or varies from an average or most common dielectric membrane thickness.
The at least one recessed region may be located between the heating element and an edge of the dielectric membrane.
An edge of the dielectric membrane may refer to a perimeter edge of the dielectric membrane, in other words, the area where the dielectric membrane meets or joins the semiconductor substrate. The area of the dielectric region above the semiconductor substrate may refer to the area of the dielectric region outside the dielectric membrane.
The recessed region may be located between the heating element and the edge of the dielectric membrane spaced from the heating element. In particular, the recessed regions maybe defined such that there is one recessed region between the heating element and the edge of the membrane, one recessed region between the first temperature detector element and the edge of the membrane, and no recessed region between the heater and the first temperature detector element.
The recessed regions may be holes (perforations) through the dielectric membrane. This would be advantageous, as the thermal conduction path through the solid of the dielectric membrane will be impeded and this will mean that the thermal conduction will occur through the holes (mainly via conduction) or above the holes (via both conduction and convection), thus facilitating the measurement of the composition of the fluid based on the different thermal conductivity of each of the components of the fluid.
11649661-1 18
There may be at least one hole through the membrane to connect the upper side of the membrane to the lower side of the membrane via the fluid to be sensed. The at least one hole also disrupts the thermal conduction path through the solid dielectric membrane, forcing more heat to dissipate via convection and conduction through the environment. The presence of the at least one hole also helps to reduce the power consumption of the device (for the same heater temperature), because of the reduction in the heat conduction losses (through the solid membrane). Furthermore, the presence of the at least one hole allows for a lower thermal mass of the membrane thus reducing the time needed for the heater to heat up and cool down.
The at least one hole or recessed region may be used to enhance the sensitivity/selectivity to any fluid or component of the fluid (e.g. air with a concentration of CO2) with a thermal conductivity that is different to that of a reference fluid or another component of the fluid (e.g. air).
An arrangement and specific design of different holes and different sensing elements is provided to enhance the sensitivity to any fluid or component of the fluid (e.g. air with a concentration of CO2) with a thermal conductivity that is different to that of a reference fluid or another component of the fluid (e.g. air).
The arrangement of different holes or slots (or recessed regions) may be placed symmetrically around the heating element and the second temperature sensing element.
The at least one recessed region may comprise one or more holes. The holes may refer to apertures, perforations or slots extending through an entire height or depth or thickness of the dielectric membrane. This forms a fluid flow path and provides fluid connection between area above and area below membrane.
The at least one of the one or more holes may comprise an elongate slot extending towards opposite edges of the dielectric membrane. The elongate slot may not extend completely to the edges of the dielectric membrane or completely isolate the dielectric membrane either side of the elongate slot. The elongate slot increases thermal isolation across a width of the dielectric membrane of the device. Optionally the elongate slot may
11649661-1 19 be extending in a same direction as one or more heating elements and/or sensing elements. The elongate slots may be, for example, rectangular, square, or semicircle.
The one or more holes may comprise an array of perforations. The perforations may comprise individual holes significantly smaller than a width of the dielectric membrane of the device. The array of perforations may can extend substantially across a width of the device.
The at least one recessed region may comprise a partial recess within the dielectric membrane. The partial recess or trench may extend from a top surface of the dielectric membrane or may extend from a bottom surface of the dielectric membrane. The partial recess may extend partially through a height or depth or thickness of the dielectric membrane. The at least one perforation may be in the form of a trench formed from the top or the bottom surface but not penetrating the other surface.
The discontinuities may be referred to as a gap in the membrane from the top surface to the bottom surface. Though, not as effective in terms of the thermal performance, a discontinuity could also refer to a trench or partial hole created from either the top or the bottom surface (if an upside-down membrane is used) without penetrating the other surface. The advantage of such partial holes is that they could impact less the mechanical strength of the membrane and in some cases they may be easier to be manufactured. Moreover, such partial holes could be used to hermetically seal the bottom side of the membrane or allow no fluid penetration below the membrane.
The at least one recessed region may have a meander shape. In other words, the discontinuity may have a non-standard shape such as a concertina or corrugated shape formed of a series of regular sinuous curves, bends, or meanders.
The etched region of the semiconductor substrate may have sloped sidewalls. The etched region of the semiconductor substrate may not extend through the entire depth of the semiconductor substrate.
The semiconductor substrate may comprise an additional etched portion, and the dielectric layer may comprise an additional dielectric membrane located over the additional etched portion of the semiconductor substrate. The sensor may further
11649661-1 20 comprise an additional heating element located within the additional dielectric membrane, and an additional first temperature sensing element.
The additional heating element and the additional first temperature sensing element may operate similar to the heating element and first temperature sensing element. This increases sensitivity of the device.
The heating element and the additional heating element may be connected in series. The additional first temperature sensing element and the first temperature sensing element may be connected in series. The sensor may comprise an additional second temperature sensing element connected in series to the second temperature sensing element.
The heating elements may be connected in series and operated substantially at the same temperature.
The heating elements may be connected in series and the second temperature sensing elements may also be connected in series. The first temperature sensing elements may also be connected in series. In this case, a differential signal between the series combination of the first resistive temperature detectors and the second resistive temperature detector is obtained and used to determine the concentration of a fluid or particular fluid components based on different thermal conductivities. This allows the sensitivity of the sensor to be increased (by scaling up with the number of membranes, heating elements, and temperature sensing element) and also lowers the minimum resolution of the concentration of a particular gas component that can be sensed based on its difference in thermal conductivity compared to the rest of the fluid.
The heating element and the additional heating element may be configured to operate at different temperatures.
Each sensing element in combination with a corresponding first and second temperature sensing elements may operate independently and preferably at different temperatures to improve selectivity to different gases.
11649661-1 21
The heating element may be driven at more than one temperature, to increase the selectivity of the device. Gas thermal conductivity varies with temperature, and this variation is different dependent on the gas. In one drive mode, the heater can be driven at a temperature where the thermal conductivity of air and carbon dioxide are identical, and then used to detect another gas (e.g. hydrogen or methane). In this scenario, there will be known unwanted response from present carbon dioxide and thus the selectivity of the device is improved. The heater can also be run at the temperature that provides the optimum sensitivity for the gas that is being measured.
The fluid sensor may comprise an array of multiple dielectric membranes located over multiple etched portions of the semiconductor substrate, each membrane having: a heating element located within the dielectric membrane; a second resistive temperature detector element located in the proximity of the heating element and within the dielectric membrane. For each membrane of the array of membranes, the fluid sensor may comprise a first resistive temperature detector element located outside of the membrane and used as a reference. A differential signal may be measured between the at least one first resistive temperature detector element and the at least one second resistive temperature detector such that the differential signal is a function of the thermal conductivity of the fluid around the sensor and the concentration of particular fluid components with different thermal conductivities.
The array may contain one or several first resistive temperature detectors outside the dielectric membrane.
The fluid sensor may further comprise a covering located on a surface of the sensor, where the covering may comprise a hole configured to allow fluid travel from an outer surface of the covering to a fluid channel above the dielectric membrane.
The fluid sensor may further comprise a further temperature sensing element located outside the membrane region. The further temperature sensing element may be thermally isolated from the heating element.
An additional or further temperature sensor may be placed outside the dielectric membrane as a reference temperature sensing element to measure the ambient temperature or the temperature of the fluid, and the signal from the further temperature
11649661-1 22 sensor may be used for temperature compensation for a more accurate calculation of the concentration of one or more specific components of the fluid.
The reference temperature sensing element (resistive temperature detector) could be used as part of a combination sensor (or a sensor fusion system) to read multiple physical properties of the environment (fluid composition and concentration of different components, fluid temperature or ambient temperature, or fluid velocity of fluid flow rate). Alternatively, a separate temperature sensor could be integrated on-chip as an extra resistive temperature detector, a diode or a transistor. An ambient temperature sensor could also be provided as part of the ASIC as a VPTAT or IPTAT sensor based on bandgap reference.
The temperature compensation can be done by using both the temperature reading from the additional temperature sensing element/elements and the differential reading between the first and second resistive temperature sensors. This can be implemented by either a formula (within an algorithm) to adjust the final reading, or using a look up table and interpolation to determine the final reading.
The fluid sensor may further comprise an additional first temperature sensing element outside the membrane region and an additional second temperature sensing element located on or within the dielectric membrane region.
The fluid sensor may further comprise a pair of temperature sensing elements located on the dielectric membrane, wherein a first temperature sensing element of the pair of temperature sensing elements may be located on a first side of the heating element and a second temperature sensing element of the pair of temperature sensing elements may be located on a second side of the heating element.
The device is able to simultaneously sense properties of the fluid flow such as speed, mass, volume, shear stress as well as the composition of the flow (e.g., whether the fluid, in this case, the gas, has a certain CO2 or hydrogen or methane percentage/ppm within air).
Therefore, the fluid sensor may comprise a first pair of sensing elements and a second pair of sensing elements, and a differential signal between the first pair of further sensing
11649661-1 23 elements may be configured to measure a property of a composition of the flow (such as different components of the fluid and their concentrations based on their different thermal conductivities), and a differential signal between the second pair of sensing elements may be configured to measure a flow property (such as flow rate, flow direction, velocity or flow mass or flow volume rates).
The flow could be measured by employing the pair of temperature sensing elements displaced on either side of the heating element within the same dielectric membrane, and optionally used as a differential pair. The differential pair may be formed of one upstream sensing element and one downstream sensing element.
Holes or discontinuities (also referred to as recessed regions) may be placed so that they affect less the differential signal between the pair of temperature sensing elements that measure the properties of the flow but they affect significantly more the differential signal between the sensing elements that measure the composition of the flow.
According to a further aspect of the present disclosure, there is provided a fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising: a semiconductor substrate comprising a first etched portion and a second etched portion, wherein the first etched portion and the second etched portion are substantially identical in size and shape; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first etched portion of the semiconductor substrate and a second dielectric membrane located over the second etched portion of the semiconductor substrate; a single active heating element, wherein the active heating element is located only within the first dielectric membrane; a first temperature sensing element located within the second dielectric membrane; and a second temperature sensing element located on or within the first dielectric membrane, wherein the second temperature sensing element is substantially identical in shape and size to the first temperature sensing element, and wherein the separation between the second temperature sensing element and the first temperature sensing element introduces a temperature difference between the second temperature sensing element and the first temperature sensing element, such that a differential signal between the first temperature sensing element and the second temperature sensing element is indicative of the concentration or composition of the fluid based on a thermal conductivity of the fluid.
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The first temperature sensing element may be placed on a second dielectric membrane wherein the second dielectric membrane does not comprise an active heating element. The two membranes may be located side by side, laterally spaced from each other, and may be identical in size and shape. The first temperature sensing element and the second temperature sensing element may be placed in a similar or identical position inside each of their respective dielectric membranes. Providing the temperature sensing elements in identical membranes improves matching characteristics.
The sensor comprises a single active heating element, wherein the active heating element is located only within the first dielectric membrane. Therefore, the sensor may comprise only one active heating element, such that there is no active heating element or electrically connected or powered heating element in the second dielectric membrane.
The second temperature sensing element may be a separate temperature sensing element, or the heating element may be configured to operate as the second temperature sensing element.
The sensor may further comprise an auxiliary structure located within the second dielectric membrane, and the auxiliary structure may be electrically isolated. The auxiliary structure may be configured such that the first dielectric membrane and the second dielectric membrane have the same mechanical and thermal stress properties.
Moreover, an auxiliary structure (also referred to as a dummy layer) (not connected electrically) may be located on or within the second dielectric membrane, such that the two temperature sensing elements have similar or identical structures in their proximity (i.e. neighbouring structures) and the two dielectric membranes with their respective embedded structures have substantially the same mechanical and thermal mass properties. The auxiliary structure may be electrically isolated, in other words the dummy structure in the second dielectric membrane may be not connected to any electrical signal. This provides the advantage that the two temperature sensing elements are very well matched (they are both on identical membranes, they have similar neighbouring structures around them) in terms of their characteristics, including stress, or deformations. Moreover, the two sensing elements see similar mechanical stress profile and therefore common mode effects such as ambient pressure or vibrations can be
11649661-1 25 removed. Furthermore, the dynamic characteristics of the temperature sensing elements will be better matched because of their identical thermal mass.
According to a further aspect of the disclosure, there is provided a sensor assembly comprising the fluid sensor as described above and an application specific integrated circuit (ASIC) coupled to the sensor.
The control circuitry can be located on the same chip as the sensor (monolithically integrated), or can have an application specific integrated circuit (ASIC) coupled to the sensor. The ASIC can be on a separate chip, but within the same package, as a hybrid, co-packaged or using system in package (SIP) solutions. Alternatively, the ASIC could be placed outside the package, on a PCB (Printed Circuit Board) or within the same case/box.
The ASIC may be located underneath the sensor, for example using a die stack technique. Alternatively, the ASIC may be located side by side with the sensor or elsewhere. The ASIC may be connected to the sensor using wire bonding and pads, or using through-silicon-vias (TSV) extending through the semiconductor substrate. Alternatively, the sensor and the ASIC can be located on the surface of a common PCB or embedded in a PCB.
An ASIC may be provided within the same system or the same package or on-chip to provide electronic circuitry to drive, read-out signals and process signals from the sensor. The ASIC may be placed in a stack die configuration under the sensor and the sensor and ASIC are placed within a manifold or an open package, to allow contact to the fluid.
According to a further aspect of the disclosure, there is provided a sensor assembly comprising a sensor housing; and a fluid sensor as described above located within the flow sensor housing.
The fluid sensor housing may comprise an inlet and an outlet, and a fluid flow path for directing a fluid flow through the sensor. The sensor may be packaged within a packaging house or manifold with an inlet, outlet and a channel to provide more accurate measurements of the flow or the composition of the fluid.
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According to a further aspect of the disclosure, there is provided a sensor assembly comprising the fluid sensor as described above, wherein the fluid sensor may be packaged on a printed circuit board in a flip-chip configuration.
The device may be packaged in a metal TO type package, in a ceramic, metal or plastic SMD (surface mount device) package. The device may also be packaged directly on a PCB, or with a flip-chip method. The device may also be embedded in a substrate, such as a customised version of one of the previously mentioned package, a rigid PCB, a semi-rigid PCB, flexible PCB, or any other substrate, in order to have the device surface flush with the substrate surface. The package can also be a chip or wafer level package, formed for example by wafer-bonding.
In particular, the package maybe designed such that there is a surface very close to the membrane, on one side or both sides of the membrane, for example in a flip-chip scenario, such that the surface is less than 50um from the membrane. This increases the power loss through the fluid and improves the sensitivity of the sensor.
According to a further aspect of the disclosure, there is provided a method of measuring a concentration or composition of a fluid using a fluid sensor as described above, the method comprising: applying an electrical bias to the heating element; and monitoring the electrical bias applied to the heating element and using the value of the electrical bias applied to the heating element and the differential signal to determine the concentration or composition of the fluid based on thermal conductivity of the fluid.
Applying an electrical bias to the heating element may comprise applying an electrical bias such that the differential signal between the first temperature sensing element and the second temperature sensing element may be minimised. Minimised may refer to reducing the differential signal to zero or substantially zero.
The electrical power, current, or voltage applied to the heating element may be adjusted to bring to zero or substantially zero the differential signal between the first and second temperature detector elements (by varying the heating element power, current, or voltage could be such that the resistances of the two temperature detectors or the voltages across the temperature detectors are equal). This may be done during the calibration of the sensor or during the operation of the sensor. This could be set as
11649661-1 27 calibrated point, giving a zero differential signal. Alternatively, this could be set during the operation and the heater power/current/voltage could be measured as an indication of the fluid compositions or the concentration of its components. The change in the electrical power, voltage or current through the heater may be monitored to measure one or more concentrations of specific components of the fluid based on their different thermal conductivities.
The first and second temperature sensing elements, and optionally the heating element, may be connected to a differential amplifier, a Wheatstone bridge, a lock-in amplifier, or a current reversal-based method type circuit such that the differential signal may be used to measure one or more concentrations of specific components of the fluid based on their different thermal conductivities.
The measurement of the differential signal (for example, the differential resistance) can be performed in a number of ways. A first way is to apply a constant current to both the first and second temperature sensing elements (temperature resistive detectors) and measure the voltage difference between them using a differential amplifier. Further methods include the use of a Wheatstone bridge or other type of bridges, or current reversal-based techniques. For all these methods, a calibration can be done initially to set a zero-point value. This can either set a differential voltage value when the target fluid (or component of the target fluid) is not present, or modify the current to one of the resistors to ensure the differential voltage is at zero when the target fluid is not present. Alternatively, the calibration can be done initially to set a zero-point value of the differential signal when the component of the fluid (e.g. CO2) is known (e.g. 400 ppm of CO2 in air) by using an external precision C02 device (e.g. NDIR sensor).
The method may comprise driving the heating element in pulse mode or AC mode to modulate the temperature of the heating element to vary the differential signal; and using the differential signal to selectively differentiate between different fluid components and/or determine the concentration of the different components. In implementations, this may comprise heating the sensor to a first temperature where the thermal conductivity of air and the thermal conductivity of the target gas (for example carbon dioxide) are the same. This then facilitates the determination of the effect of other gases in the air (such as water vapour). The sensor may then operate at a second (different) temperature, and the effect of other gases may be accounted for (i.e. reduced or cancelled) using e.g. a
11649661-1 28 look up table or a formula, so that only the effect of the target gas is determined. The formula and/or look up table may be predetermined for the target gas. The device may therefore comprise circuitry ora control system facilitating temperature modulation of the sensor using e.g. the heating element(s) or other suitable heater.
In implementations, the thermal conductivity fluid sensor can be used in a mode where, instead of a constant DC temperature, the temperature is varied.
Thermal conductivity sensors generally work at a constant (DC) operating temperature and measure the heat loss to the surrounding medium. This DC method is typically most effective when there is a single known gas (such as hydrogen) and the signal can be directly related to the concentration of the known gas. However, if the gas type is unknown then the concentration of the target gas cannot be determined using this method. Moreover, if another gas is present as well as the known target gas, then again the gas concentration cannot be determined. That means it is not possible to determine the concentration of the target gas in a mixture (e.g. H2 and He) or where the target gas is an unknown gas.
However, if the temperature of the thermal conductivity sensor is changed, then the thermal time constant associated with the transition depends not only upon the thermal conductivity l of the gas, but also on the specific heat capacity c of the gas (and the density of the gas p). The equation for the thermal time constant is: pcV Tth = lA
Where ¾ is the thermal time constant, V is the volume of a body and A is surface area of the body.
As such, by varying the temperature of the sensor, the differences in density and heat capacity can be used to distinguish between different gases, in addition to the differences in the thermal conductivity.
The temperature of the heating element may be modulated by varying the current, voltage or power to different levels and/or with different electrical pulses such as to vary the differential signal between the first and second resistive temperature detectors in
11649661-1 29 order to selectively differentiate between different fluid components and/or to provide information regarding the concentration of such components.
The temperature of the heater may be modulated and the voltage difference between the first and second temperature sensing elements at different temperatures may be assessed against reference values, and the difference between the two may be indicative of the flow composition.
The heating element temperature may be modulated by applying different power levels to increase sensitivity and selectivity to different fluid components based on their thermal conductivity variation with temperature. For example, the difference between the thermal conductivities of CO2 and the air is higher at room temperature than at high temperatures. The opposite is true for Methane, so the difference between the thermal conductivities of methane and the air is lower at room temperature than at high temperatures. Hydrogen has also a different variation of the thermal conductivity with temperature than that of CO2 or air. By running the heater at different temperature levels (i.e. modulating the temperature of the heater), it is entirely possible to differentiate between the contributions of different concentrations of fluid components in the fluid. In this way, for example, Hydrogen and CO2 contributions can be decoupled and their concentration values can be found.
The heater (also referred to as the heating element) may be operated in a pulse mode (e.g. driven with a square wave, sinusoidal wave, Pulse Width Modulated wave (PWM), Pulse Density Modulation, etc.) or continuous mode. The pulse mode has, among others, the advantage of reduced power consumption, reduced electromigration for enhanced device reliability/lifetime, and improved fluid properties sensing capabilities. Pulses could be used in different polarities to further reduce the impact of electromigration on the heating element.
Different drive modes and measurement modes are possible. For example, the heater can be driven using PWM, and the off time of the PWM can be used to measure heater resistance, and/or differential signal. This measurement can be done in a very short time, faster than the thermal time constant of the membrane to avoid self-heating.
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Selectively differentiating between different fluid components and/or determining the concentration of the different components may comprise using a neural network.
An algorithm containing machine learning and artificial intelligence may be implemented. For example, the sensor or a fluid sensing system may further comprise a controller or a processing system comprising a neural network. The neural network may be trained using data from different known gases or mixture of gases at different temperatures. The use of a trained neural network to identify known gases or a mixture of gases can improve accuracy, sensitivity and selectivity of the fluid sensor.
The neural network may be trained to recognise the composition of a gas mixture based on the differential signal between the first and second temperature sensing elements. The neural network could be trained using supervised learning based on a set of data of sensor output values for known gas mixtures at a set of heating element temperatures,. The inputs to the neural network could be the sensor output values at a predetermined set of temperatures. The neural network may be configured to process each differential signal from the first and second temperature sensing elements in order to determine the components of the gas mixture and the concentrations of each component in the gas mixture. The outputs from the neural network could be the fraction of each gas in the mixture. Synthetic training data could be generated to enhance the training by providing, for example, many more combinations of gases than would be practically realisable in a real laboratory. A support-vector machine could be trained to discriminate between different gases.
The method may comprise: applying a modulated function to the heating element, the first temperature sensing element, or the second temperature sensing element; measuring the modulation, the time delay, or the phase shift of the differential signal between the first temperature sensing element and the second temperature sensing element; and determining a concentration or composition of the fluid using the measured modulation, time delay or phase shift.
A transient, modulated, or pulsed signal may be applied to either the heater element or the first or second temperature sensing elements, and the signals from the first or second temperature detectors will consequently be transient, and their time shape, time delay, or phase shift depends on both the thermal conductivity and the thermal diffusivity of the
11649661-1 31 fluid around the sensor and its concentration of particular fluid components with different thermal conductivities and the thermal diffusivities
The heaters or the first or second resistive temperature detectors can be biased with a transient signal (e.g. AC, square wave, pulsed, step). Using transient based signals, the thermal diffusivity can be determined using the measured values from the first and second temperature sensing elements. In this way, more information can be extracted from the environment.
In a method of transient fluid sensor drive modes, a step change in input current can be applied to the heater and the time constant for the temperature rise in the heater can be measured. This time constant can give information about the thermal conductivity and diffusivity of the environment. Both can be used to identify gas concentration.
In another method of transient sensor drive modes, a sinusoidal wave can be applied to the heater. The change in amplitude and change in phase shift can provide information on thermal conductivity and thermal diffusivity, thus providing information on the gas concentration.
Additionally or alternatively, the heating element(s) may be provided with a DC bias point onto which a small AC signal (such as e.g. an AC, square wave, pulsed or step signal) can be superimposed. By using small AC based signals, the thermal diffusivity, conductivity and/or thermal capacity of the target fluid can be determined using the measured values from the temperature sensing elements of the first and second dielectric material. The changes in the amplitude, phase shift and/or changes in frequency of the measured values can provide information on thermal conductivity and/or thermal diffusivity, thus providing information on the gas concentration, or facilitating the selection between different components of the gas.
Any of the resistive temperature detectors may be driven in short pulses of power, voltage or current. The temperature sensing elements (resistive temperature detectors) may be driven in a pulse mode (e.g. driven with a square wave, sinusoidal wave, Pulse Width Modulated wave, Pulse Density Modulation, etc.) or continuous mode. The pulse mode has, among others, the advantage of reduced self-heating of the temperature sensing elements, which minimises the noise and increases the sensitivity or the signal
11649661-1 32 to noise ratio. This is particularly important for the second sensing temperature element (which is closer to the heating element), which suffers more from the self-heating effect than the first temperature element. This however could also be important for the first temperature sensor, especially if the first temperature sensor is placed on a dielectric membrane (the same membrane as the heating element and the second temperature sensing element, or a different membrane).
Whilst several methods are described, any other method of driving the sensor that can provide information on the environment that is being measured may be used.
Further described herein is a fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising: a semiconductor substrate comprising a first etched portion and a second etched portion; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first etched portion of the semiconductor substrate and a second dielectric membrane located over the second etched portion of the semiconductor substrate; two or more active heating elements, wherein a first active heating element is within the first dielectric membrane and a second active heating element is located within the second dielectric membrane; a first heat diffuser located over the first heating element, wherein an edge of the first heat diffuser extends beyond an edge of the first heating element; a second heat diffuser located over the second heating element, wherein an edge of the second heat diffuser is approximately aligned with an edge of the second heating element; and one or more conductive elements located on or over one or both of the first and second dielectric membranes, the conductive elements arranged such that a conductive heat loss through the second dielectric membrane is greater than a conductive heat loss though the first dielectric membrane.
A differential signal between the first heating element and the second heating element is related to a thermal conductivity of the fluid or air mixture. As a result, this differential signal is indicative of the concentration or composition of the fluid or air mixture being measured, and may be used to determine a concentration or composition of the fluid.
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The conductive elements and heat diffusers may be configured such that the total heat loss from the first dielectric membrane is approximately or substantially equal to the total heat loss from the second dielectric membrane. In other words, and for example, the additional heat loss to air from the first active heating element (due to the larger area of the first heat diffuser compared to the second heat diffuser) may be approximately equal to the additional conductive heat loss from the conductive elements located on the second dielectric membrane, such that the total heat loss from the two membranes is substantially the same.
The conductive elements may comprise a material with a higher thermal conductivity than the thermal conductivity of the first and second dielectric membranes. Alternately, the conductive elements may comprise holes or slots within the first and/or second dielectric membranes. In some implementations, the conductive elements may comprise materials with a lower thermal conductivity that the thermal conductivity of the first and/or second dielectric membranes, to thereby reduce the conductive heat loss of one or both of the first and second dielectric membranes.
The first and second etched portions may be substantially identical, i.e. such that they have approximately the same dimensions and/or shape.
Each heating element of the two or more active heating elements may be substantially identical in e.g. shape and/or size. Alternatively, one or more of the heating elements may be different to the other heating elements.
The first and second heat diffusers may be any element suitable for spreading heat, such as heat spreading plates. Some or all of the edges of the first heat diffuser may extend beyond corresponding edges of the first heating element, such that a size of the first heat diffuser is greater than the size of the first heating element. Similarly, some or all of the edges of the second heat diffuser may approximately align with corresponding edges of the second heating element, such that the second heat diffuser and second heating element are approximately or substantially the same size. As a result, the heat loss to air from the first heating element may be greater than the heat loss to air from the second heating element.
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The first and second heat diffusers may comprise a material with higher thermal conductivity that the thermal conductivity of the first and second dielectric membranes.
According to a further aspect of the present disclosure, there is provided a fluid sensing system comprising a fluid sensor as described above; and a controller configured to perform a method as described above.
The fluid sensing system may include a hardware or software interface wherein an algorithm is implemented to facilitate to selectively differentiate between different fluid components and/or to provide information regarding the concentration of such components.
A software algorithm configured to perform any of the methods as described above could be implemented to differentiate between these components and increase sensitivity related to each of the components of the fluids. The software algorithm could be implemented in a local microprocessor. Calibrated data could be stored in a memory device or integrated circuit. Alternatively, the software could be incorporated within an ASIC and driving of the sensor and processing of the signal could be done within an ASIC.
Processing of the signal could also be done remotely in a sensor hub, or on an external server accessed using the Internet (for example, the cloud).
Sampling and averaging of the data, as well as ways to remove outliers from the data could also be used as part of an algorithm and could be implemented in hardware using different electronic components such as micro-controllers, memories or could be done using an ASIC.
Readings from the sensor may be averaged in several ways, for example using a moving mean average or a moving median average. A moving mean average is useful for removing random noise from the signal. A moving median average is useful for removing outliers.
According to a further aspect of the present disclosure, there is provided a method of manufacturing a fluid sensor as described above, the method comprising: forming a first dielectric membrane located over a first etched portion of a semiconductor substrate semiconductor substrate comprising a first etched portion; forming a heating element
11649661-1 35 located within the first dielectric membrane; forming a first temperature sensing element spatially separated from the heating element, wherein the first temperature sensing element is located outside of the first dielectric membrane and over the semiconductor substrate, or wherein the first temperature sensing element is located on or within the first dielectric membrane and wherein the fluid sensor comprises at least one recessed region within the first dielectric membrane configured to thermally isolate the heating element from the first temperature sensing element.
Brief Description of the Figures
Some embodiments of the disclosure will now be described by way of example only and with reference to the accompanying drawings, in which:
Figure 1 shows a top view of a thermal conductivity fluid sensor with one circular resistor within a membrane, and one circular resistor outside the membrane, and circuitry to control & measure the sensor;
Figure 2 shows a top view of a thermal conductivity fluid sensor with two circular resistors within the membrane;
Figure 3 shows the cross-section of a thermal conductivity fluid sensor shown in Figure
2;
Figure 4 shows the top view of a thermal conductivity fluid sensor with recessed regions formed of slots within the membrane;
Figure 5 shows the cross-section of a thermal conductivity fluid sensor with slots in the membrane, as shown in Figure 4;
Figure 6 shows the top view of a thermal conductivity fluid sensor with recessed regions formed of arrays of circular holes;
Figure 7 shows the top view of a thermal conductivity sensor fluid with one wire resistive temperature detector within a membrane, and one wire resistor outside the membrane;
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Figure 8 shows the top view of a thermal conductivity fluid sensor with two wire resistive temperature detectors within a membrane;
Figure 9(a) shows the top view of a thermal conductivity fluid sensor with two wire resistive temperature detectors within a membrane and recessed regions shaped as slots;
Figure 9(b) shows an alternative thermal conductivity fluid sensor with a greater number of recessed regions within the dielectric membrane;
Figure 9(c) shows an alternative thermal conductivity fluid sensor having a resistive wire having a meander shape within the same layer as the heater;
Figure 9(d) shows an alternative thermal conductivity fluid sensor having a wire having a meander shape and a connecting element connecting two portions of the wire;
Figure 10(a) shows the cross-section of the device in Figure 9(a);
Figure 10(b) shows the cross-section of the device in Figure 9(c);
Figure 11 shows the top view of a thermal conductivity fluid sensor with two wire resistive temperature detectors within a membrane, and recessed regions comprising arrays of circular holes;
Figure 12(a) shows the top view of a thermal conductivity fluid sensor with the circuitry on the same chip;
Figure 12(b) shows the top view of an alternative thermal conductivity fluid sensor, where the first temperature sensing element is placed on a second dielectric membrane;
Figure 12(c) shows the top view of an alternative thermal conductivity fluid sensor, where the first temperature sensing element is placed on a second dielectric membrane, having a dummy element;
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Figure 13 shows the cross-section of a thermal conductivity fluid sensor with sloping sidewalls of the etched semiconductor substrate;
Figures 14(a), 14(b), and 14(c) show cross-sections of alternative thermal conductivity fluid sensors where the etched portion of the substrate does not extend through the entire thickness of the substrate;
Figure 15 shows the top view of a thermal conductivity fluid sensor where the second resistive temperature detector element (shown as a resistive wire) is also on the membrane;
Figures 16(a) and 16(b) shows two alternative thermal conductivity fluid sensors comprising of an array of membranes and resistive temperature detectors;
Figure 17 shows a thermal conductivity fluid sensor packaged such that there is a very thin channel above the membrane;
Figure 18 shows a thermal conductivity fluid sensor packaged in a flip-chip configuration;
Figure 19 shows the top view of a thermal conductivity fluid sensor with identical meander shaped resistive temperature detectors on and off the membrane;
Figure 20 shows a plot of gas thermal conductivity function with respect to the temperature for various gases;
Figure 21a shows a circuit diagram for measuring the thermal conductivity fluid sensor comprising a Wheatstone bridge;
Figure 21b shows an alternative circuit diagram for measuring the thermal conductivity fluid sensor comprising a Wheatstone bridge;
Figure 21c shows a circuit diagram for measuring the thermal conductivity fluid sensor with a Wheatstone bridge, where the thermal conductivity sensor comprises an array of identical membranes;
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Figure 22 shows a circuit diagram using constant current sources for both the resistive temperature detectors and the heating element;
Figure 23 shows a circuit diagram where the differential current between the two resistive temperature sensors is measured;
Figure 24 shows a circuit diagram comprising a Wheatstone bridge where each arm of the bridge can have a different voltage applied to keep the bridge balanced;
Figure 25 shows a circuit diagram with a Wheatstone bridge and a balancing resistor in the branch with the reference resistive temperature detector;
Figure 26 shows a circuit diagram with a Wheatstone bridge, with the reference resistive temperature detector in series with a transistor;
Figure 27 shows a flow chart giving a method that can be used to electronically balance the Wheatstone bridge;
Figure 28 shows a top view of the thermal conductivity sensor, with an additional on-chip temperature sensor to determine the ambient temperature, or the die temperature;
Figure 29(a) shows a circuit diagram where a single resistor is used both as the heater and the first resistive temperature sensor element of the fluid sensor;
Figure 29(b) shows the current through the heater of the sensor of Figure 30(a);
Figure 30 shows a top view of the thermal conductivity fluid sensor where there are two resistive temperature detectors within the membrane region and two resistive temperature detectors outside the membrane region;
Figure 31 shows a circuit diagram of the thermal conductivity fluid sensor for the configuration where there are two resistive temperature detectors within the membrane region, and two outside the membrane region;
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Figure 32 shows a top view of a thermal conductivity fluid sensor having a further pair of sensing elements configured to operate as a flow sensor;
Figure 33 shows the cross-section of a fluid sensor assembly having a thermal conductivity fluid sensor contained in a package;
Figure 34 shows the cross-section of an alternative fluid sensor assembly having a thermal conductivity fluid sensor contained in a package;
Figure 35 shows the cross-section of an alternative fluid sensor assembly having a thermal conductivity fluid sensor contained in a package;
Figure 36 shows a circuit diagram of a thermal conductivity fluid sensor where the heater is controlled via a feedback loop from the differential amplifier;
Figure 37 shows the top view and cross-section of a thermal conductivity fluid sensor comprising two recessed regions designed with different thermal properties;
Figure 38 shows an alternate circuit diagram for a thermal conductivity fluid sensor;
Figure 39 shows the top view of an alternate thermal conductivity fluid sensor.
Figure 40 shows a graph with thermal conductivities of air and different gases at different temperatures.
Figure 41 shows the top view and cross-section of a thermal conductivity fluid sensor comprising two sealed cavities, with one containing holes.
Figure 42 shows an alternate circuit diagram for a thermal conductivity fluid sensor. Figure 42a shows the current through the heater for the circuit in Figure 42.
Figure 43 shows an alternate circuit diagram for a thermal conductivity fluid sensor.
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Figure 43a shows the current through the heater and the voltage across it, as well as the reading computation for the circuit in Figure 43.
Figure 44 and 45 show alternate circuit diagrams for a thermal conductivity fluid sensor based on Figure 43a.
Figure 46 shows a comparison between DC and current-reversal DC voltage measurements.
Figure 47 shows a circuit schematic for driving the sensors in who different current directions.
Figure 48 shows a table of thermal properties of different gases.
Detailed Description of the Preferred Embodiments
Some examples of the disclosed device are given in the accompanying figures.
Figure 1 shows a top view of a thermal conductivity fluid sensor. It comprises a chip 1 made of a semiconductor substrate and a dielectric layer or region suspended on or over an etched portion of the semiconductor substrate, defining a region of the dielectric layer above the etched portion as a dielectric membrane 4. There is a resistor 2 embedded within the membrane, and track 7 connect it to bond pads 6. The resistor 2 is configured to operate as a heating element 2, and in this embodiment, it also acts as a resistive temperature detector element. There is another temperature detector element (also referred to as a thermal detector element) 3 outside the dielectric membrane. The fluid sensor also includes circuitry 5, that uses a differential signal from the two temperature detector elements 2, 3 to determine the composition of the fluid based on its thermal conductivity.
Due to the spatial separation between the heating element 2 and the first temperature sensing element 3, the heater 2 operates at a higher temperature than the first temperature sensing element 3 even in zero flow (or when no flow is present) when the heater 3 is powered up. The temperature of the first temperature sensing element 3 is dependent on the ambient temperature, and the temperature of the heating element 2
11649661-1 41 can vary depending on the heat loss to the surrounding fluid - which is dependent on the thermal conductivity of the fluid. The temperature differential (differential signal) between the heating element 2 and the first temperature sensing element 3 may be proportional to the concentration of a fluid.
For example, if CO2 is present in the sensor, the thermal conductivity of the CO2 is smaller than that of air, the temperature difference between the heater 2 and the first temperature sensing element 3 will be greater as the thermal conductivity of the CO2 is smaller than that of air.
The temperature difference between the heating resistor 2 and the first temperature sensing element 3 could be translated into a voltage difference or resistance difference, depending on the temperature sensing element employed. For diodes supplied with constant current, or for thermopiles, the voltage difference is appropriate. For Resistive Temperature Detectors (RTD), several read-out techniques could be employed such as using instrumentation bridges to measure change in the resistance or using current mirrors and sensing the voltage difference.
In this figure, the membrane is shown as circular. However, it can be rectangular, rectangular with rounded corners or any other shape. Similarly the resistors 2 and 3 are shown as circular, but can be any shape including ring, meander or rectangular. The resistor maybe made of a CMOS metal such as aluminium, tungsten, titanium or copper, or a non-CMOS metal such as gold or platinum, or from polysilicon or single crystal silicon.
Figures 2 shows a top view of an alternative thermal conductivity fluid sensor, and Figure 3 illustrates a cross-section of the sensor of Figure 2.
The thermal conductivity fluid sensor of Figures 2 and 3 has two circular resistors 2, 8 within the membrane region 4. One of the resistors is configured to operate as a heater element 2, and the other resistor within the dielectric membrane is configured to operate as temperature detector element 8. As shown in Figure 3, the two resistors 2, 8 are made of different layers within the dielectric layer 10 and can be in close proximity to each other so that they are at substantially the same temperature. The substrate 11 is a
11649661-1 42 semiconductor and the resistive temperature detectors are embedded within the dielectric layer 10.
Due to the spatial separation between the heating element 2 and the second temperature sensing element 8 (both on or within the dielectric membrane 4) and the first temperature sensing element 3, the second temperature sensing element 8 operates at a higher temperature than the first temperature sensing element 3 even in zero flow (or when no flow is present) when the heater 2 is powered up.
Figures 4 shows a top view of an alternative thermal conductivity fluid sensor, and Figure 5 illustrates a cross section of the sensor of Figure 4.
The thermal conductivity fluid sensor of Figures 4 and 5 has a circular resistive heater 2 acting as both a heater element and a temperature detector element, and has two recessed regions within the membrane, which are shown as two slots 12. The slots are circular around the heater.
The recessed regions minimise the thermal path through the solid dielectric membrane, forcing more heat to dissipate via convection and conduction through the environment (mostly above the membrane via conduction and convection), but partly also via heat conduction through the space formed by the slots or below the membrane. In this way a larger proportion of the heat loss of the heating element is to the surrounding fluid. So when there is a change in the thermal conductivity of the fluid the change in temperatures of the heating element and the second temperature sensing element are increased - thus the recessed regions increase the sensitivity of the device.
The presence of the slots also helps to reduce the power consumption of the device (for the same heater temperature), because of the reduction in the total heat losses. Furthermore, the slots help to reduce the thermal response time (increase the speed at which the heater heats up when supplied with an electrical power pulse) due to the decrease in the thermal mass of the membrane.
Figure 6 shows a top view of a thermal conductivity fluid sensor with a circular resistive heater 2 acting as both a heater element and a temperature detector element, and several recessed regions around the resistor in the shape of small circular holes 13.
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Similar to the slots of Figures 4 and 5, the circular holes 13 increase the proportion of power loss to the fluid and there improve sensitivity, reduce thermal response time and power consumption.
Figure 7 shows a top view of a thermal conductivity sensor with a wire shaped resistor 2 within the membrane region, and one wire shaped resistor outside the membrane 3, with the resistor within the membrane 2 acting as both a heater element and as a first temperature detector element. The membrane 4 is in case is a rectangular membrane with rounded corners - but can also be of any other shape. This sensor operates similarly to the sensor of Figure 1.
Figure 8 shows a top view of a thermal conductivity sensor with two wire resistors within the membrane region, with one operating as a heater element 2, and the other operating as a temperature detector element 8.
Figures 9 (a) to 9(d) each show a top view of an alternative thermal conductivity fluid sensor with recessed region 12 within the dielectric membrane. Recessed regions reduce the thermal losses from the heater, and increase the percentage of power loss to the fluid, thus improving the device sensitivity.
Figure 9a shows sensor where the heater element 2 is a wire resistor, and a second thermal detector element 8 is also a wire resistor.
Figure 10a shows the cross-section of the device in figure 9a. There are two wire resistors, one as a heater element 2, and one as a first thermal detector element 8.
Figure 9b shows a sensor where there are four recessed regions 12 on the membrane, two on either side of the heater and second temperature detector element. By increasing the number of recessed regions, the sensitivity of the device is increased.
In Figure 9c, the second thermal detector element 8 has a meander shape and is designed such that it is located on both sides of the heater 2. In this configuration, the second thermal detector element 8 is located in a different layer of the dielectric layer than the heater 2. The shape of the first temperature sensing element 3 is also the same as the second temperature sensing element 8.
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Figure 10b shows the cross-section of the device in figure 9c. The first thermal detector element 8 is in two parts, and split either side of the heater.
Figure 9d shows a sensor in which the second thermal detector element 8 also has a meander shape and is located on both sides of the heater 2. The second temperature detecting element 8 is formed of two portions, with a first portion of the second temperature detecting element 8 located on a first side of the heating element 2, and a second portion of second temperature detecting element 8 located on a second, opposite side of the heating element. A connection between the two portions of the second temperature detecting element is located outside the dielectric membrane 4, and is formed of a connecting element 20. This allows the first thermal detector element 8 to be made within the same layer of the dielectric layer as the heater element 2, with only the connecting element 20 located within a different layer of the dielectric region, and used to bridge the two portions of the second thermal detector element 8. The shape of the first temperature sensing element 3 is also the same as the second temperature sensing element 8.
Figure 11 shows a top view of a thermal conductivity fluid sensor with a recessed region 13 including arrays of circular holes within the dielectric membrane region 4.
Figure 12(a) shows a top view of a thermal conductivity fluid sensor with circuitry 5 located on the same chip as the heating element 2, and the first and second temperature sensing elements 3, 8. The circuitry 5 is used to control and drive the heater 2, and also measure the differential signal between the first temperature detector element 3 and the second temperature detector element 8. It may comprise a constant current or constant resistor drive circuit, a constant current source, a Wheatstone bridge, an amplifier, an Analog to Digital convertor, a Digital to Analog Convertor and/or a microcontroller.
Figure 12(b) shows a top view of a further fluid sensor where the first temperature sensing element 3 is on a second membrane, 4a, separate and identical in dimensions with the first membrane 4. The second dielectric membrane 4a has no active heating element. Common mode effects such as extra temperature rise due to self-heating when the two temperature sensing elements are biased can be removed. The effect of pressure and/or residual stress/strain in the membranes can also be cancelled out.
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Finally, the thermal mass associated with the two temperature sensing elements can be identical (or very similar) and thus dynamic mismatching effects can be minimised.
Figure 12(c) shows a similar fluid sensor to the one shown in Figure 12(b). Here an auxiliary structure 201 including one or more further dummy layers or elements is located on or within the second dielectric membrane 4a, such that the two temperature sensing elements have similar structures in their proximity (i.e. neighbouring structures) and the two membranes with their respective embedded structures seem to be identical from a mechanical and thermal mass perspective. For example, the auxiliary structure 201 may have identical size, shape and materials as the heating element 2. However, the dummy elements 201 in the second membrane are not connected to any electrical signal, and are therefore electrically isolated. The advantage of this fluid sensor is that the two temperature sensing elements 3, 8 are very well matched (they are both on identical membranes, they have similar neighbouring structures around them) in terms of their characteristics, including stress, or deformations. Moreover, the two sensing elements see similar mechanical stress profile and therefore common mode effects such as ambient pressure or vibrations can be removed.
Figures 13, 14(a), and 14(b) show etched regions within the semiconductor substrate 11 a thermal conductivity fluid sensor. In figure 13, the etched region has sloping sidewalls, which can be achieved by use of KOH or TMAH etching. Such an etching method is cheaper, but requires a larger chip area.
Figures 14(a) and 14(b) show thermal conductivity fluid sensors where the etched region does not extend through the entire semiconductor substrate 11. This can be achieved by etching from the front side of the substrate. This process results in a membrane or bridge structure supported by a dielectric beam. This results in a sensor with lower thermal power losses, but also with lower mechanical robustness compared to the sensor of Figure 13.
In Figure 14a etching is performed such that it stops at the crystal plane of the semiconductor substrate 11, resulting in an etched region having a triangular profile. In figure 14b, the etching is isotropic, resulting in an etched region having a rounded profile. In figure 14c the etching is performed similar to Figure 14a in that it stops at the crystal planes of the semiconductor substrate 11, but the stop point of the etching process is
11649661-1 46 also controlled (for example by timing) so that it does etch completely, resulting in an etched region having a trapezoid profile.
Figure 15 shows a top view of a thermal conductivity sensor design where both the first and second thermal detector elements 3, 8 are located on or within the same dielectric membrane 4. The heater element 2 and the second temperature detector 8 element are both located between two slotted recessed regions 12. The first temperature detector element 3 is thermally isolated from the first temperature detector element 8 and the heating element 2 by one of the slotted recessed regions 12. In this configuration, the heater element 2 and the second thermal detector element 8 are at substantially the same temperature during operation of the sensor, while the first thermal detector element 3 is at a different temperature, and is closer to the ambient temperature.
Figures 16a and 16b shows top views of two thermal conductivity fluid sensors each comprising an array of membranes.
In Figure 16a there are three dielectric membranes 4, and the heater 2 and thermal detector elements 3, 8 from each membrane are connected in series. Each of the heating elements 2 are connected in series, each of the first temperature sensing elements 3 are connected in series, and each of the second temperature sensing elements 8 are connected in series. If this system is operated in a constant current mode for the heater 2 and the thermal detector elements 3, 8, then the differential voltage signal will be higher. In this example with three membranes and corresponding heating elements and temperature sensing elements, the differential voltage signal will be multiplied by three compared to sensors having a single dielectric membrane with a single heating element and first and second temperature detecting elements). This is given as an example, but greater or fewer number of membranes can also be used within the fluid sensor.
Figure 16b shows an alternative thermal conductivity fluid sensor comprising 4 membranes, but the elements in each membrane are connected separately to bond pads. This allows much more flexibility in the design and use of the sensor. The four heaters 2 could be driven separately, for example at different temperatures, or with different drive modes. Alternatively, the heating elements 2 can be connected in series externally in a manner similar to figure 16a to increase the output signal.
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Figure 17 shows a cross section of a thermal conductivity fluid sensor where there is a covering 25 forming a very thin fluid channel 26 above the dielectric membrane of the dielectric layer 10. One or more holes 30 through the covering layer 25 allow fluid of various concentrations to diffuse or flow into the fluid channel region 26. The thin channel 26 increases the thermal losses from the heating element 2 to the fluid (from the membrane 4 to the covering 25) as the heat transferred through the fluid only needs to travel a smaller distance from the heating element 2 to the covering 25, the amount of heat loss through the fluid is increased. In embodiment without the covering 25, the heat needs to travel a greater distance to the closest solid surface (which may be the chip surface, as the heat transfer isn’t required to be in a straight line). Therefore, the covering 25 increases the sensitivity of the device. The covering 25 can be a semiconductor bonded by wafer bonding. It can also be glass, or plastic.
Figure 18 shows a cross section of a thermal conductivity fluid sensor packaged in a flip chip method. Solder balls 36 form electrical connections to a Printed Circuit Board (PCB) 35. This also forms a thin channel 26 between the membrane of the dielectric layer 10 and the PCB 35, allowing for an increase in sensitivity of the device to fluid concentration similar to the device in figure 18.
Figure 19 shows a top view of a thermal conductivity fluid sensor with the resistive temperature sensors 3, 8 having a meander shape. In particular, the second resistive temperature sensor 8 is configured such that the wire element of the second resistive temperature sensor 8 loops around one of the bond pads of the heater 2 and the second resistive temperature sensor 8 has two bond pads located on either side of the other bond pad of heater 2. This means that the second temperature sensing element 8 can be made in a single layer, and preferably within the same material layer as the heater 2. In this configuration, the first resistive temperature sensor 3 is the same shape as the second resistive temperature sensor 8, but is located outside the membrane region 4.
Figure 20 shows how the thermal conductivity measured by the fluid sensor varies with temperature for the gases of air, carbon dioxide, hydrogen and methane. This figure illustrates that the temperature dependence of gas thermal conductivity is different for different gas compositions. This means that heaters can be used at an optimum temperature for sensitivity of the device to different gases. In addition to this, the inset shows a detailed view of the temperature that air and carbon dioxide have the same
11649661-1 48 value of thermal conductivity. This can be advantageous for the device selectivity, and multiple heater temperatures can be used to help identify, or ignore, certain gases (e.g. running the device at the temperature where carbon dioxide and air are identical eliminates any response to carbon dioxide in air).
Figure 21a shows an example circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor. This circuitry could be used in conjunction with any of the sensors described above having a heater 2, and first and second temperature sensing elements 3, 8. The heater 2 is driven by a current source. The first and second resistive temperatures sensors 3, 8 are located on sides of a Wheatstone bridge along with two additional resistors 40 and 41. One side of the bridge (between the first and second resistive temperatures sensors 3, 8) is connected to a reference voltage 50, while the other side 60 is grounded. A differential amplifier 55 measures the differential voltage between the two legs of the Wheatstone bridge.
The heater 2 may be drive with a constant current. When the concentration of the target gas changes, then the temperature of the heater 2, and hence the temperature and resistance of the second resistive temperature sensor 8 will change. This will change the differential voltage between the two arms of the Wheatstone bridge and can be detected. The circuit may be calibrated in a standard environment (for example, with no target gas present) to know what the nominal or calibrated differential voltage is. Deviation from this calibrated differential voltage indicates presence of the target gas.
Preferably the resistors 40 and 41 are chosen such amplifier 55 outputs a zero voltage at a normal of calibrated condition (for example Oppm of the target gas in air, or in case the target gas is carbon dioxide then in 400ppm of carbon dioxide in air). The resistors 40 and 41 maybe trimmed during the calibration of the device. If resistors 40 & 41 are not chosen in such a way, then they may be calibrated to know what the differential voltage will be in the calibration conditions.
Another way to drive the fluid sensor is to control the current through the heater 2 such that the differential voltage across the Wheatstone bridge is always constant. In this case, the change in current required within the heater 2 could be measured to indicate the presence of a target gas.
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Figure 21b shows another arrangement of the Wheatstone bridge where the resistors 3 and 40 are swapped. Besides this many other arrangements of the bridge are possible.
Figure 21c shows an example of circuitry for driving a thermal conductivity fluid sensor and measuring the output from the fluid sensor comprising an array of membranes and, heater and temperature sensor elements connected in series. This circuitry could be used in conjunction with either of the sensors shown in Figures 17a and 17b. The Wheatstone bridge configuration of Figure 22b can be used in a similar manner to that described in relation to Figure 22a. The devices connected in this way can amplify the sensitivity due to increased voltage changes.
Figure 22 shows an example circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor. The heater 2, and the first and second resistive temperature sensors 3, 8 are each driven by their own, separate current sources 45, 46, 47. Preferably, the fluid sensor is first calibrated in a standard, predetermined environment, and current sources 46 and 47 are adjusted such that the output from the differential amplifier 55 is zero. During operation, the current sources 46 and 47 are driven at the calibrated current levels, and the deviation of the output from zero of the differential amplifier 55 indicates the presence of the target gas.
Figure 23 shows alternative circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor, having a Wheatstone bridge similar to Figure 21. However, the heater 2 is driven by a voltage source VHTR. Additionally the arm of the Wheatstone bridge that has the first resistive temperature sensor 3 also has a variable resistor 44 in series with the first resistive temperature sensor 3. The first and second resistive temperature sensors 3, 8 can have different resistances during heater operation, but during calibration the variable resistor 44 can be adjusted such that the output from the differential amplifier 55 is zero. The variable resistor 44 can be adjusted manually or electronically.
Figure 24 shows alternative circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor, however each side of the bridge has a different supply voltage. One side is kept constant at VREF, while the other is kept at an adjustable voltage VBAL. During calibration, VBAL can be adjusted so that the differential amplifier 55 gives an output of zero volts. This VBAL value can then be stored in either in firmware or
11649661-1 50 software of the sensor. This VBAL value is then applied whenever the device is operated, and deviation of the differential amplifier output from zero indicates the presence and concentration of the target gas. In an alternate configuration, VBAL can be controlled during operation to keep the output signal at zero, and changes in the required VBAL value can be measured to indicate the presence of a gas.
Figure 25 shows alternative circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor comprising a Wheatstone bridge with a variable resistor 44 similar to figure 23, but the heater is driven using a current source.
Figure 26 shows alternative circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor. The first and second resistive temperature sensors 3, 8 are both in the bottom side of the Wheatstone bridge. Furthermore, the branch comprising the first resistive temperature sensing element 3 also has a Field Effect Transistor (FET) 65 in series with the first resistive temperature sensing element 3. The FET 65 is similar to the variable resistor of Figure 25, however this can be controlled electronically allowing calibration without manual intervention.
Figure 27 shows steps in of a method of balancing the Wheatstone bridge shown in figure 24. This method uses a fixed number of iterations. A counter is set to the maximum number of iterations. At each iteration, the counter is reduced by 1. If the value of the counter is negative then the current VBAL value is set as the balance voltage. Otherwise, the output from the differential amplifier is checked. If the output is positive, then the VBAL value is increased, otherwise it is decreased.
This method can be used in two ways. It can be used in calibration of the fluid sensor to determine the required balance voltage at a standard environment. Alternatively, it can be used during operation of the fluid sensor to keep the Wheatstone bridge balanced, and the VBAL value can be measured to determine presence and concentration of gas.
Other similar method or algorithms can also be used; for example, counting up to a maximum number of iterations, or performing iterations until the absolute output from the differential amplifier is within the required range, or a mixture of the method described above.
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Figure 28 shows the top view of a thermal conductivity fluid sensor where there is an additional temperature sensing element 75 on the chip, and outside the dielectric membrane 4. This additional temperature sensing element 75 can be used to compensate for effects of ambient temperature changes. Most effects of ambient temperature changes will be cancelled out due to the differential measurement method of the fluid sensor. However, compensating for ambient temperature changes using a temperature sensing element 75 will further improve accuracy. The additional temperature sensing element 75 shown in the figure is a resistive temperature sensor. However, it can also be a diode, transistor or a standard temperature measurement circuit such as an iptat or a vptat circuit.
Figure 29a shows alternative circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor where a single resistor 2 is used as the heater and the second resistive temperature sensing element, similar to the sensor shown in Figure 1. In this sensor there is a bridge circuit having two sides. One side has the first resistive temperature sensing element 3, and an additional resistor 41. The other side comprises the heating resistor 2 and a further additional resistor 40, with resistor 40 ideally identical to resistor 41. When using the resistor 2 to measure sensing, the current from the current source 45 flows through resistors 2 and 40. The signal at the output of the differential amplifier 55 will be dependent on the temperature of resistor 2. When using resistor 2 for heating, a switch 70 is closed, allowing a larger current to flow.
Figure 29b shows a PWM (Pulse Width Modulation) waveform which shows with time the current and/or voltage in the heater of Figure 29a. The pulses have a high frequency such that there is little change in heater temperature during the “off” mode of the pulsed heater. During the ’’off” mode the resistance of the heater can be measured to determine the heater temperature, and a differential signal between the temperature of the heating element and the fist temperature sensing element can be used to determined thermal conductivity of the fluid in the sensor. This method can be used in cases where the heater is also used as the second resistive temperature detector element as shown in figure 29a, by opening and closing the switch 70.
Figure 30 shows a top view of a thermal conductivity fluid sensor where in addition to the first and second temperature detector elements 3, 8 there are two additional temperature detector elements 3A, 8A. Temperature sensing elements 8 and 8A are both on or within
11649661-1 52 the dielectric membrane 4 and in close proximity to the heater 2, whereas temperature sensing elements 3 and 3A are outside the membrane region 4.
Figure 31 shows circuitry for driving the thermal conductivity fluid sensor and measuring the output from the thermal conductivity fluid sensor shown in figure 31. The second temperature detector elements that are on the membrane 8, 8A are placed on opposite sides of the Wheatstone bridge. Similarly, both the first temperature detector elements outside the membrane region 3, 3A are also placed on opposite sides to each other. This configuration doubles the sensitivity of the thermal conductivity sensor.
Figure 32 shows the top view of a thermal conductivity fluid sensor comprising two additional resistive elements 100 either side of the heater 2 and the second temperature detector element 8. This allows the device to be used as not only a thermal conductivity sensor, but also a flow sensor. One resistive element of the pair of resistive elements 100 is located upstream of the heating element 2 and another resistive element of the pair of resistive elements 100 is located downstream of the heating element 2. The heating element 2 extends in a direction substantially perpendicular to the direction of flow through the sensor. When the fluid passes over the top of the membrane 4, the heater 2 cools down due to heat convention losses. In the presence of the flow, the downstream sensing element sees a higher temperature than the upstream sensing element. The temperature difference between the pair of resistive elements 100 increases with the flow rate (or flow velocity). In the presence of a fluid flow, there will be a difference in resistance between the two additional resistive elements 100 depending on the speed and direction of the fluid flow. Whilst shown as resistive elements, the two additional elements 100 for flow sensing can be based on other temperature detection principles such as diode based temperature detectors, or a thermopile temperature detector.
Figure 33 shows a cross-section of a thermal conductivity fluid sensor assembly. It comprises a package base 101 and a package lid 102. Within the package is an ASIC (Application Specific Integrated Circuit) chip 103 that is used to control and measure the thermal conductivity sensor chip. Above this ASIC chip 103 is the thermal conductivity sensor chip comprising a substrate 10 and dielectric region or layer 11. The sensor chip may include any fluid sensor as described above. Wire bonds 104 electrically connect the thermal conductivity sensor fluid chip to the ASIC chip 103, and wire bonds 105
11649661-1 53 electrically connect the ASIC 103 to the package base 101. A hole 108 within the package lid 102 allows the ambient air or gas to diffuse into the package and around the thermal conductivity sensor. More than one hole may be present within the package lid, and the size and shape of the hole 108 can be varied, and filters may be placed around or within the hole 108 or holes to protect against particles or liquids.
Figure 34 shows a cross-section of an alternative thermal conductivity fluid sensor assembly. The ASIC chip 103 and the fluid sensor chip are not stacked on top of each other, but are located side by side within the package. Wire bonds 106 connect the sensor chip to the ASIC chip 103.
Figure 35 shows a cross-section of an alternative thermal conductivity fluid sensor assembly. Compared to the sensor assemblies shown in Figures 33 and 34, the lid 102 has two ports, one as an input port 106 and one as an output port 107.
Figure 36 shows alternative circuitry for driving a thermal conductivity fluid sensor and measuring the output from the sensor. There is a circuit block 80 to control the heater 2. The output from the instrumentation amplifier 55 is part of a feedback loop into the heater control 80. The heater 2 can then be controlled such that it keeps the output of the instrument amplifier 55 at zero voltage. The bias or control signal required to the heater 2 is then used to determine presence and concentration of gas within the fluid sensor.
Figure 37 shows a top view and cross-section of an example thermal conductivity fluid sensor. In this example there are two dielectric membranes, a first membrane 4 and a second membrane 4a. Both membranes have identical heaters 2 and 2a which can also be configured to act as temperature sensors. There is an additional layer 200 on the chip which forms structures 203, 203a and 202. Layer 200 is preferably made of a material with thermal conductivity higher than the dielectric membranes 4, 4a. Structure 203 is located above the first membrane 4, and is a circle, or a plate shape that is larger than the size of the heater 2 within the first membrane. Structure 203a is located above the second membrane 4a, and forms a circle or a plate that is the same or a similar size as the heater 2a. It will be understood that structures 203 and 203a do not have to be circular, and may be any other shape, such as rectangles. However, in some implementations, circular structures 203, 203a may provide a more uniform heat distribution and/or enhanced mechanical stability, in comparison to other structure
11649661-1 54 shapes. Structure 202 is located above the chip, but is absent above the bond pads 6 and some areas of the membranes 4, 4a. As shown, an edge of structure 202 may be approximately aligned with to the edge of the first membrane area 4, but structure 202 does not extend above the first membrane area 4. However, structure 202 is does extent above the second membrane 4a area, leaving a circular portion in the middle of the second membrane 4a that is uncovered.
Such a construction can be configured so that the power consumption of both the heaters 2 and 2a is the same or approximately the same for a given temperature. However, the ratio of power loss to air or other fluids as compared to power loss through the membrane(s) 4, 4a may be different for each of the heaters 2, 2a. As a result, both heaters may give a different response when in the presence of a target fluid such as a target gas, and a differential signal between them (e.g. using the heaters 2, 2a as temperature sensors) can therefore be used to determine the concentration or composition of the target gas.
Figure 38 shows an example circuit for measuring the thermal conductivity fluid sensor shown in Figure 37. In this circuit, a Wheatstone bridge is used with the heaters 2 and 2a connected in the circuit along with two fixed resistors 40, 41. The differential voltage measured between the two arms of the bridge can be used to determine the concentration of the target gas.
Figure 39 shows a top view of another thermal conductivity fluid sensor. In this implementation there is a single dielectric membrane 4 which has an active heater 2. The active heater 2 can also be configured to act as a temperature sensor. A first temperature sensor 3 is placed between the active heater 2 and the edge of the dielectric membrane 4. An elongated slot 12 may optionally be placed next to the heater on the opposite side from the first temperature sensor. A second temperature sensor 8 is placed on the semiconductor chip 1 outside the dielectric membrane 4.
In this implementation, the temperature of the first temperature sensor 3 may be a fixed portion of the difference between the temperature of the active heater 2 and the second temperature sensor 8. As the ratio is fixed, a reference can be constructed in the circuitry 5 using the resistances of the heater 2 and the second temperature sensor 8 and the fixed ratio to compare with the resistance of the first temperature sensor 3. The
11649661-1 55 difference between the constructed reference and the signal from the first temperature sensor can be used to determine the concentration of the target gas.
Figure 40 shows a graph plotting the thermal conductivity of air and different gases across different temperatures. The graph illustrates that if the heater is driven at around 800K, then air and carbon dioxide have the same thermal conductivities, and any deviation from normal is caused by other effects, such as humidity. The heater can then be run at a lower temperature where in addition to other effects, carbon dioxide also causes a deviation in signal. Using algorithms or a look up table the deviation due to other effects can then be cancelled to determine the deviation solely due to carbon dioxide.
Figure 41 shows another example thermal conductivity fluid sensor. In this implementation there are two dielectric membranes, a first membrane 4 and a second membrane 4a. The two membranes are identical, apart from the two (or more) holes 12 on membrane 4. Membrane 4a has no holes. Both membranes have identical heaters 2 and 2a, and sensing elements 3 and 3a. The thermal conductivity fluid sensor is located on a base of a package 8, where the cavities 210 and 210a under the membranes 4 and 4a are both sealed. Such a construction can be designed so that the sensing element 3a, which is exposed to the gas only on one side of the membrane 4a can be used as a reference for the sensing element 3, which is exposed to the gas on both sides of the membrane 4. This design could lead to a faster and more reliable temperature compensation, since both sensing elements are directly exposed to the same environment, however only sensing element 3 is exposed to a higher gas concentration.
Figure 42 shows an example circuit for measuring a thermal conductivity fluid sensor such as that shown in Figure 41. In this circuit two AC current sources 45 and 45a are used with heaters 2 and 2a respectively. Both current sources can independently generate square wave signals, as shown in Figure 42a, with adjustable intensity and frequency. The differential voltage signal, measured across heaters 2 and 2a, is then processed by a lock-in amplifier 55, or a fast Fourier transform (FFT)-based digital signal processing (DSP) circuit.
Figure 43 shows another example circuit for measuring a thermal conductivity fluid sensor such as that shown in Figure 41. In this circuit two reversible DC current sources
11649661-1 56
45 and 45a are used with heaters 2 and 2a respectively. Both current sources can independently generate currents with alternating polarities, as shown in Figure 43a, with adjustable intensity and frequency. In this case the voltage measurements on each heater are performed based on a three-step delta method as detailed in Figure 43a. This three-step delta method may offer significant advantages over other DC resistance measurement techniques in reducing the impact of or overcoming errors due to changing temperature.
Figure 44 shows a variation of the circuit presented in Figure 43. In this circuit two DC current sources 45 and 45a are used with heaters 2 and 2a respectively, while two reversible DC current sources 46 and 46a, providing a much smaller current, are used with sensing elements 3 and 3a respectively. The much smaller current level provided by the reversible DC current sources 46 and 46a when compared to e.g. the circuit of Figure 43, aids in reducing transient effects due to current switching and thus may allow for a faster and more accurate delta reading.
Figure 45 shows variation of the circuit presented in Figure 44. In this circuit the sensing element 3 and 3a are connected in series and driven by a single reversible DC current source 46. Using a single current source may improve the circuit immunity to common mode noise while simplifying circuit overall.
Figure 46 shows a comparison between -1200 DC voltage measurements of a -60 W heater made with -8 mA test current taken approximately over 120 seconds. The DC measurements fluctuate with a voltage error of up to 30%, whereas the three-point DC reversal method measurements fluctuate with less than 5% error. These figures can be further significantly improved by using a smaller (e.g. less than a few mA) test current.
Figure 47 shows a circuit schematic for driving the sensors in two different current directions. There is a control 301 that provides the electrical bias to the sensors 302. The sensors are read by a read out circuit 303. The transistors 305,306,307 and 308 control the direction of current within the sensors 302. When transistors 305 and 306 are on, and 307 and 308 are off then the current flows in one direction through the sensors. While when the transistors 305 and 306 are off, and transistors 307 and 308 are on, the current flows in the opposite direction. This method can be used to improve the accuracy of measurements using the delta method. The control system 301 can be just a current
11649661-1 57 source, or a voltage source, or a more complex circuit. The sensors 302 may be e.g. a temperature sensing resistor, or may comprise more than one resistor. For example, the more than one resistors may be provided in a bridge configuration - where all the branches have active sensors, and/or some branches have fixed resistors. The read out circuit 303 can have a differential amplifier, filter and/or an analog to digital circuit.
Figure 48 provides the thermal properties of different example gases of interest as well as those of dry and wet air (at standard temperature and pressure). The values of nitrogen and oxygen are also provided to demonstrate how sensitive the values can be to the oxygen content of air.
Figure 48 also shows the thermal response time relative to dry air for each of these gases. For example, it can now be seen that Helium gas is 8.3x faster than dry air and 7.1x faster than wet air. It can be seen that hydrogen gas is 6.9x faster than methane. Wet air is 1.13 or 13% faster than dry air. Finally, it can be seen that C02 is 2.1x slower than dry air and 2.4x slower than wet air.
It is now possible to determine the gas type from the thermal response time and hence, knowing the gas, also determine the concentration of the gas in air. It is also possible to determine the different gases in the mixture because there will be two distinct thermal constants in the TC response. For example, one faster one for H2 and one much slower one (x6.9) for CH4.
Driving the thermal conductivity heater with an AC signal (or using a pulse) will result in different frequency responses according to the gas type and concentration. The frequency content of the signal (e.g. an FFT) will show which gas is present by a characteristic frequency and the height of the FFT peak will give its concentration. In this way we can determine the type of gas when the gas is unknown and also determine the gases present in a gas mixture as well as their concentrations.
Finally, it should be noted that the thermal time constant of dry and wet air are similar (12% difference) and very different to that of H2 and C02. In other words, the relative humidity of the air will not significantly affect the signal at the frequency for C02 or H2.
11649661-1 58
This AC method decouples the thermal signal of the target gas (for example C02) from that of a variable background gas (for example, other components of air such as nitrogen and/or oxygen ), and therefore provides a much more accurate way of measuring a gas concentration or composition than DC techniques.
The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘above’, ‘overlap’, ‘under’, ‘lateral’, etc. are made with reference to conceptual illustrations of a device, such as those showing standard cross- sectional perspectives and those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to a device when in an orientation as shown in the accompanying drawings.
Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the disclosure, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.
Reference Numerals
1 Semiconductor chip 35 Printed Circuit Board
2 Resistive heating element 36 Solder balls 2A Additional heating element 40, 41 Additional resistor
3 First temperature sensing element 42, 43 Additional resistor
3A Additional first temperature sensing 44 Variable resistor element 45, 45a, 46, 46a, 47 Current source
4 Dielectric membrane 50 Reference voltage
4a Second dielectric membrane 55 Differential amplifier
5 Circuitry 60 Ground
6 Bond pads 65 Field Effect Transistor
7 T rack 70 Switch
11649661-1 59
8 Second temperature sensing element 75 Ambient temperature sensing element 8A Additional second temperature 80 Heater control sensing element 100 Pair of temperature sensing
9 T racks elements
10 Dielectric layer 101 Package base
11 Semiconductor substrate 102 Package lid
12 Elongate slot 103 ASIC
13 Hole 104, 105 Wire bonds
20 Connecting element 106 Inlet
25 Covering layer 107 Outlet
26 Fluid channel above membrane 108 Hole through package lid 30 Hole through covering layer 110 Lid
200 Additional layer
201 Dummy elements
202, 203, 203a Additional structures 210, 210a: Etched portion of Substrate
301 Control Circuitry
302 Sensor(s)
303 Read out Circuitry
305, 306, 307, 308 switching transistors
11649661-1

Claims

60 CLAIMS:
1. A fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising: a semiconductor substrate comprising a first etched portion; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first etched portion of the semiconductor substrate; a heating element located within the first dielectric membrane; and a first temperature sensing element spatially separated from the heating element, wherein the first temperature sensing element is located outside of the first dielectric membrane and over the semiconductor substrate, or wherein the first temperature sensing element is located on or within the first dielectric membrane and wherein the fluid sensor comprises at least one recessed region within the first dielectric membrane configured to thermally isolate the heating element from the first temperature sensing element, wherein the heating element is further configured to operate as a second temperature sensing element, and wherein the separation between the second temperature sensing element and the first temperature sensing element introduces a temperature difference between the heating element and the first temperature sensing element, such that a differential signal between the first temperature sensing element and the second temperature sensing element is indicative of the concentration or composition of the fluid based on a thermal conductivity of the fluid.
2. A fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising: a semiconductor substrate comprising a first etched portion; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first etched portion of the semiconductor substrate; a heating element located within the first dielectric membrane; a first temperature sensing element spatially separated from the heating element, wherein the first temperature sensing element is located outside of the first dielectric membrane and over the semiconductor substrate, or
11649661-1 61 wherein the first temperature sensing element is located on or within the first dielectric membrane and wherein the fluid sensor comprises at least one recessed region within the first dielectric membrane configured to thermally isolate the heating element from the first temperature sensing element; and a second temperature sensing element located on or within the first dielectric membrane, wherein the second temperature sensing element is substantially identical in shape and size to the first temperature sensing element, and wherein the separation between the second temperature sensing element and the first temperature sensing element introduces a temperature difference between the second temperature sensing element and the first temperature sensing element, such that a differential signal between the first temperature sensing element and the second temperature sensing element is indicative of the concentration or composition of the fluid based on a thermal conductivity of the fluid.
3. A fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising: a semiconductor substrate comprising a first etched portion and a second etched portion, wherein the first etched portion and the second etched portion are substantially identical in size and shape; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first etched portion of the semiconductor substrate and a second dielectric membrane located over the second etched portion of the semiconductor substrate; a single active heating element, wherein the active heating element is located only within the first dielectric membrane; a first temperature sensing element located within the second dielectric membrane; and a second temperature sensing element located on or within the first dielectric membrane, wherein the second temperature sensing element is substantially identical in shape and size to the first temperature sensing element, and
11649661-1 62 wherein the separation between the second temperature sensing element and the first temperature sensing element introduces a temperature difference between the second temperature sensing element and the first temperature sensing element, such that a differential signal between the first temperature sensing element and the second temperature sensing element is indicative of the concentration or composition of the fluid based on a thermal conductivity of the fluid.
4. A fluid sensor according to claim 1 , wherein the first temperature sensing element and the second temperature sensing element are both located on or within the first dielectric membrane, and wherein the fluid sensor comprises at least one recessed region within the first dielectric membrane configured to thermally isolate the heating element and the second temperature sensing element from the first temperature sensing element.
5. A fluid sensor according to claim 1, wherein the second temperature sensing element is located in a same layer of the dielectric region as the heating element and wherein the second temperature sensing element laterally surrounds the heating element, or wherein the second temperature sensing element is located below or above the heating element.
6. A fluid sensor according to claim 1 , wherein the first temperature sensing element is configured to have a higher resistance at room temperature than a resistance of the second temperature sensing element at room temperature, and wherein the first temperature sensing element and the second temperature sensing element are configured to have substantially the same resistance at an operating temperature of the sensor without a fluid present.
7. A fluid sensor according to claim 1, wherein the heating element is a resistive heating element; and/or wherein at least one of the first temperature sensing element and the second temperature sensing element are resistive temperature sensing elements.
11649661-1 63
8. A fluid sensor according to claim 1, further comprising circuitry configured to determine the concentration or composition of the fluid based on the differential signal; and optionally wherein the circuitry may be located on a same chip as the fluid sensor.
9. A fluid sensor according to claim 8, wherein the circuitry comprises one or more of: a constant current or constant resistor drive circuit, a constant or alternating current source, a Wheatstone bridge, an amplifier, an Analog to Digital convertor, a Digital to Analog Convertor, or a microcontroller.
10. The fluid sensor of claim 8, wherein the first temperature sensing element and the second temperature sensing are located on two sides of a bridge circuit, and wherein the sensor is configured such that an output of the bridge circuit is a function of the thermal conductivity of the fluid around the sensor.
11. A fluid sensor according to claim 1, wherein the semiconductor substrate comprises an additional etched portion, and wherein the dielectric layer comprises an additional dielectric membrane located over the additional etched portion of the semiconductor substrate, and wherein the sensor further comprises: an additional heating element located within the additional dielectric membrane; and an additional first temperature sensing element; and optionally wherein the heating element and the additional heating element are connected in series, and/or wherein the additional first temperature sensing element and the first temperature sensing element are connected in series; and optionally wherein the heating element and the additional heating element are configured to operate at different temperatures.
11649661-1 64
12. A fluid sensor according to claim 1, further comprising a covering located on a surface of the sensor, wherein the covering comprises a hole configured to allow fluid to travel from an outer surface of the covering to the fluid channel above the dielectric membrane.
13. A fluid sensor according to claim 1, further comprising a further temperature sensing element located outside the membrane region.
14. A fluid sensor according to claim 1, further comprising an additional first temperature sensing element outside the membrane region and an additional second temperature sensing element located on or within the dielectric membrane region.
15. A fluid sensor according to claim 1, further comprising a pair of temperature sensing elements located on the dielectric membrane, wherein a first temperature sensing element of the pair of temperature sensing elements is located on a first side of the heating element and a second temperature sensing element of the pair of temperature sensing elements is located on a second side of the heating element.
16. A fluid sensor according to claim 3, wherein the sensor further comprises an auxiliary structure located within the second dielectric membrane, wherein the auxiliary structure is electrically isolated, and wherein the auxiliary structure is configured such that the first dielectric membrane and the second dielectric membrane have the same mechanical and thermal stress properties.
17. A sensor assembly comprising the fluid sensor of claim 1 and an application specific integrated circuit (ASIC) coupled to the sensor.
18. A sensor assembly comprising: a flow sensor housing; and a sensor according to claim 1 located within the flow sensor housing.
11649661-1 65
19. A sensor assembly comprising the fluid sensor of claim 1 , wherein the fluid sensor is packaged on a printed circuit board in a flip-chip configuration.
20. A method of measuring a concentration or composition of a fluid using a sensor of claim 1 , the method comprising: applying an electrical bias to the heating element; and monitoring the electrical bias applied to the heating element and using the value of the electrical bias applied to the heating element and the differential signal to determine the concentration or composition of the fluid based on thermal conductivity of the fluid.
21. A method according to claim 21 , wherein applying an electrical bias to the heating element comprises applying an electrical bias such that the differential signal between the first temperature sensing element and the second temperature sensing element is minimised.
22. A method according to claim 21 , comprising: driving the heating element in pulse mode or AC mode to modulate the temperature of the heating element to vary the differential signal; and using the differential signal to selectively differentiate between different fluid components and/or determine the concentration of the different fluid components; and optionally wherein differentiating between different fluid components and/or determining the concentration of the different components comprises using a neural network.
23. A method according to claim 21 , wherein the method comprises: applying a modulated function to the heating element, the first temperature sensing element, or the second temperature sensing element; measuring the modulation, the time delay, or the phase shift of the differential signal between the first temperature sensing element and the second temperature sensing element; and determining a concentration or composition of the fluid using the measured modulation, time delay or phase shift.
11649661-1 66
24. A fluid sensing system comprising: a fluid sensor according to claim 1 ; and a controller configured to perform the method of claim 20.
25. A method of manufacturing a fluid sensor according to claim 1, the method comprising: forming a first dielectric membrane located over a first etched portion of a semiconductor substrate semiconductor substrate comprising a first etched portion; forming a heating element located within the first dielectric membrane; forming a first temperature sensing element spatially separated from the heating element, wherein the first temperature sensing element is located outside of the first dielectric membrane and over the semiconductor substrate, or wherein the first temperature sensing element is located on or within the first dielectric membrane and wherein the fluid sensor comprises at least one recessed region within the first dielectric membrane configured to thermally isolate the heating element from the first temperature sensing element.
26. A fluid sensor according to claim 3, comprising a second active heating element located only within the second membrane.
27. A fluid sensor according to claim 26, comprising one or more conductive elements located over one or both of the first and second dielectric membranes, the conductive elements arranged such that a conductive heat loss through the second dielectric membrane is greater than a conductive heat loss though the first dielectric membrane.
28. A fluid sensor according to claim 27, wherein the first and second dielectric membranes are configured such that a total heat loss from each of the first and second dielectric membranes is approximately equal, such that a differential signal between the first active heating element and the second active heating element is indicative of the concentration or composition of the air mix based on a thermal conductivity of the air mix.
29. A fluid sensor according to claim 28, comprising:
11649661-1 67 a first heat diffuser located over the first heating element, wherein an edge of the first heat diffuser extends beyond an edge of the first heating element; and a second heat diffuser located over the second heating element, wherein an edge of the second heat diffuser is approximately aligned with an edge of the second heating element.
30. A fluid sensor according to claim 11 , wherein: the first and second etched portions form sealed regions; and one of the first and second dielectric membranes comprises one or more holes exposing the sealed region of the respective etched portion to the fluid via the one or more holes such that the concentration or composition of the fluid may be determined based on a differential signal between the first temperature sensing element and the first additional temperature sensing element.
31. A method according to claim 21, comprising: driving the heating element in AC mode to modulate the temperature of the heating element to vary the differential signal; monitoring the differential signal at the modulation frequency using a lock-in amplifier and/or based on a Fourier transform-based technique; and selectively differentiating between different fluid components and/or determining the concentration of the different fluid components based on the differential signal.
32. A method according to claim 21 comprising: driving the heating element or a sensing element adjacent to the heating element in a reversible current DC mode; and monitoring the differential signal based on a two point or a three point DC reversal-based technique; and selectively differentiating between different fluid components and/or determining the concentration of the different fluid components based on a differential signal.
33. A method according to claim 21, driving the heating element by one or more current sources with alternating polarities; and
11649661-1 68 monitoring the differential signal based on a two point or a three point DC reversal-based technique; and selectively differentiating between different fluid components and/or determining the concentration of the different fluid components based on a differential signal wherein the heating element is driven.
34. A method according to claim 33, wherein the heating element is driven by a single current source, and wherein the fluid sensor comprises switches configured to change the direction of a current in terminals of the heating element or the sensing element.
35. A fluid sensor according to claim 1, comprising a control unit configured to drive the heating element in an AC bias or a pulse bias, and determine the concentration and type of gas or gases present based on the frequency content of a resulting signal.
11649661-1
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