WO1998029711A1

WO1998029711A1 - Sensor arrangement and method of biasing a sensor

Info

Publication number: WO1998029711A1
Application number: PCT/EP1997/007291
Authority: WO
Inventors: Eric Perraud; Marc Osajda; Pierre Collette
Original assignee: Motorola Semiconductors S.A.
Priority date: 1996-12-31
Filing date: 1997-12-22
Publication date: 1998-07-09
Also published as: FR2757942A1

Abstract

A sensor arrangement (30) comprises a sensor (32) having supply contacts (38) and a supply signal generator (34) for generating a supply signal (42) comprising pulses and for applying the pulsed supply signal to the supply contacts such that in use the sensor (32) is biased periodically for a predetermined period (Ton) and a sensor output signal of the sensor is measured during at least some of the predetermined periods. The supply signal may comprise for example a plurality of alternate positive and negative pulses, or a plurality of positives pulses.

Description

SENSOR ARRANGEMENT AND METHOD OF BIASING A SENSOR

Field of the Invention

The invention relates to a sensor arrangement and a method of biasing a sensor.

Background of the Invention

Sensors, such as pressure sensors, are sometimes used in wet environments. Pressure sensors are used, for example, in applications which need to sense the pressure of a fluid or vapours such as in pressure cookers, water pumps, and central heating systems.

Low cost semiconductor pressure sensors cannot generally be used in such wet environments as the sensor die and supply contacts of such sensors would be in contact with the fluid or vapour. In such cases, when a supply voltage is applied to the sensor supply contacts, due to the wet environment, electrical corrosion occurs at the pads with the result that the device fails after some working hours. Several solutions have been proposed which try and prevent water contacting the semiconductor sensor die and supply contacts.

The MPX906 pressure sensor supplied by Motorola, Inc. is arranged such that pressure can be applied on the backside of the sensor. This means that the die and the aluminium contact pads are not exposed to the fluid or vapour surrounding the sensor and so corrosion can be avoided.

However, this solution presents some major drawbacks: it can work only with low pressure ranges (less than 100 KPa) otherwise the die may be detached from the package; it cannot work in alkaline solutions (pH > 9) in order to avoid any etching of the wafer; and the electrical performances are not as good compared to arrangements wherein the pressure is applied to the topside of the sensor. Since a huge amount of the market for water compatible pressure sensors is in the industrial market which require mid or high pressures ranges (200KPa - 1000 KPa), in view of some of the above drawbacks the 'backside' pressure sensor does not provide a solution to the corrosion problem for such 'high' pressure applications. Another known solution comprises mounting the sensor die on an intermediate diaphragm which is hermetic to water and filling the gap between the diaphragm and the die with an oil or gel which is not compressible, for example silicon oil. Such an arrangement is described in UK Patent Application No. GB-A-2266152.

Although this solution reduces the corrosion problems, it requires special housing with several seals which makes the final sensor product expensive. In addition, the manufacturing and assembly processes are complex and not easy to implement in high volume production since many operations, such as gel filling, are required.

There is therefore a need for an improved sensor arrangement which is reliable in wet environments, even at high pressures, and which is inexpensive to manufacture.

Summary of the Invention

In accordance with the present invention there is provided a sensor arrangement as recited in claim 1 of the accompanying claims.

In accordance with the present invention there is provided a method of biasing a sensor as recited in claim 8 of the accompanying claims.

Brief Description of the Drawings

Sensor arrangements and a method of biasing a sensor having supply contacts in accordance with the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: FIG. 1 is a schematic cross-section diagram of a pressure sensor; FIG. 2 is an enlarged schematic cross-section of the sensor die and lead frame of FIG. 1; FIG. 3 is a block schematic diagram of a sensor arrangement in accordance with the present invention;

FIG. 4 is a graphical representation of a supply signal for biasing a sensor in accordance with a first embodiment of the present invention;

FIG. 5 is a graphical representation of sensor lifetime versus biasing 'on- time'; FIG. 6 is a block schematic diagram of a first sensor arrangement in accordance with the first embodiment of the present invention;

FIG. 7 is a graphical representation of a control signal for controlling a supply switch of the first sensor arrangement of FIG. 6; FIG. 8 is a graphical representation of a signal for clocking part of the first sensor arrangement of FIG. 6;

FIG. 9 is a graphical representation of a supply signal for biasing a sensor in accordance with a second embodiment of the present invention;

FIG. 10 is a block schematic diagram of a second sensor arrangement in accordance with the second embodiment of the present invention;

FIG. 11 is a graphical representation of a control signal for controlling supply switches of the second sensor arrangement of FIG. 10;

FIG. 12 is a graphical representation of a signal for clocking part of the second sensor arrangement of FIG. 10; FIG. 13 is a block schematic diagram of an alternative sensor arrangement in accordance with the second embodiment of the present invention;

FIG. 14 is a block schematic diagram of a third sensor arrangement in accordance with a third embodiment of the present invention; FIG. 15 is a graphical representation of a supply signal for biasing a sensor in accordance with the third embodiment of the present invention;

FIG. 16 is a graphical representation of a control signal for controlling supply switches of the third sensor arrangement of FIG. 14;

FIG. 17 is a graphical representation of a signal for clocking part of the third sensor arrangement of FIG. 14.

Detailed Description of the Drawings

FIG. 1 is a schematic cross-sectional diagram of a pressure sensor 2, such as a MPX2000 series sensor supplied by Motorola, Inc., comprising a die 4 mounted on an epoxy polymer case 6. A gel 12, such as a silicone polymer, surrounds the die 4 to protect the die and a lead frame 8 and wire bond 10 connects the pressure sensor 2 to a supply Vcc. The sensor 2 transforms a difference in pressure between PI and P2 into a voltage signal by deformation of the diaphragm 5 and through a strain gauge transducer 14 (FIG. 2). FIG. 2 shows the lead frame 8 and die 4 in more detail. The die 4 comprises the transducer 14 and electronic circuitry (not shown) for calibration and compensation which are protected by a silicon nitride passivation layer 16. Typically, the passivation layer 16 has a thickness 20 of 0.4 microns. Wire bond 10 is bonded at one end to the lead frame 8, for example by ultrasonic bonding, and at another end to supply contacts 18 of the sensor 2, for example by ball bonding. The supply contacts provide electrical connections between the lead frame 8 and the die 4 and are typically aluminium contacts. In order to make bonding feasible, the supply contacts are not protected by the passivation layer 16 and typically have a thickness 22 of 1.1 microns. Gel 12 surrounds the described assembly.

Generally all materials are permeable to aqueous solutions and steam after a defined time. This time is in the range of minutes and hours for polymers and years for steal and glasses. Gels are particularly permeable due to the very low level of cross linking and the 'large¹ material net structure which allows the water molecules to go very easily through the gel coating. This means that water and other fluids and particularly steam can penetrate the gel 12 and reach the die 4.

When the sensor die 4 is in contact with an aqueous solution, oxydoreduction reactions between aluminium Al and aluminium ion Al³⁺ at the supply contacts 18 and some oxydoreduction couples of the water start as soon as the sensor is biased by the supply Vcc. After some working hours, the aluminium contact of the die 4 is eventually destroyed causing an open circuit on the wire bond 10. This is the aluminium electrocorrosion failure phenomena mentioned in the introduction. Once water has penetrated the gel 12 and reached the die 4, the failure phenomena may be quite fast, since the supply voltage is usually very large, 5VDC, compared to the oxydoreduction potentials of the aluminium (-1.66V for A1/A13+).

There are also some other failure causes such as galvanic corrosion which occurs between the different types of metals. But the mean-time-to-failure (MTTF) due to these other causes is much longer than the MTTF caused by electrocorrosion of the sensor supply contacts 18.

Although the solutions mentioned in the introduction address the electrocorrosion failure problem, they suffer from significant problems themselves. The present invention seeks to increase the sensor lifetime in aqueous media even at pressures of greater than 200KPa without requiring expensive packaging, complex gels and intricate process steps.

Referring now to FIG. 3, a sensor arrangement 30 in accordance with a preferred embodiment of the present invention comprises a sensor 32 having supply contacts 38 coupled to a supply signal generator 34. The supply signal generator 34 generates a supply signal comprising pulses such that the sensor 32 is biased periodically for predetermined periods. Preferably, an output of the sensor 32 is coupled to sample and hold circuitry 36 which is clocked by a clock signal CLK synchronised with the pulses of the supply signal such that the sample and hold circuitry 36 samples the sensor output signal at an output 40 of the sensor 32 when the sensor is biased only. The sample and hold circuitry 36 provides an output signal Sout representative of a parameter sensed by the sensor 32. As the sample and hold circuitry 36 is clocked by a clock signal CLK which is synchronised to the supply signal pulses, the output signal Sout is an analog signal and so the pulsed biasing of the sensor appears transparent to a user of the sensor arrangement 30. The sample and hold circuitry 36 may be omitted but in this case the sensor output signal at the output 40 would be a pulsed signal, with the magnitude of the pulses carrying the pressure value.

FIG. 4 shows an example of a pulsed supply signal 42 which may be used to bias the sensor 32 in accordance with a first embodiment of the present invention.

The predetermined periods for which the sensor 32 is biased is the on- time Ton. The periods for which the sensor is not biased is the off-time Toff. The period T of the pulsed supply signal 42 is T = Ton + Toff. The supply signal 42 is arranged to have a short duty cycle so that Ton is small.

Since electrocorrosion is stopped when a sensor is not biased, the invention, by utilising a pulsed supply signal to bias the sensor for a short periods of time during which measurements can be taken, reduces the electrocorrosion of the supply contacts 38. The value of T will depend on the application and on the response time of the sensor required by the user. If the parameter to be sensed by the sensor changes slowly, a fast response time is not needed so that T can be large. If the parameter to be sensed by the sensor changes quickly, T will have to be small. The on-time Ton must be long enough so that the sensor output signal can be measured but not long enough to start significant corrosion at the supply contacts 38. The inventors of the present invention found by experimental tests with different sensors and different on-times that instead of the lifetime of the sensor being determined by the cumulative time of the sensor when biased or the ratio Ton/T, the sensor lifetime versus on-time Ton appears like an avalanche phenomena as can be seen in FIG. 5. Since the tests are destructive tests, the curve 44 shown in FIG. 5 was obtained with different sensors so it is difficult to extrapolate a generic law of the sensor lifetime versus the bias time. However, the same trend was obtained with another set of sensors. There is clearly a sensor biasing time Ton (20 to 40 ms) under which the sensor lifetime is greatly increased. One explanation could be that when a biasing pulse is short, very few electrons may penetrate the supply contacts and oxidise them. Also, with aluminium contacts a thin layer of aluminium oxide over the aluminium contacts may slow down the corrosion but as soon as this layer is destroyed and the aluminium contacts directly exposed to electrocorrosion , the speed of reaction increases very quickly. Thus, provided that Ton is small in the order of 10 ms, a significant increase in the lifetime of a sensor can be obtained.

FIG. 6 shows a first sensor arrangement 46 in accordance with the first embodiment of the invention which is an implementation of the sensor arrangement 30 of FIG. 3 wherein the sensor 32 is a strain-gauge pressure sensor 47 such as the MPX2300D pressure sensor supplied by Motorola, Inc. and the supply signal comprises a plurality of positive pulses as shown in FIG. 4. The supply signal generator 34 comprises clock circuitry 48 which generates a clock signal CLK to clock the sample and hold circuitry 36 as shown in FIG. 7 and a control signal Control (as shown in FIG. 8) to control a voltage supply signal provided by a reference voltage supply Vcc via a switch 50. Switch 50 is coupled between a first supply contact 54 and a first reference voltage terminal (preferably ground) of the reference voltage supply. A second reference voltage terminal (Vcc) of the reference voltage supply is coupled to a second supply contact 56. When the control signal Control is high, the switch 50 is closed and the pressure sensor 47 is biased with the voltage supply signal Vcc coupled to the second supply contact 56. The control signal Control therefore ensures that a pulsed supply signal is applied to the pressure sensor 47.

An amplifier 52 is coupled to outputs Vs4- and Vs- of the pressure sensor 47. An output of the amplifier 52 is sampled by the sample and hold circuitry 36 to provide the analog output signal Sout. Preferably, the signals are arranged so that there is a delay between the rising edge of the control signal Control and the rising edge of the clock signal CLK in order to ensure that the sensor has settled before the output of the amplifier 52 is sampled (i.e. before a measurement is taken).

The period T of the control signal (which corresponds to the period of the supply signal) determines the response time of the pressure sensor 47. The ratio Ton/T determines the value and hence the size of the capacitor (not shown) of the sample and hold circuitry 36. Since pressure is a slow moving parameter compared to the speed of electronic operations, such as sampling, a control signal period T of 100ms and an on-time Ton of 10ms is a good compromise between the sensor lifetime, sensor response time (100ms) and the size of the sensor arrangement 46. For a period T of 100ms, a capacitor of a few nF is needed for the sample and hold circuitry 36. This means that the circuitrv 36 can therefore be mounted in a surface mount technology package and can thus provide a very small and integrated arrangement. In the first sensor arrangement 46, electrocorrosion only occurs at the supply contact coupled to the positive electrode (Vcc) of the supply signal generator 34 with the result that this supply contact corrodes faster than the other supply contact.

A second embodiment of the invention addresses this problem by generating a supply signal comprising alternate positive 58 and negative 60 pulses as shown in FIG. 9. By alternately changing the polarity of the supply signal, one of supply contacts will only be corroded when the sensor is positively biased and the other supply contact will only be corroded when the sensor is negatively biased. Referring now also to FIG. 10, a second sensor arrangement 62 in accordance with a second embodiment of the invention is similar to the first sensor arrangement 46 in accordance with the first embodiment and like components are referred to by the same reference numeral.

The strain-gauge structure of the pressure sensor 47 is symmetric so that the sensor output signal is the same whether the first supply contact 54 is coupled to ground or Vcc and the second supply contact 56 is coupled to Vcc or ground respectively provided that the output signal is sensed between the Vs+ and Vs- pins and the Vs- and Vs+ pins respectively.

Thus, the second sensor arrangement 62 comprises a first switch 64 for switching the first supply contact 54 between ground and Vcc of the reference voltage supply in response to a control signal Scont (signal 61 in FIG. 11) generated by the clock circuitry 48 and a second switch 66 for switching the second supply contact 56 between Vcc and ground of the reference voltage supply in response to the control signal Scont (signal 63 in FIG. 11) generated by the clock circuitry 48. The outputs Vs+ and Vs- of the pressure sensor 47 are coupled to a multiplexer 68 which is clocked by the same clock signal CLK which clocks the sample and hold circuitry 36. The outputs of the multiplexer 68 are coupled to the sample and hold circuitry 36 via the amplifier 52. The multiplexer 68 ensures that the input signal to the sample and hold circuitry 36 is always a positive analog voltage.

When positively biased (when the first supply contact 54 is coupled to ground and the second supply contact 56 is coupled to Vcc), the multiplexer couples the sensor output signal across the outputs Vs+ and Vs- to the sample and hold circuitry 36 via the amplifier 52. When negatively biased (when the first supply contact 54 is coupled to ground and the second supply contact 56 is coupled to Vcc), the multiplexer couples the sensor output signal across the outputs Vs- and Vs+ to the sample and hold circuitry 36 via the amplifier 52.

The second sensor arrangement 62 in accordance with the second embodiment of the invention therefore ensures that the second supply contact 56 will be corroded only when the sensor 47 is positively biased and the first supply contact 54 will be corroded only when the sensor 47 is negatively biased. The lifetime of the sensor 47 of the second sensor arrangement 62 (FIG. 10) should therefore be doubled compared to the lifetime of the sensor 47 of the first sensor arrangement 46 (FIG. 6). However, the architecture of the second sensor arrangement 62 in accordance with the second embodiment is more complex than the first sensor arrangement 46 in accordance with the first embodiment. The analog multiplexer 68 needs to have very low differential voltage drops and hence is expensive to implement. An alternative sensor arrangement 72 in accordance with the second embodiment, which arrangement avoids the need to use an expensive multiplexer 68, is shown in FIG. 13.

The sensor arrangement 72 is similar to the second sensor arrangement 62 except that the multiplexer 68 and amplifier 52 are replaced by first 74 and second 76 amplifiers. Like components are referred to by the same reference numeral. The non-inverting input of the first amplifier 74 is coupled to the output Vs+ of the sensor 47 and the inverting input of the first amplifier 74 is coupled to the output Vs- of the sensor 47. The non-inverting input of the second amplifier 76 is coupled to the output Vs- of the sensor 47 and the inverting input of the second amplifier 76 is coupled to the output Vs+ of the sensor 47. The outputs of the first 74 and second 76 amplifiers are coupled to the sample and hold circuitry 36. The first 74 and second 76 amplifiers are clocked by the clock signal CLK generated by the clock circuitry 48 and ensure that, like the multiplexer 68, the input signal to the sample and hold circuitry 36 is always a positive analog voltage.

The sensor arrangement 72 operates in a similar manner to that of the second sensor arrangement 62. When positively biased (when the first supply contact 54 is coupled to ground and the second supply contact 56 is coupled to Vcc), the first amplifier 74 is biased and couples the sensor output signal across the outputs Vs+ and Vs- to the sample and hold circuitry 36 and the second amplifier 76 is disabled. When negatively biased (when the first supply contact 54 is coupled to ground and the second supply contact 56 is coupled to Vcc), the second amplifier 76 is biased and couples the sensor output signal across the outputs Vs- and Vs+ to the sample and hold circuitry 36 and the first amplifier 74 is disabled. The sensor arrangement 72 in accordance with the second embodiment of the invention therefore ensures that the second supply contact 56 will be corroded only when the sensor 47 is positively biased and the first supply contact 54 will be corroded only when the sensor 47 is negatively biased. Referring now to FIG. 14, a third sensor arrangement in accordance with a third embodiment of the invention avoids the need for a multiplexer 68 and first 74 and second 76 amplifiers by generating a supply signal 78, as shown in FIG. 15, comprising a negative pulse after a positive pulse and by only measuring the sensor output signal during a positive pulse. Like components to those of the sensor arrangements 46, 62, 72 are referred to by the same reference numeral.

The supply signal 78 is generated using the first 64 and second 66 switches and in response to the control signal Scont (signals 80 and 82 respectively of FIG. 16) generated by the clock circuitry 48. The clock signal CLK (see FIG. 17) generated by the clock circuitry 48 ensures that the sample and hold circuitry 36 only samples the sensor output signal during a positive pulse. FIG. 15 shows a negative pulse immediately following a positive pulse. The principle of the third embodiment may equally be applied to a supply signal as shown in FIG. 9 but wherein the sensor output signal is only measured during the positive pulses of the supply signal. In addition, the sensor arrangement in accordance with the present invention may be arranged such that a negative pulse occurs at any time between two positive pulses. The method of biasing the sensor in accordance with the third embodiment (FIG. 14) was found by experiment to increase the lifetime of the sensor 47 compared to the method in accordance with the first embodiment (FIG. 3). One explanation could be that the chemical agents coming from the aqueous solution and causing electrocorrosion do not penetrate the aluminium pad but are very close to the surface since the positive pulse is very short and these agents are then removed from the aluminium pad during the negative pulse thereby radically stopping any electrocorrosion. In summary, the present invention generates and applies a supply signal comprising pulses so that the sensor is only biased for a predetermined period during which the sensor output signal can be measured. Since the sensor is not biased all the time, the electrocorrosion of the supply contacts is considerably reduced thereby increasing the lifetime of the sensor. The present invention utilises simple circuitry to generate the supply signal and so is much simpler and cheaper to implement compared to the solutions of the known arrangements described above.

For pressure sensors, the sensor arrangement in accordance with the present invention can be used at any pressures. The sensor and additional circuitry may be implemented on one die or as a module.

By utilising clocked sample and hold circuitry, the pulsed biasing method of the present invention can be made transparent to the user of the sensor arrangement. The principle of the present invention applies to any sensor having supply contacts which are exposed to aqueous media. For example, pressure sensors which are to be used in pressure cookers, chemical sensors which are to be used to detect certain chemicals in a solution, temperature sensors and humidity sensors. In the field of chemical sensors, which sensors comprise a sensitive layer and a heater for heating the sensitive layer, the principle of applying a pulsed supply signal to the sensitive layer in accordance with the invention could also be utilised but for another purpose: to limit the drift of the sensitive layer which may occur when the sensitive layer is under voltage. By biasing the sensitive layer only when measurements are run or by biasing the sensitive layer with alternate positive and negative pulses, a reduction in the drift of the resistance of the sensitive layer should be obtained.

Claims

1. A sensor arrangement comprising: a sensor having supply contacts; and a supply signal generator for generating a pulsed supply signal comprising positive and negative pulses and for applying the pulsed supply signal to the supply contacts such that in use the sensor is biased periodically for a predetermined period and a sensor output signal of the sensor is measured during at least some of the predetermined periods.

2. A sensor arrangement according to claim 1 further comprising sample and hold circuitry coupled to an output of the sensor for receiving the sensor output signal and having a clock input for receiving a clock signal generated by the supply signal generator which signal is synchronised with the pulsed supply signal, the sample and hold circuitry in response to the clock signal for sampling the sensor output signal during at least some of the predetermined periods and for providing a sampled output signal representative of a parameter sensed by the sensor.

3. A sensor arrangement according to claim 2 wherein the supply signal generator comprises: clock circuitry for generating the clock signal and a control signal synchronised to the clock signal; and a reference voltage supply coupled to the supply contacts of the sensor for periodically supplying the supply signal to the supply contacts in response to the control signal.

4. A sensor arrangement according to claim 3 wherein the supply signal generator further comprises: a first switch coupled to a first supply contact for coupling the first supply contact to first or second reference voltage terminals of the reference voltage supply in response to the control signal; and a second switch coupled to a second supply contact for coupling the second supply contact to the second or first reference voltage terminals respectively of the reference voltage supply in response to the control signal such that the supply signal comprises a plurality of positive and negative pulses, each of the positive and negative pulses lasting the predetermined period and having the same amplitude.

5. A sensor arrangement according to claim 4 wherein the clock signal is synchronised with the positive pulses only such that the sensor output signal is measured during the positive pulses only.

6. A sensor arrangement according to claim 4 wherein the clock signal is synchronised with the positive and negative pulses such that the sensor output signal is measured during the positive and negative pulses and the sensor arrangement further comprises a multiplexer coupled between the sensor and the sample and hold circuitry, the multiplexer being clocked by the clock signal.

7. A sensor arrangement according to any preceding claim wherein the supply signal generator is part of a MCU.

8. A method of biasing a sensor having supply contacts, the method comprising the steps of: generating a pulsed supply signal comprising positive and negative pulses; applying the pulsed supply signal to the supply contacts so that the sensor is biased periodically for a predetermined period; and measuring a sensor output signal during at least some of the predetermined periods.

9. A method of biasing a sensor according to claim 8 wherein the measuring step comprises the steps of: generating a clock signal synchronised to the pulsed supply signal; sampling the sensor output signal in response to the clock signal during at least some of the predetermined periods; and providing a sampled output signal representative of a parameter sensed by the sensor.

10. A sensor arrangement according to claim 1, 2, 3, 4, 5, 6 or 7 or a method of biasing a sensor according to claim 8 or 9 wherein the pulsed supply signal has a short duty cycle and wherein each of the positive and negative pulses lasts the predetermined period and has the same amplitude.

11. A sensor arrangement according to claim 10 or a method of biasing a sensor according to claim 10 wherein the pulsed supply comprises a plurality of alternate positive and negative pulses.

12. A sensor arrangement according to claim 11 or or a method of biasing a sensor according to claim 11 wherein a negative pulse follows right after a positive pulse.

13. A sensor arrangement according to claim 10, 11 or 12 or a method of biasing a sensor according to claim 10, 11 or 12 wherein the clock signal is synchronised with the positive pulses only, such that the sensor output signal is measured during the positive pulses only.

14. A sensor arrangement according to claim 10, 11 or 12 or a method of biasing a sensor according to claim 10, 11 or 12 wherein the clock signal is synchronised with the positive and negative pulses such that the sensor output signal is measured during the positive and negative pulses.

15. A sensor arrangement according to any preceding claim or a method of biasing a sensor according to any preceding claim wherein the predetermined period is less than a period after which substantial corrosion of supply contacts starts.

16. A sensor arrangement according to any preceding claim or a method of biasing a sensor according to any preceding claim wherein the sensor comprises a chemical sensor having a sensitive layer for sensing predetermined chemicals, the supply contacts being coupled to the sensitive layer for supplying the supply signal thereto, wherein in use, a signal across the sensitive layer provides the sensor output signal of the sensor.