Pressure sensing element for a pressure sensing device, method of operation thereof and pressure sensor device

Abstract

A pressure sensing element (100) for a pressure sensing device, the pressure sensing element (100) comprising a substrate (102), a hollow (104) formed in the substrate (102), and a thermally deformable membrane (106) disposed over an opening of the hollow (104) in the substrate (102), the thermally deformable membrane (106) defining a thermally deformable membrane (200) over the hollow (104) and comprising one or more heating element (216, 218, 220). The thermally deformable membrane (200) is responsive to a change in temperature of the one or more heating element (216, 218, 220) and oscillatably deformable between a first deformation position distal from an undeformed neutral position in a first direction and a second deformation position distal from the undeformed neutral position in a second direction opposite to the first direction. The thermally deformable membrane (200) also comprises a plurality of strain-sensing piezoresistors (206, 208, 210, 212).

Description

PRESSURE SENSING ELEMENT FOR A PRESSURE SENSING DEVICE, METHOD OF OPERATION THEREOF AND PRESSURE SENSOR DEVICE

[0001] The present invention relates to a pressure sensing element for a pressure sensor device, the pressure sensing element being of the type that, for example, employs resonance in combination with a membrane to sense pressure in a medium. The present invention also relates to a method of operating a pressure sensing element for a pressure sensor device, the method being of the type that, for example, comprises oscillating a membrane of a pressure sensing element. The present invention further relates to a pressure sensor device of the type that, for example, comprises a pressure sensing element having an oscillatable membrane.

[0002] In the field of electronic sensors, the use of silicon to form sensors that provide a signal that is a measure of surface strain has been very successful: silicon exhibits elasticity whilst possessing high degrees of stiffness when in monocrystalline form. As such, reliable membranes for sensing pressure can be formed from silicon as part of a sensor device structure.

[0003] Silicon also offers a very high piezoresistive response to mechanical strain, a feature that is generally exploited by using p-type doped resistors at a silicon surface of mechanical structures where the silicon surface strain changes with a quantity to be measured, for example pressure, and the silicon surface strain is proportional to the quantity to be measured. A well-known example is the silicon pressure sensor with a silicon membrane disposed over a cavity with p-type doped silicon piezoresistors formed on the surface of the membrane at the edges of the cavity. Such structures provide a very stable and reliable way of sensing a strain that is caused by a pressure or force.

[0004] Since silicon does not exhibit any ductility below 600°C, silicon resonators are known to outperform quartz resonators and so this capability can be exploited to form silicon sensors whose principle of operation employs resonance. Such sensors contain a flexible part that can resonate, an actuator to make the flexible part resonate and a sensing part to measure the amplitude of the deflection so that the output of the sensing part can be used as part of a feedback loop to maintainthe flexible part of the sensor in resonance. Known examples of this type of sensor are electrostatic accelerometers and gyroscopes, for example a toothcomb micromachined gyroscope, which have a mass suspended by flexible silicon beams and peripheral capacitive structures that are used for sensing the position and actuating the mass at the same time. The capacitive structures are generally defined by gaps that are etched through a silicon layer with a thickness of between about 3 to 50 pm and the effective electrodes of the capacitive structures are in fact the walls that are created by this etching. The entire resonator consisting of the suspended mass, the beams and the capacitive structures are all formed from monocrystalline or polycrystalline silicon. However, these capacitive structures must be protected from the environment by a cap that hermetically seals the capacitive gaps, because the environmental atmosphere negatively influences the system. Consequently, such resonating structures cannot be exposed to a medium to be monitored.

[0005] “Thermally actuated silicon tuning fork resonators for sensing applications in air” (L.A. Beardslee, et al., MEMS 2012, Paris, FRANCE, 29 January - 2 February 2012) describes a MEMS tuning fork device comprising four measurement resistors arranged as a so-called “Wheatstone bridge” and two thermal resistors to actuate the device. The device is exposed to air and fingers of the tuning fork can move in antiphase by activating the thermal resistors. A feedback loop is provided, the thermal resistors serving as actuators and the Whetstone bridge being used to measure an oscillation frequency of the fingers. Resonance is obtained when the thermal resistors are powered when the fingers of the tuning fork are farthest apart and consequently exert a compressive force on the thermal resistors. It is therefore difficult to distinguish between strain and temperature signals using the measurement. Moreover, the device is prone to crosstalk owing to heat pulses generated by the thermal resistors being detected by the Whetstone bridge independent of strain signals.

[0006] “Novel resonant pressure sensor based on piezoresistive detection and symmetrical in-plane mode vibration” (Han et al. Microsystems & Nanoengineering (2020) 6:95 Microsystems & Nanoengineering) describes an electrostatic resonator for a pressure sensor. An all-silicon resonator, sealed with a glass cap,is formed beneath a membrane. When pressure is applied from the back of the device, stress is created on the structure of the resonator via a silicon island and an anchor mechanically coupled to the membrane. The amplitude of the resonance is measured by piezoresistors on the resonator in combination with reference resistors to form a Wheatstone bridge, which measures oscillation frequency. The structure described is somewhat structurally complex and a mechanical connection through the silicon island and anchor structure transfers force caused by pressure to the resonator. Differential pressure cannot be measured as a vacuum is required for the resonator, which also serves as a reference pressure. Hence, the device structure is limited to absolute pressure measurements and so is not particularly flexible.

[0007] “All-Quartz High Accuracy MEMS Pressure Sensor Based on Double- Ended Tuning Fork Resonator” (Wang et al., Eurosensors 2015, Procedia Engineering 120 (2015) 857 - 860) relates to a piezoelectric resonator for pressure sensing. The structure described is formed from quartz as opposed to silicon: three quartz wafers are used to form, inter alia, a membrane and a resonator. Actuation of the resonator is achieved by metal contacts patterned on both sides of the quartz resonator. Pressure is transferred from the membrane by two butts to a double- ended tuning fork structure formed in a separate quartz layer. The membrane is connected to the quartz resonator by glass frit, which is less mechanically stable than quartz or silicon.

[0008] The metal electrodes on the quartz resonator exert a level of mechanical stress on the quartz resonator that influences the oscillatory behaviour of the quartz resonator. This stress is susceptible to change over time owing to creep and plastic deformation of the metal from which the electrodes are formed. Furthermore, the mechanical properties of quartz are inferior to silicon and forming structures from quartz is also known to be more challenging than forming structures from silicon. However, the authors favoured quartz as a material from which to form the resonator, because silicon does not exhibit piezoelectric characteristics. As in the case of the double-ended tuning fork resonator structure mentioned above, differential pressure cannot be measured, because a vacuum that serves as areference pressure is required for the resonator. Consequently, this device structure is also not particularly flexible.

[0009] According to a first aspect of the present invention, there is provided a pressure sensing element for a pressure sensor device comprising: a substrate; a hollow formed in the substrate; and a thermally deformable membrane disposed over an opening of the hollow in the substrate, the thermally deformable membrane comprising one or more heating element; wherein the thermally deformable membrane is responsive to a change in temperature of the one or more heating element and oscillatably deformable between a first deformation position distal from an undeformed neutral position in a first direction and a second deformation position distal from the undeformed neutral position in a second direction opposite to the first direction; and the thermally deformable membrane also comprises a plurality of strain-sensing piezoresistors.

[0010] The one or more heating element may be respectively dimensioned to support oscillatory motion of the thermally deformable membrane at a predetermined frequency of at least 300KHz. The one or more heating element may occupy 5% or less of a surface area of the thermally deformable membrane. The one or more heating element may occupy 0.5% or less of a surface area of the thermally deformable membrane. The one or more heating element may occupy 0.2% or less of a surface area of the thermally deformable membrane. The one or more heating element may occupy 0.1 % or less of a surface area of the thermally deformable membrane.

[0011] The oscillation, when in use, of the thermally deformable membrane may be in a fundamental resonant mode of the thermally deformable membrane.

[0012] The oscillation, when in use, of the thermally deformable membrane may be in a higher resonant mode of the thermally deformable membrane than the fundamental resonant mode of the thermally deformable membrane.

[0013] The plurality of strain-sensing piezoresistors may be configured to sense, when in use, strain applied to the thermally deformable membrane.

[0014] The first deformation position may correspond to a first maximum extent of deformation in the first direction.

[0015] The second deformation position may correspond to a second maximum extent of deformation in the second direction.

[0016] The substrate may comprise a common electrical connection node; the one or more heating elements may be operably coupled to the common electrical connection node.

[0017] The one or more heating elements may be disposed on the thermally deformable membrane and over the hollow.

[0018] The one or more heating elements may be disposed at least partially in the thermally deformable membrane and over the hollow.

[0019] A first heating element of the one or more heating elements may be disposed over the centre of the hollow.

[0020] A number of the one or more heating elements may be disposed over the hollow at a periphery thereof.

[0021] The thermally deformable membrane may further comprise a temperature sensing element configured to measure a temperature of the thermally deformable membrane.

[0022] The one or more heating elements may be one or more resistors and / or diodes.

[0023] The thermally deformable membrane may be a layer of monocrystalline silicon; the plurality of strain-sensing piezoresistors may be diffused into the thermally deformable membrane; and the one or more heating elements may be formed substantially simultaneously with the plurality of strain-sensing piezoresistors in a lattice of the thermally deformable membrane.

[0024] The temperature sensing diode may be formed substantially simultaneously with the plurality of strain-sensing piezoresistors in a lattice of the thermally deformable membrane.

[0025] Each of the plurality of strain-sensing piezoresistors may be formed at least partially inside the thermally deformable membrane over the hollow; a heating element of the one or more heating elements may be located at a periphery of the membrane; and the heating element may be located at an opposite side of the membrane with respect to a piezoresistor of the plurality of piezoresistors.

[0026] According to a second aspect of the present invention, there is provided a pressure sensor device comprising: a pressure sensing element as set forth above in relation to the first aspect of the invention; a readout circuit comprising: a strain sensing circuit operably coupled to the plurality of piezoresistors; a signal tracking circuit operably coupled to the strain sensing circuit; and a signal extraction circuit.

[0027] The signal extraction circuit may be configured to generate, when in use, an oscillating timing signal adaptively.

[0028] According to a third aspect of the present invention, there is provided a method of operating a thermally deformable membrane of a pressure sensing element as set forth above in relation to the first aspect of the invention, the method comprising: applying voltage or current pulses at a predetermined frequency to the one or more heating elements.

[0029] The voltages or currents may be applied at a frequency greater than the fundamental resonant mode of the thermally deformable membrane.

[0030] The thermally deformable membrane may deform in response to heat pulses applied, when in use, by the one or more heating element driven by the pulses, thereby causing the thermally deformable membrane to oscillate.

[0031] The pulses may be applied before the thermally deformable membrane reaches the first deformation position.

[0032] The pulses applied to the number of the one or more heating elements may be applied before the thermally deformable membrane reaches the second deformation position.

[0033] The first heating element may be driven in an alternating manner with the number of the one or more heating elements.

[0034] The method may further comprise: generating a membrane strain measurement signal, the membrane strain signal being an oscillatory electrical signal comprising a number of harmonic frequencies corresponding to resonant modes of the thermally deformable membrane; selecting a desired harmonic frequency at which to drive the thermally deformable membrane; configuring the predetermined frequency to apply the voltage or current pulses to match the selected harmonic frequency in order to achieve oscillation of the thermally deformable membrane at a resonant mode corresponding to the harmonic frequency selected.

[0035] The thermally deformable membrane may oscillate at an oscillation frequency in response to the pulses applied by the one or more heating elements; and the method may further comprise: generating an oscillating timing signal to control application of the pulses at the predetermined frequency; a frequency of the oscillating timing signal may be half the oscillation frequency of the thermally deformable membrane.

[0036] The pulses may be a square waveform and may have a duty cycle of between about 10% and about 20%.

[0037] The phase-locked loop circuit may generate an output signal in response to electrical signals generated by the piezoresistors. The signal extraction circuit may extract a first sensing signal and a second sensing signal from the output signal generated by the phase-locked loop circuit; the first sensing signal may be proportional to an oscillation frequency of the thermally deformable membrane and the second sensing signal may be proportional to an externally applied pressure to the thermally deformable membrane.

[0038] The method may further comprise: measuring a temperature of the thermally deformable membrane; and controlling power of the pulses to maintain the temperature of the thermally deformable membrane at a constant temperature.

[0039] It is thus possible to provide a pressure sensing element and method of manufacture thereof that permits formation of elements to sense and actuate a membrane of the pressure sensing device on or at least partially in the monocrystalline silicon of the membrane. As such, aging of the actuation elements, the sensing elements and the membrane does not affect performance of the pressure sensing device. Indeed, formation of the elements to sense and actuation the membrane from the same material as the membrane avoids actuation of the membrane being hampering by construction-related effects, such as mismatches of thermal expansion of different materials, plastic deformation at interfaces of different materials over time, and / or ductility of certain materials. The oscillatable membrane of the pressure sensing device can be exposed directly to a medium in which pressure is to be monitored, thereby avoiding the need to form “transfer structures” as part of the pressure sensing device to communicate force to the membrane, such transfer structures being formed from materials, for example glues, glass frit and ductile metals, which have properties that can undesirably change over time. The pressure sensing device is also capable of being used for differential pressure sensing. The method of manufacture does not require additional processing steps to specifically form a separate resonator structure, because oscillation is achieved by way of formation of heating elements on or in the membrane at the same time as formation of sensing piezoresistors. The use of multiple heating elements also facilitates maximisation of deformation of the membrane when oscillating. Furthermore, the use of alternating electrical signals to drive the heating elements minimises crosstalk between the heating elements and the strain-sensing piezoresistors. Additionally, the amplitude of the power of the heating elements can be controlled to maintain the membrane at a constant temperature during operation. Electrically coupling the heating elements to a common electrical connection node also minimises the use of electrical interconnects in the pressure sensing device.

[0040] At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0041] Figure 1 is a schematic side view of a pressure sensor element for a pressure sensing device constituting an embodiment of the invention;

[0042] Figure 2 is a schematic side view of another pressure sensor element for a pressure sensing device constituting another embodiment of the invention;

[0043] Figure 3 is a schematic plan view of the pressure sensor elements of Figures 1 and 2 constituting a further embodiment of the invention;

[0044] Figure 4 is a schematic plan view of the pressure sensor elements of Figures 1 and 2 constituting yet another embodiment of the invention;

[0045] Figure 5 is a block diagram of a driver circuit and a sensing circuit constituting yet a further embodiment of the invention;

[0046] Figure 6 is a flow diagram of a method of manufacturing the pressure sensing elements of Figures 1 and 2 and constituting another embodiment of the invention;

[0047] Figure 7 is a flow diagram of a method of operating the pressure sensing elements of Figures 1 and 2 having the heating element layout of Figure 3 and constituting a further embodiment of the invention;

[0048] Figure 8 is a flow diagram of a method of operating the pressure sensing elements of Figures 1 and 2 having the heating element layout of Figure 4 and constituting yet a further embodiment of the invention;

[0049] Figure 9 is a graph of timing, drive and strain measurement signals in relation to the method of Figure 7; and

[0050] Figure 10 is a graph of a second, alternative, set of timing, drive and strain measurement signals in relation to the method of Figure 8.

[0051] Throughout the following description, identical reference numerals will be used to identify like parts.

[0052] Referring to Figure 1 , a sensing element 100, for example as used in a pressure sensor device, comprises a semiconductor substrate 102 formed, for example, from silicon. The sensing element 100 comprises a hollow 104 formed therein and a membrane layer 106 that overlies a first surface 108 of the substrate 102, including an aperture of the hollow, in order to form a closed cavity 110. In this example, the membrane layer 106 is formed from monocrystalline silicon.

[0053] The structure of the sensing element 100 is particularly suitable for applications where measurement of absolute pressure is required. However, when it is desirable to measure relative pressures, a different structure is employed and a backside 112 of the substrate 102 is etched in order that the hollow 104 extends to the backside of the substrate 102 and is no longer closed in contrast to the structure of Figure 1 .

[0054] Referring to Figure 3, in one example, the hollow 104 has a circular aperture and the portion of the membrane layer 106 overlying the aperture constitutes a membrane 200, which in this example is therefore circular. However, in other examples, the membrane 200 can be rectangular, the aperture of the hollow 104 being rectangular.

[0055] A first pair of piezoresistors 202 and a second pair of piezoresistors 204 are integrally formed with the membrane 200, for example by diffusion. The first and second pairs of piezoresistors 202, 204 are provided to measure mechanical strain and are disposed at the periphery of the membrane 200. In this regard, the first and second pairs of piezoresistors 202, 204 are disposed substantially over the hollow 104 opposite the closed aperture of the hollow 104. The first and second pairs of piezoresistors 202, 204 are formed from monocrystalline silicon. In the examples set forth above, the first pair of piezoresistors 202 comprises a first piezoresistor 206 and a second piezoresistor 208, the first and second piezoresistors extending in orthogonal directions with respect to each other. Additionally, in this example, the first piezoresistor 206 extends in part along a notional chord of the circular membrane 200, and the second piezoresistors 208 is, as mentioned above, substantially orthogonal to the first piezoresistors 206 and extends towards a circumference of the membrane 200 and in some examplescrosses the circumference. The second pair of piezoresistors 204 comprises a third piezoresistor 210 and a fourth piezoresistor 212, the third and fourth piezoresistors 210, 212 extending in orthogonal directions with respect to each other. Additionally, in this example, the third piezoresistor 210 extends in part along another notional chord of the circular membrane 200, and the fourth piezoresistor 212 is, as mentioned above, substantially orthogonal to the third piezoresistors 210 and extends towards the circumference of the circular membrane 200 and in some examples crosses the circumference. The first and second pairs of piezoresistors 202, 204 are circumferentially spaced about a periphery 214 of the circular membrane 200 and are disposed approximately at opposite sides of a quadrant of the circular membrane 200.

[0056] A first heating element 216 is disposed at substantially at the centre of the membrane 200 and is also formed from monocrystalline silicon. As the membrane 200 overlies the aperture of the hollow 104, the first heating element 216 is also disposed substantially over the centre of the hollow 104 in this example. The membrane 200 comprises the first heating element 216, which can be formed in or at least partially in the membrane 200 and is sited so as to be over the hollow 104. The first heating element 216 can be a resistor or a diode, a heating diode advantageously dissipating power in a very small space. The heating diode can be formed from a p-n junction or a metal-silicon junction. When a heating resistor is employed, the first heating element 216 can be formed from an n-type diffusion, a p-type diffusion or a metal line. As will be explained later, in some examples, the first heating element 216 and the first and second pairs of piezoresistors 202, 204 are formed using the same diffusion in the lattice of the membrane 200, for example of monocrystalline silicon, or at least substantially simultaneously in the lattice of the membrane 200. The layout of the first heating element 216 defines a heat distribution over the first heating element 216. In this regard, by shaping the first heating element 216 to heat, when in use, a predetermined region of the membrane 200, the location of heating of the membrane 200 can be controlled with precision. For example, the first heating element 216 can be an elongate diffusion of monocrystalline silicon comprising a central constriction where most of the heat generated by the first heating element 216 originates, assuming a homogeneousbulk resistance of the material used to form the first heating element 216. Additionally, changes in temperature of the first heating element 216 have to be sufficiently rapid to support oscillation of the membrane 200 at a resonant frequency from about 300KHz to up to about 10MHz, for example, and thus a time constant of between about 0.1 ps and about 3.4ps. As such, the surface area of the membrane 200 being heated has to be sufficiently small in order for such low time constants to be achieved. In this regard, the first heating element 216 is sized to support this time constant, which can be a single value or a range of values. Of course, other suitable shapes can be employed, for example square-shaped diffusions to serve as the first heating element 216. Polysilicon can also be employed to define the first heating element 216. In some examples, one of the contacts of the heating element can be a substrate contact (not shown) in order to reduce the number of metal interconnect lines employed when forming the sensing element 100. In this regard, the substrate contact serves as a common electrical connection node to which the first heating element 216 is connected.

[0057] The first, second, third and fourth piezoresistors 206, 208, 210, 212 are interconnected in a Wheatstone bridge arrangement to provide a pair of outputs that are provided to a readout circuit 300 (not shown in Figures 1 to 3) of the pressure sensor device for signal detection purposes, and the first heating element 216 is operably coupled to an actuation drive circuit 302 (also not shown in Figures 1 to 3).

[0058] Referring to Figure 4, the sensing element 100 of Figures 1 and 2 comprise additional heating elements integrally formed with the membrane 200 as compared with the arrangement of Figure 3. In this respect, the additional heating elements are disposed at the periphery of the membrane 200. A second heating element 218 and a third heating element 220 are thus disposed at the periphery of the membrane 200, for example over the hollow 104, and circumferentially spaced with respect to each other. The membrane 200 comprises the second and third heating element 218, 220, which can be formed in or at least partially in the membrane 200. In a like manner to the first and second pairs of piezoresistors 202, 204, the second and third heating elements 218, 220 are located adjacent the periphery of the membrane 200 and sited approximately at opposite sides of a quadrant of thecircular membrane 200. However, the second heating element 218 and the first pair of piezoresistors 202 are also approximately provided at opposite sides of a quadrant of the circular membrane 200, and the third heating element 220 and the second pair of piezoresistors 204 are also approximately provided at opposite sides of another quadrant of the circular membrane 200.

[0059] The arrangement of the pairs of piezoresistors 202, 204 and the peripheral heating elements 218, 220 is thus that for each pair of piezoresistors 202, 204 placed at the periphery of the membrane 200, a heating element 218, 220 is sited on an opposite side of the membrane 200 to the pair of piezo resistors 202, 204. The first and second pairs of piezoresistors 202, 204 and the second and third heating elements 218, 220 are circumferentially spaced about the membrane 200 such that the first and second pairs of piezoresistors 202, 204 are neighbouring each other and the second and third heating elements 218, 220 are neighbouring each other. This configuration of placing piezoresistors at opposite sides of the membrane 200 from a heating element maximises the distance between the heating elements and the piezoresistors, thereby mitigating any cross-talk, when in use, between the heating elements and the piezoresistors.

[0060] As in relation to the example of Figure 3, the second and third heating elements 218, 220 are operably coupled to the actuation drive circuit 302 (not shown in Figure 4).

[0061] The second and third heating elements 218, 220 can be resistors or diodes, heating diodes advantageously dissipating power in a very small space. The heating diodes can be formed from p-n junctions or metal-silicon junctions. When heating resistors are employed, the second and third heating elements 218, 220 can be formed from n-type diffusions, p-type diffusions or metal lines. As will be explained later, in some examples, the second and third heating element 218, 220 and the first and second pairs of piezoresistors 202, 204 are formed using the same diffusion in the lattice of the membrane 200, for example of monocrystalline silicon and / or the second and third heating element 218, 220 and the first and second pairs of piezoresistors 202, 204 are formed substantially simultaneously in the lattice of the membrane 200. The layout of the second and third heatingelements 218, 220 defines respective heat distributions over the second and third heating elements 218, 220. In this regard, by shaping the second and third heating elements 218, 220 to heat, when in use, other predetermined regions of the membrane 200, the locations of heating of the membrane 200 can be controlled with precision. For example, the second and third heating elements 218, 220 can respectively be elongate diffusions of doping ions in monocrystalline silicon comprising a central constriction where most of the heat generated by each of the second and third heating elements 218, 220 originates, assuming again a homogeneous bulk resistance of the material used to form the second and third heating element 218, 220. Again, changes in temperature of the second and third heating elements 218, 220 respectively have to be sufficiently rapid to support oscillation of the membrane 200 at the resonant frequency from about 300KHz to up to about 10MHz, for example, and thus with the time constant of between about 0.1 ps and 3.4ps. As such, the surface areas of the membrane 200 being heated have to be sufficiently small in order for such low time constants to be achieved. In this regard, the first heating element 216 is sized to support this time constant, which can be a single value or a range of values. Of course, other suitable shapes can be employed, for example square-shaped diffusions to serve as the second and third heating element 218, 220. Polysilicon can also be employed to define the second and third heating elements 218, 220. In some examples, one of the contacts of the heating element can be a substrate contact (not shown) in order to reduce the number of metal interconnect lines employed when forming the sensing element 100. In this regard, the substrate contact serves as a common electrical connection node to which the first, second and third heating elements 216, 218, 220 are connected.

[0062] Referring to Figure 5, the readout circuit 300 of the pressure sensor device comprises a sensing circuit 304 operably coupled to signal sense input pads 306 associated with the terminals of the Wheatstone bridge arrangement of the piezoresistors 206, 208, 210, 212. Although not shown, the sensing circuit 304 comprises a preamplification circuit, for example an instrumentation amplifier circuit, operably coupled to the signal sense input pads 306. The output of the instrumentation amplifier circuit is operably coupled to further amplification circuitryand analogue-to-digital conversion circuitry to provide a digital sense signal output 308. The digital sense signal output 308 is operably coupled to a signal tracking circuit, for example a Phase-Locked Loop (PLL) circuit 310 for tracking the frequency of an electrical signal present at the digital sense signal output 308. The PLL circuit 310 is operably coupled to a signal extraction circuit 312. More specifically, the PLL circuit 310 comprises a Phase Detector (PD) subcircuit operably coupled to a summation subcircuit, the summation subcircuit being operably coupled to a Proportional-Integral-Differential (PID) subcircuit. An output 314 of the PID subcircuit is operably coupled to the signal extraction circuit 312. An input to the PD subcircuit, which can be the digital sense signal output 308, is also operably coupled to the signal extraction circuit 312. The signal extraction circuit 312 comprises an oscillator subcircuit operably coupled to the output 314 of the PID subcircuit, the oscillator subcircuit being also operably coupled to a digital signal mixer subcircuit and the digital signal mixer subcircuit is operably coupled to a filter subcircuit, for example a low-pass filter subcircuit. The digital signal mixer subcircuit is also operably coupled to the input of the PD subcircuit of the PLL circuit. A tap into the oscillator subcircuit provides a frequency signal output, which is operably coupled to a frequency signal output pad 316, an output of the low-pass filter providing an amplitude signal output, which is operably coupled to an amplitude signal output pad 318. A frequency signal generated at the frequency signal output, when in use, constitutes a first sensing signal that is proportional to an oscillation frequency of the thermally deformable membrane and an amplitude signal generated at the amplitude signal output constitutes a second sensing signal that is proportional to an externally applied pressure to the thermally deformable membrane when in use.

[0063] The signal extraction circuit 312 is operably coupled to the actuation drive circuit 302, for example by way of the frequency signal output of the oscillator subcircuit of the signal extraction circuit 312. The actuation drive circuit 302 comprises power circuitry that is operably coupled to a plurality of heating element contact pads 320. The number of contact pads of the plurality of heating element pads 320 depends upon the number of heating elements employed. For example, the configuration of Figure 3 only requires two contact pads, whereas theconfiguration of Figure 4 requires 4 contact pads to drive a greater number of heating elements. One of the contact pads can be common electrical connection node mentioned above.

[0064] In this example, the readout circuit 300 and the actuation drive circuit 302 are not provided on the same die as the sensing element 100, but instead are packaged together and connected by wire bonding to the sensing element 100. However, in other examples, the readout circuit 300 and the actuation drive circuit 302 can be provided on the same die as the sensing element 100 or any other suitable arrangement can be employed to satisfy application requirements.

[0065] Turning to Figure 6, manufacture of the sensing elements 100 of Figures 1 to 4 will now be described.

[0066] The substrate 102 is grown (Step 400) from a bulk material, for example silicon, using any suitable known technique for fabricating a Micro- Electromechanical Systems (MEMS) substrate. The membrane layer 106 is also grown (Step 402) from, in this example, monocrystalline silicon, using any suitable semiconductor fabrication process. The membrane layer 106 is thermally deformable, and responsive in this regard to changes in temperature.

[0067] The hollow 104 is then formed (Step 404) in the substrate 102, for example using any suitable patterning and etching process. In this regard, to form the sensing element 100 of Figure 1 , the hollow 104 extends into the substrate 102 but not through the substrate 102 in order to form a recess or open cavity. In this regard, the hollow 104 comprises an opening. For the sensing element 100 of Figure 2, the substrate 102 is etched from the backside 112 of the substrate 102 in order to create a tapered shape in cross-section that widens towards the backside 112 of the substrate 102 and narrows towards the first surface 108 of the substrate 102. In this example, the hollow 104 has a first opening at the first side 108 and a second opening at the backside 112 of the substrate 102. However, as mentioned above, the hollow 104 can be rectangular in lateral cross-section to make the membrane 200 rectangular in shape, or any other suitable shape can be employed depending upon application requirements.

[0068] Once the hollow 104 has been formed, the membrane layer 106 is applied (Step 406) to the first surface 108 of the substrate 102, for example using any suitable bonding technique and then etching the membrane layer 106 to a desired thickness. Of course, the skilled person will appreciate that other suitable techniques can be employed to apply the membrane 200 to the first surface 108 of the substrate 102 for example by etching a substrate from a backside thereof to form a thinned region that serves as a membrane. In the case of the sensing element 100 of Figure 1 , the membrane layer 106 closes the open cavity formed by the hollow 104. In the case of the sensing element 100 of Figure 2, the membrane layer 106 closes one side of the hollow 104, leaving the hollow 104 open at the backside 112 of the substrate 102 to allow relative pressure to be sensed when in use.

[0069] Once the membrane layer 106 has been applied, the first and second pairs of piezoresistors 202, 204 and one or more heating elements 216, 218, 220 are formed (Step 408) in the membrane layer 106 using any suitable semiconductor diffusion technique using, in this example, monocrystalline silicon. As such, the first and second pairs of piezoresistors 202, 204 and the one or more heating elements 216, 218, 220 are integrally formed with the membrane layer 106 and formed during the same diffusion process in the lattice of the membrane 200 and / or substantially simultaneously in the lattice of the membrane 200. However, the skilled person will appreciate that other techniques can be employed that can result in the first and second pairs of piezoresistors 202, 204 and one or more heating elements 216, 218, 220 being attached to an exposed surface of the membrane layer 106 rather than being integrally formed with the membrane layer 106. In relation to the application of the first and second pairs of piezoresistors 202, 204 and one or more heating elements 216, 218, 220, the number of heating elements formed depends upon whether the sensing element 100 of Figure 3 is being formed, which requires just the first heating element 216, or the sensing element 100 of Figure 4, which additionally requires the second and / or third heating elements 218, 220.

[0070] Once the membrane layer 106 of the sensing element 100 has been populated with the first and second pairs of piezoresistors 202, 204 and one ormore heating elements 216, 218, 220, the sensing element 100 is subjected to further processing (Step 410), which can comprise packaging and any other processing steps required to form a pressure sensor device. However, for the sake of clarity and conciseness of description only the salient aspects of manufacture of the sensing element 100 will be described herein.

[0071] In operation (Figures 7 and 9), and in relation to the configuration of Figure 3, the readout circuit 300 and the actuation drive circuit 302 are initialised (Step 500) and the oscillator subcircuit of the signal extraction circuit 312 generates (Step 502) a first oscillating timing signal 600 (Figure 9) having a first timing control frequency that is provided to the power circuitry of the actuation drive circuit 302 to control the frequency of generation (Step 504) of a first pulsed heater drive signal 602, for example a square wave pulse. In this example, the first pulsed heater drive signal 602 is a voltage signal, but in other examples the first pulsed heater drive signal 602 can be a current signal. The first pulsed heater drive signal 602 has an alternating polarity. In this example, the first pulsed heater drive signal 602 has a duty cycle of between about 10% and about 20%, which prevents excess overall heat energy being generated that is not translated into mechanical energy. However, other duty cycles can be employed if beneficial.

[0072] Substantially contemporaneously with the generation of the first pulsed heater drive signal 602, the sensing circuit 304 generates a membrane strain measurement signal, which is provided to the PLL circuit 310. The PLL circuit 310, in cooperation with the signal extraction circuit 312, generate a membrane oscillation signal 604 indicative of the degree of deflection undergone by the membrane 200 in response to application of the first pulsed heater drive signal 602. The membrane oscillation signal 604 is used to monitor (Step 506) oscillation of the membrane 200 and hence ensure that the first pulsed heater drive signal 602 is timed so as to result in the application of heat to the membrane 200 in a manner that achieves resonance of membrane 200. In this regard, when the centre of the membrane 200 is heated by the first heating element 216 under control of the first pulsed heater drive signal 602, the membrane 200 responds to the change in temperature applied by the first heating element 216 and experiences periodic local compressive stresses beneath the first heating element 216. For example, a localcompressive stress induced by a first pulse 606 of the first pulsed heater drive signal 602 causes buckling of the membrane 200 and therefore movement in a first direction. In this regard, localised dissipation of heat from the first heating element 216 is, for example, over about 5% or less of surface area of the membrane 200 adjacent the first heating element 216. This proportion of the membrane 200 can be smaller, for example about 0.5% or less of the surface area of the membrane 200 or about 0.2% or less of the surface area of the membrane 200 or about 0.1 % or less of the surface area of the membrane 200. Since power dissipation by the first heating element 216 is independent of direction of current, heating of the membrane occurs when both polarities of the first pulsed heater drive signal 602 are applied. Consequently, when the first pulsed heater drive signal 602 applies a second pulse 608 with an opposite polarity to the first pulse 606, the membrane 200 is again urged in the first direction for the same reason. Owing to the above independence of direction of current, the frequency of the first oscillating timing signal 600 used to generate the first pulsed heater drive signal 602 is half the resonant frequency of the membrane 200. Between the first and second pulses 606, 608, the membrane 200 relaxes and travels in a second, opposite, direction to the first direction. This can be seen in Figure 9 by the membrane oscillation signal 604 that is provided by the signal extraction circuit 312 at the amplitude signal output pad 318. As will be appreciated by the skilled person, heating the membrane 200 with heat pulses at a sufficiently large frequency contributes to resonance of the membrane. The membrane 200 undergoes oscillatory deformation between a first deformation position distal from an undeformed neutral position in the first direction, for example when the membrane 200 is at rest, and a second deformation position distal from the undeformed neutral position in the second direction, which as mentioned above is opposite to the first direction.

[0073] When the membrane 200 deforms, this strain can be sensed by two or more of the piezoresistors 206, 208, 210, 212 in the Wheatstone bridge configuration. Hence, once oscillation of the membrane 200 is underway, the membrane strain measurement signal generated by the sensing circuit 304 is used by the PLL circuit 310 to control a phase offset of the first oscillating timing signal 600 in order to stimulate the membrane 200 so that the membrane 200 is inresonance. In order to achieve resonance, the positive and negative pulses 606, 608 should occur when the centre of the membrane 200 is moving towards its highest position or greatest amplitude, and just after the respective pulses 606, 608 peak, the greatest deflections of the membrane 200 are obtained. In this regard, in order to achieve the amplitude maxima of the membrane 200, which are the maximum extents of deformation of the membrane 200 in the first and second directions, the surface of the membrane 200 has to deform an additional amount with respect to existing deformation resulting from oscillatory motion of the membrane 200 when heat is not being applied. To achieve such additional deformation and thus greatest deflection of the membrane 200, heat is dissipated from the first heating element 216 to a localised region of the membrane 200 in response to application of the positive and negative pulses 606, 608 at the times relative to amplitude of the membrane 200 deformation indicated above. In order to achieve resonance, the PLL circuit 310 locks onto the membrane strain measurement signal (not shown) generated by the sensing circuit 304 mentioned above. The membrane strain measurement signal is an oscillatory electrical signal comprising a number of harmonic frequencies corresponding to resonant modes of the membrane 200. The output 314 of the PID subcircuit of the PLL circuit 310 provides a locked membrane strain signal that the oscillator subcircuit of the signal extraction circuit 312 uses to compare (Step 508) with the first oscillating timing signal 600 in order to determine whether the pulses of the first pulsed heater driver signal 602 are being applied to the membrane 200 at the correct point in time prior to maximum deformations of the membrane 200 being attained. The oscillator subcircuit therefore determines whether a phase difference between the first oscillating timing signal 600 and the locked membrane strain signal is correct (Step 510). If the phase difference is correct, no action is taken and the oscillator subcircuit continues to compare and assess the phase difference between the first oscillating timing signal 600 and the locked membrane strain signal (Steps 508 and 510). However, if the phase difference is incorrect, the oscillator subcircuit adjusts (Step 512) a phase offset of the first oscillating timing signal 600 in order to achieve the correct phase difference, thereby adapting the first timing signal 600, and then continues to compare and assess the phase difference between the first oscillating timing signal 600 and the locked membrane strain signal (Steps 508 and 510). Thisprocess continues while the pressure sensing element 100 is in use to measure pressure. Where resonance of the membrane 200 at a higher order mode above the fundamental mode is required, the signal extraction circuit 312 can select a harmonic frequency that is desired and set the first oscillating timing signal 600 to a frequency of application of the positive and negative pulses 606, 608 that consequentially causes the membrane 200 to oscillate at the harmonic frequency selected, thereby causing the harmonic frequency selected to dominate. In this regard, the PLL circuit 310 can be configured to lock to the harmonic required within the membrane strain measurement signal. The locked membrane strain signal thus provided to the oscillator subcircuit of the signal extraction circuit 312 can be compared by the oscillator subcircuit with the first oscillating timing signal 600 in order to determine whether the pulses of the pulsed heater driver signal 602 are being applied to the membrane 200 at the correct point in time in order to achieve and maintain oscillation of the membrane 200 at the selected harmonic frequency. In order to generate a selected resonance mode of the membrane 200, in some examples, the first heating element 216 can be placed at a location on the membrane 200 where the strain in or near the membrane 200 reaches a maximum or minimum for the specific resonance mode selected.

[0074] The first and second pulses 606, 608 generated by the power circuitry of the actuation drive circuit 302 have to be sufficiently high to obtain a required resonance amplitude that is detectable. Consequently, provided the first and second heat pulses 606, 608 are greater than a predetermined minimum voltage or current value to achieve resonance of the membrane 200, the amplitude of the first and second pulses 606, 608 can be used to manage the temperature of the membrane 200 and maintain the temperature of the membrane 200 at a substantially constant temperature, thereby supporting device stability. In this regard, the membrane 200 can comprise a temperature sensing element (not shown), for example a piezoresistor or a diode formed from the same diffusions as and / or substantially with the piezoresistors 206, 208, 210, 212 and the heating element 216 in order to measure the temperature of the membrane 200 as part of a feedback loop to manage the membrane 200 thermally.

[0075] In the above example, a single heating element 216 is disposed substantially at the centre of the membrane 200 in order to drive the membrane in a resonant mode. In this regard, the frequency of oscillation of the membrane 200 is dictated by the position of placement of the first heating element 216 on the membrane 200. Consequently, if oscillation of the membrane 200 at a harmonic overtone frequency is required, the first heating element 216 can be placed at another location on the membrane 200, for example towards the periphery of the membrane 200, depending upon the harmonic to be achieved.

[0076] Turning to Figures 8 and 10, and in relation to the configuration of Figure 4, the readout circuit 300 and the actuation drive circuit 302 are initialised (Step 700) and the oscillator subcircuit of the signal extraction circuit 312 generates (Step 702) a first oscillating timing signal 800 (Figure 10) having a first timing control frequency. The oscillator subcircuit of the signal extraction circuit 312 also generates (Step 704) a second oscillating timing signal 804 based upon the first oscillating timing signal 800. The first oscillating timing signal 800 is provided to the power circuitry of the actuation drive circuit 302 to control the frequency of generation (Step 706) of a first pulsed heater drive signal 802, for example a square wave pulse. The first pulsed heater drive signal 802 has an alternating polarity. In this example, the first pulsed heater drive signal 802 has a duty cycle of between about 10% and about 20%, which prevents excess overall heat energy being generated that is not translated into mechanical energy. However, other duty cycles can be employed if beneficial. In this example, the first pulsed heater drive signal 802 is a voltage signal of a predetermined frequency corresponding to a frequency of oscillation of the membrane 200 to be achieved, but in other examples the first pulsed heater drive signal 802 can be a current signal of the predetermined frequency corresponding to a frequency of oscillation of the membrane 200 to be achieved. The second oscillating timing signal 804 having a second timing control frequency is also provided to the power circuitry of the actuation drive circuit 302 to control the frequency of generation (Step 706) of a second pulsed heater drive signal 806, for example a square wave pulse. The second pulsed heater drive signal 806 has an alternating polarity. In this example, the second pulsed heater drive signal 806 has a duty cycle of between about 10% and about 20%, whichprevents excess overall heat energy being generated that is not translated into mechanical energy. However, other duty cycles can be employed if beneficial. In this example, the second pulsed heater drive signal 806 is a voltage signal of a predetermined frequency corresponding to the frequency of oscillation of the membrane 200 to be achieved, but in other examples the second pulsed heater drive signal 806 can be a current signal of the predetermined frequency. The oscillator subcircuit is configured so that pulses of heat are applied to the membrane 200 every quarter of a cycle of the first and second oscillating timing signals 800, 804 in an alternating manner between the central first heating element 216 and the peripheral second and third heating elements 218, 220.

[0077] Substantially contemporaneously with the generation of the first and second pulsed heater drive signal 802, 806, the sensing circuit 304 generates a membrane strain measurement signal, which is provided to the PLL circuit 310. The PLL circuit 310, in cooperation with the signal extraction circuit 312, generates a membrane oscillation signal 808 indicative of the degree of deflection undergone by the membrane 200 in response to application of the first and second pulsed heater drive signal 802, 806. The membrane oscillation signal 808 is used to monitor (Step 708) oscillation of the membrane 200 and hence ensure that the first and second pulsed heater drive signals 802, 806 are timed so as to result in the application of heat to the membrane 200 in manner that achieves resonance of the membrane 200. In this regard, when the centre of the membrane 200 is heated by the first heating element 216 under control of the first pulsed heater drive signal 802, the membrane 200 responds to the change in temperature applied by the first heating element 216 and the membrane 200 experiences periodic local compressive stresses beneath the first heating element 216 and a local compressive stress induced by a first pulse 810 of the first pulsed heater drive signal 802 causes buckling of the membrane 200 and therefore movement in a first direction. In this regard, as described above in relation to the example of Figure 3, localised dissipation of heat from the first heating element 216 is, for example, over about 5% or less of the surface area of the membrane 200 adjacent the first heating element 216. This proportion of the membrane 200 can be smaller, for example about 0.5% or less of the surface area of the membrane 200 or about 0.2% or lessof the surface area of the membrane 200 or about 0.1 % or less of the surface area of the membrane 200. When the first pulsed heater drive signal 802 applies a second pulse 812 with an opposite polarity to the first pulse 810, the membrane 200 is again urged in the first direction. Between the first and second pulses 606, 608, the periphery of the membrane 200 is heated by the second and third heating elements 218, 220 under the control of the second pulsed heater drive signal 806. In this regard, when the periphery of the membrane 200 is heated by the second and third heating elements 218, 220 under control of the second pulsed heater drive signal 806, the membrane 200 responds to the changes in temperature applied by the second and third heating elements 218, 220 and the membrane 200 experiences periodic local compressive stresses beneath the second and third heating elements 218, 220 and local compressive stresses induced by a third pulse 814 of the second pulsed heater drive signal 806 causes buckling of the membrane 200 that, in combination with relaxation of the membrane 200 towards the neutral state after termination of application of the first pulse 810, creates movement in a second, opposite, direction to the first direction. In this regard, respective localised dissipations of heat from the second and third heating element 218, 220 are each, for example, over about 5% or less of the surface area of the membrane 200 adjacent the second and third heating element 218, 220, respectively. This proportion of the membrane 200 can be smaller, for example about 0.5% or less of the surface area of the membrane 200 or about 0.2% or less of the surface area of the membrane 200 or about 0.1 % or less of the surface area of the membrane 200. When the second pulsed heater drive signal 806 applies a fourth pulse 816 with an opposite polarity to the third pulse 814, the membrane 200 is again urged in the second direction. This can be seen in Figure 10 by the membrane oscillation signal 808 that is provided by the signal extraction circuit 312 at the amplitude signal output pad 318. Since power dissipation by the first, second and third heating element 216, 218, 220 is independent of direction of current, heating of the membrane 200 occurs when both polarities of the first pulsed heater drive signal 802 and the second pulsed heater drive signal 806 are applied. As such, the predetermined frequency of the application of pulses by the first and second pulsed heater drive signals 802, 806 is higher than the respective frequencies of each of the first and second pulsed heater drive signals 802, 806, i.e. substantially atfundamental resonant frequency of the membrane 200, for example twice the respective frequencies of each of the first and second pulsed heater drive signals 802, 806. As will be appreciated by the skilled person, heating the membrane 200 with heat pulses at this predetermined frequency contributes to the membrane 200 resonating. The membrane 200 therefore undergoes oscillatory deformation between the first deformation position distal from the undeformed neutral position in the first direction, for example when the membrane 200 is at rest, and the second deformation position distal from the undeformed neutral position in the second direction, which as mentioned above is opposite to the first direction.

[0078] When the membrane 200 deforms, the strain from deformation can be sensed by one or more of the piezoresistors 206, 208, 210, 212. Hence, once oscillation of the membrane 200 is underway, the membrane strain measurement signal generated by the sensing circuit 304 is used by the PLL circuit 310 to control a phase offset of the first oscillating timing signal 800 and hence also the second oscillating timing signal 804 in order to stimulate the membrane 200 so that the membrane 200 is in resonance. In order to achieve resonance, the first and second pulses 810, 812, one of which is positive and one of which is negative, should occur when the centre of the membrane 200 is moving towards its highest position or greatest amplitude, and just after the respective pulses 810, 812 peak, the greatest deflections of the membrane 200 are obtained. In this regard, in order to achieve the amplitude maximum, which is the maximum extent of deformation of the membrane 200 in the first direction, the surface of the membrane 200 has to deform an additional amount with respect to existing deformation resulting from oscillatory motion of the membrane 200 when heat is not being applied. To achieve such deformation and thus greatest deflection of the membrane 200, heat is dissipated from the first heating element 216 to a localised region of the membrane 200 in response to application of the positive and negative pulses 810, 812 at the times relative to amplitude of the membrane 200 deformation indicated above. Likewise, at the amplitude minimum, which is the maximum extent of deformation of the membrane 200 in the second direction, the surface of the membrane 200 has to deform an additional amount in the second direction with respect to existing deformation resulting from oscillatory motion of the membrane 200 when heat isnot being applied. To achieve such deformation and thus greatest deflection of the membrane 200, heat is dissipated from the second and third heating elements 218, 220 to respective localised regions of the membrane 200 in response to application of the positive and negative pulses 814, 816 at the times relative to amplitude of the membrane 200 deformation indicated above. Similarly, the third and fourth pulses 814, 816, one of which is positive and one of which is negative, should occur when the centre of the membrane 200 is moving towards its lowest position of greatest amplitude, and just after the respective pulses 814, 816, peak, the greatest deflections of the membrane 200 in the second direction are obtained. By driving the first heating element 216 and the second and third heating elements 218, 220 in an alternating fashion, the amplitude of deflection of the membrane 200 can be optimised.

[0079] In order to achieve the correct timing for resonance, the PLL circuit 310 locks onto the membrane strain measurement signal (not shown) generated by the sensing circuit 304 mentioned above. The membrane strain measurement signal is an oscillatory electrical signal comprising a number of harmonic frequencies corresponding to resonant modes of the membrane 200. The output 314 of the PID subcircuit of the PLL circuit 310 provides a locked membrane strain signal that the oscillator subcircuit of the signal extraction circuit 312 uses to compare (Step 710) with the first oscillating timing signal 600 in order to determine whether the pulses of the first pulsed heater driver signal 802 are being applied to the membrane 200 at the correct point in time prior to maximum deflections of the membrane 200 being attained. The oscillator subcircuit therefore determines whether a phase difference between the first oscillating timing signal 800 and the locked membrane strain signal is correct (Step 712). If the phase difference is correct, no action is taken and the oscillator subcircuit continues to compare and assess the phase difference between the first oscillating timing signal 600 and the locked membrane strain signal (Steps 710 and 712). However, if the phase difference is incorrect, the oscillator subcircuit adjusts (Step 714) a first phase offset of the first oscillating timing signal 800 and hence also a second phase offset of the second oscillating timing signal 804 in order to achieve the correct phase difference, thereby adapting the first and second oscillating timing signals 800, 804 so that the first and secondpulsed heater driver signals 802, 806 apply the first, second, third and fourth pulses 810, 812, 814, 816 to the membrane 200 at correct times to achieve resonance. The oscillator subcircuit then continues to compare and assess the phase difference between the first oscillating timing signal 800 and the locked membrane strain signal (Steps 710 and 712). This process continues while the pressure sensing element 100 is in use to measure pressure. Where resonance of the membrane 200 at a higher order mode above the fundamental mode is required, the signal extraction circuit 312 can select a harmonic frequency that is desired and set the first and second oscillating timing signals 800, 804 to a frequency of application of the first, second, third and fourth pulses 810, 812, 814, 816 that consequentially causes the membrane 200 to oscillate at the harmonic frequency selected, thereby causing the harmonic frequency selected to dominate. In this regard, the PLL circuit 310 can be configured to lock to the harmonic required within the membrane strain measurement signal. The locked membrane strain signal thus provided to the oscillator subcircuit of the signal extraction circuit 312 can be compared by the oscillator subcircuit with the first and second oscillating timing signals 800,804 in order to determine whether the pulses of the first and second pulsed heater driver signals 802, 806 are being applied to the membrane 200 at the correct point in time in order to achieve and maintain oscillation of the membrane 200 at the selected harmonic frequency. In order to generate a selected resonance mode of the membrane 200, in some examples, the first, second and third heating element 216, 218, 220 can be placed at respective locations on the membrane 200 where the strain in or near the membrane 200 reaches respectively a maximum or minimum for the specific resonance mode selected.

[0080] In the above example, the first and second oscillating timing signals 800, 804 are not independent and a change to a phase offset of the first oscillating timing signal 800 results in a corresponding change to a second phase offset of the second oscillating timing signal 804 so that a phase difference between the first and second oscillating timing signals 800, 804 is maintained. However, in other examples, the first and second oscillating timing signals 800, 804 can be independently controlled and so adjustment of the first phase offset with respect tothe first oscillating timing signal 800 can be different to adjustment of the second phase offset with respect to the second oscillating timing signal 804.

[0081] The first, second, third and fourth pulses 810, 812, 814, 816 generated by the power circuitry of the actuation drive circuit 302 have to be sufficiently high to obtain a required resonance amplitude that is detectable. Consequently, provided the first, second, third and fourth pulses 810, 812, 814, 816 are greater than a predetermined minimum voltage or current value to achieve resonance of the membrane 200, the amplitude of the first, second, third and fourth pulses 810, 812, 814, 816 can be used to manage the temperature of the membrane 200 and maintain the temperature of the membrane 200 at a substantially constant temperature, thereby supporting device stability. In this regard, the membrane 200 can comprise a temperature sensing element (not shown), for example a piezoresistor or a diode formed from the same diffusions as the piezoresistors 206, 208, 210, 212 and the first, second and third heating elements 216, 218, 220 in the lattice of the membrane 200 and / or substantially simultaneously with these diffusions in the lattice of the membrane 200 in order to measure the temperature of the membrane 200 as part of a feedback loop to manage the membrane 200 thermally.

[0082] In relation to the examples described above employing one or more heating elements, the membrane 200 can be driven to oscillate at different frequencies, depending upon application requirements. For example, the membrane 200 can be made to oscillate at a fundamental frequency or higher overtones. Where the membrane 200 is made to oscillate at the fundamental mode, the whole membrane moves upwards and downwards in response to the first pulsed heater driver signal 602 in the case of the example of Figure 3 or the first and second pulsed heater driver signals 802, 806 in the case of the example of Figure 4. However, where a higher order overtone is required, some regions of the membrane 200 move downwards when other regions of the membrane 200 move upwards. For example, in the case of a second order resonance mode, which is the first harmonic frequency above the fundamental frequency, one part of the membrane 200 moves upwards while another part of the membrane 200 moves downwards. Thus, a differential actuation of the membrane 200 can be achievedwhere interference of the heating signals with the membrane strain measurement signal can be substantially attenuated.

[0083] The desired resonance mode can be achieved using the signal extraction circuit 312 in cooperation with the actuation drive circuit 302 to initiate oscillation of the membrane 200 at a frequency close to the required resonance mode. As mentioned above, the first heating element 216 in the case of the example of Figure 3, or in the case of the example of Figure 4, the first, second and / or third heating element 216, 218, 220 can be optimally positioned on the membrane 200 so as to enhance a certain required resonance mode. Higher modes of resonance can be beneficial in terms of reducing the effects of disturbing influences, for example humidity and / or package stress. The pressure sensor device exhibits improved stability over the use of the fundamental mode when operated at higher modes as at higher resonance modes higher Q-factors are obtained.

[0084] Once an initial level of oscillation of the membrane 200 is achieved, the membrane oscillation signal 808 generated comprises a spectrum of signals of different frequencies, including a signal at the fundamental frequency corresponding to the resonant frequency of the membrane 200 as well as signals corresponding to higher harmonic modes of the mechanical resonant frequency of the membrane 200. The signal extraction circuit 312 can be configured so that a desired overtone of the higher harmonic components is used as a reference, for example the second harmonic frequency component in the membrane oscillation signal 808 in order to control resonance of the membrane 200 to achieve resonance of the membrane 200 at the second order harmonic. Advantageously, selection of a higher order harmonic signal to control resonance of the membrane 200 obviates or at least mitigates the influence of interfering thermal and / or electrical actuation signals, since the frequency of such higher harmonic drive signals are spaced further in the spectrum from interfering signals than in the case where the membrane 200 is driven at a fundamental mode, especially when such interference changes with temperature and humidity where it is harder to compensate for such interference with a fixed offset signal.

[0085] Although in the above examples functional units have been described as part of and / or assigned to specific circuits and / or subcircuits, the skilled person will appreciate that such functional units can be implemented in many different ways and the functional units can be implemented in different and / or additional circuits and / or subcircuits not described herein.

[0086] In the above examples, the first heating element 216 is located generally centrally on the membrane 200 over the hollow 104 and, where employed, the second and third heating elements 218, 220 are located generally at the periphery of the membrane 200 over the hollow 104. However, it should be appreciated that optimal locations for disposing the second and third heating elements 218, 220 and / or the first heating element 216 on the membrane 200 exist for specific resonant modes. In this regard, for a given resonant mode, the membrane 200 has certain regions (depending upon the resonant mode) where maximum and minimum deformations of the membrane 200 are achieved. The heating elements 216, 218, 220 (depending upon how many heating elements are employed) are selectively located on the membrane 200 to coincide with the locations where the maximum and minimum deformation occur when the membrane 200 is driven in a desired resonant mode, for example a mode higher than the fundamental mode.

[0087] Although up to three heating elements are described herein, the skilled person should appreciate that in other examples a greater number of heating elements can be employed, depending upon the resonant mode required of the membrane 200.

Claims

Claims1 . A pressure sensing element for a pressure sensor device comprising: a substrate; a hollow formed in the substrate; and a thermally deformable membrane disposed over an opening of the hollow in the substrate, the thermally deformable membrane comprising one or more heating element; wherein the thermally deformable membrane is responsive to a change in temperature of the one or more heating element and oscillatably deformable between a first deformation position distal from an undeformed neutral position in a first direction and a second deformation position distal from the undeformed neutral position in a second direction opposite to the first direction; and the thermally deformable membrane also comprises a plurality of strainsensing piezoresistors.

2. A pressure sensing element as claimed in Claim 1 , wherein the one or more heating elements are disposed on the thermally deformable membrane and over the hollow.

3. A pressure sensing element as claimed in Claim 1 , wherein the one or more heating elements are disposed at least partially in the thermally deformable membrane and over the hollow.

4. A pressure sensing element as claimed in Claim 2 or Claim 3, wherein a first heating element of the one or more heating elements is disposed over the centre of the hollow.

5. A pressure sensing element as claimed in Claim 4, wherein a number of the one or more heating elements are disposed over the hollow at a periphery thereof.

6. A pressure sensing element as claimed in any one of the preceding claims, wherein the thermally deformable membrane further comprises a temperature sensing element configured to measure a temperature of the thermally deformable membrane.

7. A pressure sensing element as claimed in any one of the preceding claims, wherein the one or more heating elements are one or more resistors and / or diodes.

8. A pressure sensing element as claimed in Claim 6, wherein the thermally deformable membrane is a layer of monocrystalline silicon; the plurality of strain-sensing piezoresistors are diffused into the thermally deformable membrane; and the one or more heating elements are formed substantially simultaneously with the plurality of strain-sensing piezoresistors in a lattice of the thermally deformable membrane.

9. A pressure sensing element as claimed in Claim 2, wherein each of the plurality of strain-sensing piezoresistors is formed at least partially inside the thermally deformable membrane over the hollow; a heating element of the one or more heating elements is located at a periphery of the membrane; and the heating element is located at an opposite side of the membrane with respect to a piezoresistor of the plurality of piezoresistors.

10. A pressure sensor device comprising: a pressure sensing element as claimed in any one of the preceding claims; a readout circuit comprising: a strain sensing circuit operably coupled to the plurality of piezoresistors; a signal tracking circuit operably coupled to the strain sensing circuit; and a signal extraction circuit.

11. A method of operating a thermally deformable membrane of a pressure sensing element as claimed in Claim 1 , the method comprising: applying voltage or current pulses at a predetermined frequency to the one or more heating elements.

12. A method as claimed in Claim 11 , when dependent upon Claim 4, wherein the pulses are applied before the thermally deformable membrane reaches the first deformation position.

13. A method as claimed in Claim 11 , when dependent upon Claim 5, wherein the pulses applied to the number of the one or more heating elements are applied before the thermally deformable membrane reaches the second deformation position.

14. A method according to claim 12 or Claim 13, wherein the thermally deformable membrane oscillates at an oscillation frequency in response to the pulses applied by the one or more heating elements; and the method further comprises: generating an oscillating timing signal to control application of the pulses at the predetermined frequency, a frequency of the oscillating timing signal being half the oscillation frequency of the thermally deformable membrane.

15. A method as claimed in Claim 11 , further comprising: measuring a temperature of the thermally deformable membrane; and controlling power of the pulses to maintain the temperature of the thermally deformable membrane at a constant temperature.

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