GB2318231A - Micromechanical ring oscillator sensor - Google Patents

Abstract

A ring oscillator comprises CMOS inverters 11-15 linked by sensing resistors 40-44 so that the frequency of oscillation varies in dependence on the stimulus applied to the resistors. The resistors may be polycrystalline piezoresistive elements located on a cantilever 220 to sense acceleration. Alternatively they may sense pressure or strain or gas. The inverters 11 - 15 are supported on the substrate so that they are not subject to the applied stimulus, and there may be a matched reference ring oscillator in which the resistors are not subject to the stimulus. One of the interconnecting resistors may have a greater resistance than that of the others so that a change in its resistance dominates the output frequency. Fingerprint sensing is disclosed.

Description

A SENSOR This invention relates to a sensor for sensing a stimulus, particularly but not exclusively acceleration. In particular the invention relates to a sensor which comprises a ring oscillator circuit, the output frequency of which changes in dependence on the stimulus.

There are benefits in cost and reliability when a sensor and its associated electronics are integrated on the same semiconductor substrate. Furthermore a sensor which produces a frequency output is less susceptible to electrical interference than one which produces an analogue output. Ring oscillator circuits have been used to implement strain and pressure sensors, which generate a frequency output which alters with detection of the sensed stimulus.

A known ring oscillator is illustrated in Figure 1. Figure 1 illustrates a ring oscillator 1 comprising five inverter stages 10-14, having respectively inputs 20-24 and outputs 30-34. The inverter stages are connected in series by interconnects 41-44 with interconnect 40 feeding the output of the last inverter stage 14 in the series to the input of the first inverter stage 10 of the series, to form a ring. The ring oscillator 1 receives an input signal at input 50 which is passed to input 20 of the first inverter stage in the series, and produces a selfsustaining output signal from the output 34 of the last inverter stage 14 in the series, which is presented at the output 60. The ring oscillator may have any odd number of inverters, greater than or equal to three, in its ring.

In a first type of prior art sensor, utilising a ring oscillator of the above type, the frequency of the output signal depends upon the number of inverter stages in the ring and the average time delay introduced by each inverter stage. In Sensors and Actuators 4, (1983) 77-83, an odd number of 12Th inverters are connected to form a ring oscillator. Each I2L inverter comprises a pnp lateral transistor as a current source and an npn transistor at the output. The I2L inverters are mounted on a micromachined part and the straining of the I2L inverters themselves causes a change in the injection current of the I2L inverters and thus the output frequency of the ring oscillator.

In Sensors and Actuators 7 (1985) 167-176, a nine stage MOS ring oscillator is built on a movable membrane forming part of a silicon wafer. Movement of the membrane stresses the transistors within each stage, thus altering their channel mobility. The consequent change in the drain current of the transistors affects the time delay associated with each transistor, and thus the output frequency.

A second type of prior art sensor is described in EP-B-0455070.

This sensor utilises a ring oscillator to sense pressure changes.

A capacitor plate is connected to each of the interconnects joining the inverters in series. Each of these capacitor plates forms a capacitor with a movable micromachined part acting as the other capacitor plate. The movement of the micromachined part alters the time constant of the ring oscillator and hence its output frequency.

One problem associated with the first type of prior art sensors, is that because the stimulus to be sensed directly affects the inverters, the mechanism of the inverters is complex and depends upon the details of their construction.

One problem associated with the second type of prior art sensor, is that the formation of the micromachined capacitor plate demands high tolerances and is costly and complex.

It is an object of the present invention to provide a simple, cost-effective sensor, which can be formed by simple process techniques.

According to one aspect of the present invention there is provided a sensor for responding to a stimulus comprising a ring oscillator having a plurality of inverter stages connected in a ring via a set of interconnects, each interconnect connecting the output of one inverter stage with an input of a next inverter stage, wherein at least part of at least one said interconnects is spatially separated from circuitry constituting each inverter stage in such an arrangement that the stimulus is applied only to said part and not to the inverter stages, wherein the resistance of said part alters in response to the stimulus thereby changing the frequency of oscillation of the ring oscillator.

In the preferred embodiment, each inverter stage introduces substantially the same delay, and at least part of each interconnect is spatially separated to allow response to said stimulus.

Thus, the present invention allows in a simple yet effective manner for the change in frequency of oscillation of the ring oscillator to be dependent on the resistance of the interconnects which are subject to stimulus. The remaining components of the ring oscillator are not affected by the stimulus, and therefore it is reasonable to suppose that the change in frequency is substantially linearly related to the change in resistance incurred by the response to the stimulus.

A sensor according to the present invention can be used as an accelerometer where the interconnects extend from a supported portion of a semiconductor substrate to a cantilevered portion thereof. The remaining components of the ring oscillator are constructed on the supported portion. Thus, when the silicon substrate suffers an acceleration, the cantilevered portion reflects this and the resistance of the interconnects changes as a result of induced stresses. This changes the gate delay of the signal travelling around the ring oscillator because the time constant of the device is dominated by the product of the input capacitance of each stage and the resistance of the interconnect, which now varies with the acceleration.

Where the change in gate delay is similar for all inverter stages of the ring oscillator, the result is a change in ring oscillation output frequency which is proportional to acceleration. To compensate for possible changes in the interconnects due to temperature changes, a similar, "control", ring oscillator can be provided, having a cantilevered portion which is however supported at both ends and unable to flex significantly. Thus, apart from movement of the cantilevered portion the sensor ring oscillator and the control ring oscillator would be substantially matched. Therefore, the difference between the output frequencies of the two ring oscillators could be used as an indication of acceleration.

As an alternative, a ring oscillator with resistive interconnects formed wholly on a supported portion of the silicon substrate could be provided as a reference.

The sensor can also be used for other applications, for example to sense a pressure differential which would cause movement of the cantilever. Still further, it would be possible to use a sensor in accordance with the present invention as a gas sensor or the like, where the components of the ring oscillator with the exception of the spatially separated parts of the interconnects were shielded from the gas, so that only those parts were subject to it. Thus, once again the change in resistance of the interconnects would be indicative of the applied stimulus.

According to another aspect of the present invention there is provided a sensor for responding to a stimulus comprising a ring oscillator having a plurality of inverter stages connected in a ring via a set of interconnects, each interconnect connecting the output of one inverter stage with an input of a next inverter stage, wherein at least one of said interconnects has a resistance which is substantially greater than the operating resistance of the preceding stage, the resistance of said at least one interconnect altering in response to the stimulus by such an amount that it dominates the resulting change in frequency of oscillation of the ring oscillator.

Here, it is ensured that the effect of the resistance of the interconnects dominates the resulting frequency change by virtue of the much larger size of these resistances.

For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which: Figure 1 is a circuit diagram of a known ring oscillator; Figure 2a is a circuit diagram of a ring oscillator in accordance with one embodiment of the invention; Figure 2b is a schematic circuit diagram of an inverter stage of the ring oscillator of Figure 2a; Figure 2c is an amplified circuit diagram of a ring oscillator in accordance with one embodiment of the present invention; Figure 3 is a transistor level diagram of the inverter stage of Figure 2b; Figure 4 is a perspective view of part of a silicon wafer showing an implementation of a sensor; Figure 5 is a plan circuit diagram of the sensor of Figure 4; Figure 6 is a diagram of a chip carrying four sensors; and Figures 7a to 7d illustrate steps in the sequence of manufacture of the structure of Figure 4.

Figures 2a, 2b and 2c illustrate the concept underlying the sensors of the present invention. Figure 2a illustrates a sensor 2 comprising a ring oscillator circuit 1. The ring oscillator circuit 1 is similar to the ring oscillator circuit illustrated in Figure 1 and similar reference numerals denote similar parts.

The circuit of Figure 2, however, differs from that in Figure 1 in that in the circuit of Figure 1, the interconnects 40-44 having a very low resistance, and in fact are designed to be as conductive as possible to minimise the effect of unwanted resistance between inverter stages. In contrast, in the present invention the interconnects 70-74 provide substantial resistances of respectively R70,R71,R72,R73 and R,4 ohms. It is not essential for the resistances R70-R,4 to be of the same value, but as will become apparent, it may be advantageous. In any event, typical values for these resistances could be of the order of 102 or 103Q.

Reference is now made to Figure 2b which illustrates a schematic diagram of the inverter stage 10 of Figure 2a. Reference numeral 20 indicates the input to the inverter stage and reference numeral 30 indicates the output of the inverter stage. Reference numeral 90 denotes the effective on-state resistance of inverter 10. Reference numeral 80 denotes the effective input capacitance of the inverter stage 10. Reference numeral 10' indicates the inverting circuitry which effects a phase change or delay between the input 20 and output 30.

One possible type of inverter stage 10-14 for use in the sensor 2, is illustrated in Figure 3. This inverter stage 10 is a CMOS inverter. Reference numeral 100 indicates a p-channel FET transistor. Reference numeral 110 indicates a n-channel FET transistor. Reference numerals 20 and 30 respectively indicate the input and output of the inverter. The source of the pchannel transistor 100 is connected to a supply voltage rail Vcc; the drain of transistor 100 is connected to the output 30. The source of the n-channel transistor 110 is connected to earth and the drain of the transistor 110 is connected to the output 30.

The input 20 is connected to the gate of transistor 100 and the gate of transistor 110. Comparing Figures 3 and 2b, the input capacitance of inverter 10 represents the gate capacitance of the CMOS inverter.

Reference is now made to Figure 2c which illustrates the circuit of Figure 2a with the input capacitances and the on-state resistances of each inverter stage also illustrated. Reference numerals 90,91,92,93 and 94 respectively indicate the on-state resistances of inverter stages 10,11,12,13 and 14, having respectively resistance values of R90, R91, R92, R93 and R94 ohms.

Reference numerals 80,81,81, 83 and 84 respectively indicate the input capacitance of inverter stages 10,11,12,13 and 14, having respectively capacitance values of C80,C8l,C82,C83 and C84 farads.

The remaining reference numerals indicate the same parts as described in relation to Figure 2a.

As is well known, the phase change introduced to a voltage signal of frequency f by an RC circuit comprising a resistor of value R ohms and a capacitance of value C farads, is tan-l (2r fCR).

The ring oscillator 1 of Figure 2c will therefore oscillate at a frequency dependent on the RC value of the inverter stages.

According to the concept underlying the present invention, the variation in output frequency in response to a stimulus is made substantially dependent only on changes in the values of resistors 70-74. This is done in one embodiment by making the resistors 71-74 in such a way that they are affected by a stimulus while the remaining components of the inverter stages are not. For example, a strain, pressure or acceleration sensor is made by forming at least part of the resistors 70-74 from a piezo-resistive material and running at least some of the interconnects comprising this material over a micromachined flexible member so that they are stressed in response to the stimulus. Suitable piezo-resistive materials include metal or metallic alloys, single-crystal semiconductors and polycrystalline semiconductor films. The deflection of the micromachined members may be caused by the direct application of the stimulus to the member for example the pressure of a fluid or some other applied force. The deflection of the member may also be caused indirectly, for example, by accelerating the body supporting the micromachined member so that the inertia of the member causes it to deflect. Such a sensor is illustrated in Figure 4.

Figure 4 illustrates a micromachined sensor. A substrate 200 has a first portion which has been undercut to form a micromachined cantilever 220 and a supported portion 240 which has not been undercut. The cantilever 220 is joined to the supported portion 240 of the substrate along the line 230 and separated from the remaining portions of the substrate by the cavity 210. The cantilever is resiliently flexible and can flex in the direction indicated by arrows A. The cantilever 220 and the supported portion 240 define a surface on which or in which a sensor can be defined. The circuit diagram for the sensor of Figure 4 is shown schematically in Figure 5.

The substrate 200 is formed from crystalline silicon and the elements of the ring oscillator are formed as an integrated circuit in or on the substrate. The inverter stages 10-14 are formed on or in the supported substrate 240, and are physically separated from the cantilever 220. The inverter stages are the same. The interconnects 40-44 are formed on or in the substrate 200 and extend along the cantilever. At least those portions of the interconnects extending along the cantilever 220 beyond the line 230 are formed from a piezo-resistive material. The resistance of each of the interconnects 40-44 is substantially the same, and denoted by R1. The resistance Rl is greater than the on-state resistance of an inverter stage, and it is preferably one, two, three or more orders of magnitude greater, so that the on-state resistance of the previous inverter is negligible in comparison with the interconnect resistance. In the illustrated embodiment the on-state resistance is about 1KQ and the interconnect resistance is more than 100KQ. The input capacitance is about lpF.

In a prototype of the preferred embodiment a ring oscillator using 17 inverter stages was used. It is preferred to form the inverters as CMOS inverters from standard semiconductor processing techniques and to form the interconnects 40-44 from polycrystalline silicon, for example by etching a deposited film.

The interconnects could of course be formed from any piezoresistive material such as evaporated metal or metal alloys. It should be appreciated from the foregoing description of the circuits of Figures 2a, 2b and 2c that the change in frequency of the output signal at node 60 will be proportional to the deflection of the cantilever for small deflections. It should also be appreciated'that the change in frequency of the signal at the output of the sensor 60 will be proportional to the magnitude of the acceleration experienced by the substrate 200 in the direction of arrow B.

The sensor will therefore have application as an accelerometer, or a tactile sensor. It could also be used to detect fingerprints, because the ridges of a finger or thumb would cause movement of the cantilever while the trough would not. A problem may arise with sensor 2 due to temperature fluctuations. If the interconnects 40-41, with resistances R70-R74, are made of a material which has a resistance dependent upon temperature, a variation in temperature will effect a change in the resistance of the interconnects 40-41 and hence the output frequency.

Variations in temperature therefore introduce noise into the output of the sensor. To overcome this problem it is possible to provide a "control" ring oscillator with the same response characteristics as the ring oscillator in sensor 2. The "control" ring oscillator and the sensor 2 are both exposed to the same ambient conditions save that the "control" is shielded from the stimulus to be sensed. The output of the "control" ring oscillator and the output of the sensor are then compared to determine the contribution of a stimulus to the output of the sensor 2. In particular the difference in the output frequencies can be determined.

The "control" ring oscillator may comprise the same material and components as the sensor 2, with the interconnects 40-44 being isolated from the stimulus but not from temperature changes.

Alternatively the interconnects 40-44 of the control ring oscillator may be formed of different material which is unresponsive to the stimulus but has similar thermal properties.

It is not essential for the control ring oscillator to be on the same chip as the sensor.

Another way the problems of temperature fluctuations may be addressed is by maintaining the sensor at an elevated temperature (e.g. 60"C) so that variations in the ambient temperature do not affect it.

A chip with a number of micromachined cantilevers 220 is illustrated in Figure 6. Reference numerals 300,310,320 and 330 respectively indicate identical sensors manufactured in accordance with the present invention. The sensor 300 has elements which extend along the support 240, which bridges the cavity 210. This sensor can act as a control sensor for the sensors 310 and 320. The sensor 330 is formed completely on the substrate 240. This sensor may also act as a control sensor for sensors 310 and 320. The control sensors may be used to reduce the effect of temperature variations on the frequency outputs of sensors 310 and 320 as hereinbefore described. For example the output of sensor 300 may be compared with the output of sensor 310, to determine the bending of, or the extent of bending the cantilever 220 of sensor 310. the sensor 310 extends along a long cantilever 220. Sensor 320 extends along a shorter cantilever. The shorter cantilever will be stiffer than the longer cantilever and will respond differently to applied forces or accelerations.

Reference will now be made to Figures 7a to 7d to describe a method of manufacture of a micromachined cantilever in accordance with one embodiment of the invention. In Figure 7a reference numeral 400 denotes a single composite wafer comprising a silicon substrate 402, a layer of silicon dioxide 104 on top of the substrate and a top layer of silicon 406. The thickness of the substrate in the described example is 500m < 100 > Si, the thickness of the oxide layer is 7ym. Composite wafers having this structure are commercially available. The top layer 106 could be single crystal silicon or polysilicon. Single crystal silicon can be produced by depositing silicon and recrystallising it after deposition or by bonding two oxidized wafers and thinning one side. A layer 408 of silicon nitride is placed on top of the silicon layer 406. Photoresist 410 is used to define a pattern to be etched from the silicon, for example that shown in plan view in Figure 5 or Figure 6. The silicon layer 408 is etched using the photoresist 410 as a mask to give the structure shown in Figure 7b. The silicon nitride layer is then used as a mask for a subsequent etch through the top silicon layer 406, which will define the shape of the cantilever and hence its dynamic properties. In section the structure is as shown in Figure 7c. A wet etch using buffered hydrofluoric acid is then used to remove the oxide layer 404 from beneath the top silicon layer 406, to provide silicon components supported in a cantilevered fashion and spaced from the substrate by an undercut region 403. This is shown in section in Figure 4d. A cantilever 220 is thereby defined, suitable for receiving components of a sensor as illustrated in Figure 4.

Inverters, preferably CMOS inverters, can then be formed in the region 240 of layer 406 by standard techniques. Interconnects can then be formed on or in the silicon layer 406, by standard techniques. The interconnects may be formed from a thin film of polysilicon or a sputter deposited layer of metal or metal alloy.

It is desirable although not essential for all the interconnects to extend along the cantilever 220. It is desirable although not essential for the interconnects to have the same resistance. It will be appreciated that if polycrystalline silicon is used to define the interconnects, differential doping may be used to optimise the resistances of the interconnects. It should also be appreciated that although a particular shape of cantilever has been illustrated, the shape may be varied to control the dynamic response of the cantilever.

Claims (11)

CLAIMS:

1. A sensor for responding to a stimulus comprising a ring oscillator having a plurality of inverter stages connected in a ring via a set of interconnects, each interconnect connecting the output of one inverter stage with an input of a next inverter stage, wherein at least part of at least one of said interconnects is spatially separated from circuitry constituting each inverter stage in such an arrangement that the stimulus is applied only to said part and not to the inverter stages, wherein the resistance of said part alters in response to the stimulus thereby changing the frequency of oscillation of the ring oscillator.

2. A sensor according to claim 1 wherein each inverter stage introduces substantially the same delay, and wherein at least part of each interconnect is spatially separated to allow response to said stimulus.

3. A sensor according to claim 1 or 2 wherein said at least one of said interconnects has a resistance which is substantially greater than the operating resistance of the preceding inverter stage, the resistance of said at least one interconnect altering in response to the stimulus by such an amount that it dominates a resulting change in frequency of oscillation of the ring oscillator.

4. A sensor according to claim 3 wherein the resistance of said at least one interconnect is at least two orders of magnitude greater than the operating resistance of the respective inverter stage.

5. A sensor according to any preceding claim manufactured as an integrated circuit on a silicon substrate comprising a first, fixed portion and a second, movable portion, wherein said at least part of said at least one interconnect is located on the movable portion, the remaining components of the ring oscillator being located on the fixed portion.

6. A sensor according to claim 5 wherein the part of the or each interconnect on the movable portion comprises piezoresistive material.

7. A sensor according to any preceding claim wherein each inverter stage comprises a CMOS inverter circuit.

8. A sensor according to any preceding claim which comprises a control ring oscillator which is arranged so that none of its components can be affected by the stimulus, the output of the control oscillator being compared with the output of the sensor ring oscillator for detecting changes.

9. A sensor according to any of claims 1 to 5 when used as a gas sensor, comprising means for shielding the components of the ring oscillator with the exception of said at least part of the or each interconnect from the stimulus.

10. A sensor substantially as hereinbefore described with reference to or as shown in Figures 2c and 4 to 6.

11. A sensor for responding to a stimulus comprising a ring oscillator having a plurality of inverter stages connected in a ring via a set of interconnects, each interconnect connecting the output of one inverter stage with an input of a next inverter stage, wherein at least one of said interconnects has a resistance which is substantially greater than the operating resistance of the preceding stage, the resistance of said at least one interconnect altering in response to the stimulus by such an amount that it dominates the resulting change in frequency of oscillation of the ring oscillator.