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Sensors2008,8,2317-2330 sensors IsSN14248220 C 2008 by MDPl www.mdpiorg/sensors Full Research Paper A Micromachined Capacitive Pressure Sensor Using a Cavity-Less Structure with Bulk-Metal/Elastomer Layers and Its Wireless Telemetry application Kenichi Takahata I,* and Yogesh B Gianchandani 2 i Department of Electrical and Computer Engineering, University of British Columbia, 2332 Main Mall, Vancouver, BC V6T 1Z4, Canada; E-mail: takahata @ece. ubc.ca Department of Electrical Engineering and Computer Science, University of Michigan, 1301 Beal Ave, Ann Arbor, MI 48109-2122, USA; E-mail: yogesh @umich. edi s Author to whom correspondence should be addressed Received: 31 December 2007/Accepted: 31 March 2008/Published: 2 April 2008 Abstract: This paper reports a micromachined capacitive pressure sensor intended for applications that require mechanical robustness. The device is constructed with two micromachined metal plates and an intermediate polymer layer that is soft enough to deform in a target pressure range. The plates are formed of micromachined stainless steel fabricated by batch-compatible micro-electro-discharge machining. a polyurethane room- temperature-vulcanizing liquid rubber of 38-um thickness is used as the deformable material. This structure eliminates both the vacuum cavity and the associated lead transfer challenges common to micromachined capacitive pressure sensors. For frequency-based interrogation of the capacitance, passive inductor-capacitor tanks are fabricated by combining the capacitive sensor with an inductive coil. The coil has 40 turns of a 127-um diameter copper wire. Wireless sensing is demonstrated in liquid by monitoring the variation in the resonant frequency of the tank via an external coil that is magnetically coupled with the tank. The sensitivity at room temperature is measured to be 23-33 ppm/KPa over a dynamic range of 340 KPa, which is shown to match a theoretical estimation. Temperature dependence of the tank is experimentally evaluated

Sensors 2008, 8, 2317-2330 sensors ISSN 1424-8220 © 2008 by MDPI www.mdpi.org/sensors Full Research Paper A Micromachined Capacitive Pressure Sensor Using a Cavity-Less Structure with Bulk-Metal/Elastomer Layers and Its Wireless Telemetry Application Kenichi Takahata 1,* and Yogesh B. Gianchandani 2 1 Department of Electrical and Computer Engineering, University of British Columbia, 2332 Main Mall, Vancouver, BC V6T 1Z4, Canada; E-mail: takahata@ece.ubc.ca 2 Department of Electrical Engineering and Computer Science, University of Michigan, 1301 Beal Ave., Ann Arbor, MI 48109-2122, USA; E-mail: yogesh@umich.edu * Author to whom correspondence should be addressed. Received: 31 December 2007 / Accepted: 31 March 2008 / Published: 2 April 2008 Abstract: This paper reports a micromachined capacitive pressure sensor intended for applications that require mechanical robustness. The device is constructed with two micromachined metal plates and an intermediate polymer layer that is soft enough to deform in a target pressure range. The plates are formed of micromachined stainless steel fabricated by batch-compatible micro-electro-discharge machining. A polyurethane roomtemperature-vulcanizing liquid rubber of 38-μm thickness is used as the deformable material. This structure eliminates both the vacuum cavity and the associated lead transfer challenges common to micromachined capacitive pressure sensors. For frequency-based interrogation of the capacitance, passive inductor-capacitor tanks are fabricated by combining the capacitive sensor with an inductive coil. The coil has 40 turns of a 127-μmdiameter copper wire. Wireless sensing is demonstrated in liquid by monitoring the variation in the resonant frequency of the tank via an external coil that is magnetically coupled with the tank. The sensitivity at room temperature is measured to be 23-33 ppm/KPa over a dynamic range of 340 KPa, which is shown to match a theoretical estimation. Temperature dependence of the tank is experimentally evaluated

Sensors 2008. 8 2318 Keywords: pressure sensor, wireless, stainless steel, polyurethane, micro-electro-discharge machinin 1. Introduction Capacitive pressure sensors are favored for low-power and telemetric applications since they draw no DC power, and conveniently form passive inductor-capacitor(L-C)tank circuits for frequency based measurement of pressure [1-3]. Micromachined capacitive pressure sensors have typically used an elastic diaphragm with fixed edges and a sealed cavity in between the diaphragm and the substrate below [4, 5]. Since this configuration relies on the deflection of a relatively thin diaphragm against a sealed cavity, in some applications there is a concern of robustness of the diaphragm and leaks in the cavity seal. Lead transfer for the sealed electrode has also been a persistent challenge. This has motivated the development of innovative fabrication methods that involve multilayer deposition planarization, and other remedies, but require relatively high mask counts[6, 7]. Another approach to deal with this has been to move the sense gap outside the cavity [8 This research explores a capacitive pressure sensor that consists of two micromachined metal plates with an intermediate polymer layer. Sandwich-type constructions with deformable intermediate layers have been used in some micromachined sensors [9, 10] as well as commercial pressure mapping systems(for, e.g., seat pressure monitoring)[11]. The selected configuration aims to eliminate the need of diaphragms and cavities from the micromachined capacitive sensors. Use of polymeric material that is soft enough to deform over a target pressure range allows thickness of the polymer, or capacitance of the parallel plate capacitor, to be dependent on hydraulic pressure that surrounds the device. This capacitive change can be interrogated by either a hard-wired interface or a wireless set-up in which the sensor serves as a capacitor of an L-C tank. The inductor coil can be separately coupled with the sensor(Figure la), or it can be formed by winding an insulated wire directly on the sensor to minimize the device size(Figure 1b). The wireless interrogation is performed by an external antenna/inductor that is magnetically coupled with the L-C tank device(Figure 2 ) Proper choice of materials compatible with particular environments will offer broader opportunities such automobile and biomedical applications that respectively include air pressure monitoring in the tires [12] and bowel pressure detection [ 13]. The inherent environmental compatibility is a significant advantage because it allows us to circumvent constraints and problems associated with the packaging [14] that in general degrades device performance and cost effectiveness in the device manufacturing This paper is constituted as follows. Section 2 describes the working principle and design of the sensor. The details of the fabrication process for the L-C tank device and the results are presented in Section 3. Section 4 reports the results of experimental characterization for the elastomer material used in this effort as well as the developed L-C tank device and the demonstration of wireless sensing with the device. These experimental results are evaluated in conjunction with the theoretical analysis in Section 5, followed by discussion in Section 6. Section 7 concludes the overall effort. Portions of this paper have appeared in conference abstract form in [16, 171

Sensors 2008, 8 2318 Keywords: pressure sensor, wireless, stainless steel, polyurethane, micro-electro-discharge machining 1. Introduction Capacitive pressure sensors are favored for low-power and telemetric applications since they draw no DC power, and conveniently form passive inductor-capacitor (L-C) tank circuits for frequencybased measurement of pressure [1-3]. Micromachined capacitive pressure sensors have typically used an elastic diaphragm with fixed edges and a sealed cavity in between the diaphragm and the substrate below [4, 5]. Since this configuration relies on the deflection of a relatively thin diaphragm against a sealed cavity, in some applications there is a concern of robustness of the diaphragm and leaks in the cavity seal. Lead transfer for the sealed electrode has also been a persistent challenge. This has motivated the development of innovative fabrication methods that involve multilayer deposition, planarization, and other remedies, but require relatively high mask counts [6, 7]. Another approach to deal with this has been to move the sense gap outside the cavity [8]. This research explores a capacitive pressure sensor that consists of two micromachined metal plates with an intermediate polymer layer. Sandwich-type constructions with deformable intermediate layers have been used in some micromachined sensors [9, 10] as well as commercial pressure mapping systems (for, e.g., seat pressure monitoring) [11]. The selected configuration aims to eliminate the need of diaphragms and cavities from the micromachined capacitive sensors. Use of polymeric material that is soft enough to deform over a target pressure range allows thickness of the polymer, or capacitance of the parallel plate capacitor, to be dependent on hydraulic pressure that surrounds the device. This capacitive change can be interrogated by either a hard-wired interface or a wireless set-up in which the sensor serves as a capacitor of an L-C tank. The inductor coil can be separately coupled with the sensor (Figure 1a), or it can be formed by winding an insulated wire directly on the sensor to minimize the device size (Figure 1b). The wireless interrogation is performed by an external antenna/inductor that is magnetically coupled with the L-C tank device (Figure 2). Proper choice of materials compatible with particular environments will offer broader opportunities such as in automobile and biomedical applications that respectively include air pressure monitoring in the tires [12] and bowel pressure detection [13]. The inherent environmental compatibility is a significant advantage because it allows us to circumvent constraints and problems associated with the packaging [14] that in general degrades device performance and cost effectiveness in the device manufacturing [15]. This paper is constituted as follows. Section 2 describes the working principle and design of the sensor. The details of the fabrication process for the L-C tank device and the results are presented in Section 3. Section 4 reports the results of experimental characterization for the elastomer material used in this effort as well as the developed L-C tank device and the demonstration of wireless sensing with the device. These experimental results are evaluated in conjunction with the theoretical analysis in Section 5, followed by discussion in Section 6. Section 7 concludes the overall effort. Portions of this paper have appeared in conference abstract form in [16, 17]

Sensors 2008. 8 2320 where E is the Young's modulus of the elastomer, 2Y and 2w are the length and width of the rectangle layer, respectively, A is a constant given by 3"( and S is a geometric parameter called shape factor, which is approximately represented for the S 2r(Y+W)(1-e) (3) where 2T is the resultant thickness of the layer upon the compression and So is the original shape factor with the initial thickness(2To) before the compression. The strain can be expressed as e=l-T/To. The final thickness determines the capacitance of the structure, C=E(4YW)/(2T), where a is the permittivity of the elastomer, and then the resonant frequency of the L-C tank, f=1/(2TVLC) where L is the inductance of the tank. The permittivity of polyurethane is reported to be stable over the pressure range that is involved in this effort [19]. With these, the ratio of the resonant frequency after the compression to the original one and that for capacitance can be coupled with the strain as T where Co and fo are the original capacitance and resonant frequency prior to the compression, respectively. Therefore, the relationship between the applied pressure and the ratio in the resonant frequency, ffo=F, can be expressed using Equations(1)and(4)as P 1|-E1+ (5) 2 F 3(y2+W For the configuration illustrated in Figure 1b, the two capacitive parallel plates with the indicated dimensions were microfabricated from stainless-steel sheets by micro-electro-discharge machining (EDM) in this effort. HEDM is an electrothermal micromachining technique that can be used to cut any type of electrical conductors including all kinds of metals and alloys [20]. The machining typically uses cylindrical tungsten electrodes that are precisely shaped to have diameter ranging between 5 and 300 um. Since these plates potentially have burrs at the edges as characteristic defects of the machining technique, the top plate was designed to be slightly smaller than the base plate(50-um offset from all sides of the base plate) to minimize probability of physical/electrical contact between the two plates at the edges. Having the offset also assists with the self-alignment of the two plates in the assembly step described in the subsequent section

Sensors 2008, 8 2320 where E is the Young’s modulus of the elastomer, 2Y and 2W are the length and width of the rectangle layer, respectively, A is a constant given by ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = + − Y W Y W A 10 11 2 3 4 , (2) and S is a geometric parameter called shape factor, which is approximately represented for the structure by ( ) ( ) e S T Y W YW S − = + = 2 1 0 (3) where 2T is the resultant thickness of the layer upon the compression and S0 is the original shape factor with the initial thickness (2T0) before the compression. The strain can be expressed as e=1-T/T0. The final thickness determines the capacitance of the structure, C = ε (4YW)/(2T) , where ε is the permittivity of the elastomer, and then the resonant frequency of the L-C tank, f = 1/(2π LC) , where L is the inductance of the tank. The permittivity of polyurethane is reported to be stable over the pressure range that is involved in this effort [19]. With these, the ratio of the resonant frequency after the compression to the original one and that for capacitance can be coupled with the strain as e T T C C f f = = = − ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ 1 0 0 2 0 , (4) where C0 and f0 are the original capacitance and resonant frequency prior to the compression, respectively. Therefore, the relationship between the applied pressure and the ratio in the resonant frequency, f/f0 =F, can be expressed using Equations (1) and (4) as ( ) 2 2 2 2 2 2 4 2 0 log 3 1 1 1 1 2 F Y W Y W E F EAS P ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + − ⎟ − + ⎠ ⎞ ⎜ ⎝ ⎛ = − . (5) For the configuration illustrated in Figure 1b, the two capacitive parallel plates with the indicated dimensions were microfabricated from stainless-steel sheets by micro-electro-discharge machining (μEDM) in this effort. μEDM is an electrothermal micromachining technique that can be used to cut any type of electrical conductors including all kinds of metals and alloys [20]. The machining typically uses cylindrical tungsten electrodes that are precisely shaped to have diameter ranging between 5 and 300 μm. Since these plates potentially have burrs at the edges as characteristic defects of the machining technique, the top plate was designed to be slightly smaller than the base plate (50-μm offset from all sides of the base plate) to minimize probability of physical/electrical contact between the two plates at the edges. Having the offset also assists with the self-alignment of the two plates in the assembly step described in the subsequent section

Sensors 2008. 8 2321 3. Fabrication In this effort, room-temperature-vulcanizing(RTv) liquid rubber of polyurethane was selected to form the elastomeric layer. This material offers mechanical robustness such as high tear and abrasive resistances, chemical resistance, and controllability of its softness over a wide range. It has been extensively used in medical implant applications [21] and was also used to fabricate micro/nanostructures for MEMs applications [22-24]. Other rubber materials such as polydimethylsiloxane(PDMS )that are formed with low-viscosity liquids are also potential candidates for the elastomer layer. Of course, mechanical properties such as plasticity limit, thermal expansion coefficient, would play a role in the final selection, as would considerations about manufacturing and Integration The fabrication process is illustrated in Figure 3. As mentioned earlier, two capacitive plates were patterned with HEDM using a Panasonic MG-ED72W system(step 1). The base and top plates were cut from type -304 stainless-steel sheets with thickness of 200 um and 50 um, respectively, using cylindrical electrodes with 190-um diameter(Figure 4a). The base plate was still connected to the original sheet through two tethers after the machining as shown in Figure 4a. a two-part polyurethane RTV liquid rubber(Poly 74-20, part A: polyurethane pre-polymer, part B: polyol, Polytel Development Co., PA, USA)with the softener(part C: plasticizer), which is vulcanized to very soft (20 Shore A)and robust rubber, was used to form the intermediate polymer layer. The softness of the rubber can be adjusted by changing the proportion of the softener to be mixed. This effort used a formulation of part A B: C=1: 1: 1. The mixed solution was applied to the upper surface of the base plate (step 2), and then the top plate was placed on it( step 3). In this step, the top plate is self-aligned to the base due to surface tension of the solution. After curing, the device was released as shown in Figure 4b by mechanically breaking the tethers(step 4). The measured thickness of the cured polyurethane layer was approximately 38 um. The thickness of the layer can be adjusted by controlling the amount of the solution to be applied. (In large scale production, many of the kinds of parameters that are used to control the thickness of photoresist in photolithography polymer viscosity, substrate spin speed, etc can be used in this context as well. )Finally, the device was coupled with an inductive coil: For the device in Figure 1b, the coil was formed by winding an enamel-coated copper wire(AWG 36, 40 turns)directly on the sensor and bonding the terminals on separate stainless-steel plates with conductive adhesive(step 5). The fabricated L-C tank shown in Figure 4c was measured to have nominal capacitance of 6.3 pF and inductance of 640 nH Measured resonant frequency and quality factor of the tank, which were probed via test leads shown in Figure 4c, were 106 MHz and 1 respectively. The measured resonant frequency of the tank is close to the theoretical frequency of about 80 MHz that is obtained from the measured capacitance and inductance of the tank he capacitive structure was also coupled with and centered in a larger circular coil(5-mm diameter, 5 turns) formed using AWG 40( 80 um) enamelled copper lead. This configuration was selected for preliminary wireless testing to enlarge the magnetic coupling coefficient [25] between the device and the external antenna/coil while reducing the negative impact of eddy current generated in the stainless-steel plates. The use of conducting adhesive between the stainless-steel plates(without surface preparation)and copper leads of the coil with a conductive adhesive provided high contact resistance between them. This caused the low quality factor mentioned earlier, which limits the

Sensors 2008, 8 2321 3. Fabrication In this effort, room-temperature-vulcanizing (RTV) liquid rubber of polyurethane was selected to form the elastomeric layer. This material offers mechanical robustness such as high tear and abrasive resistances, chemical resistance, and controllability of its softness over a wide range. It has been extensively used in medical implant applications [21] and was also used to fabricate micro/nanostructures for MEMS applications [22-24]. Other rubber materials such as polydimethylsiloxane (PDMS) that are formed with low-viscosity liquids are also potential candidates for the elastomer layer. Of course, mechanical properties such as plasticity limit, thermal expansion coefficient, would play a role in the final selection, as would considerations about manufacturing and integration. The fabrication process is illustrated in Figure 3. As mentioned earlier, two capacitive plates were patterned with μEDM using a PanasonicTM MG-ED72W system (step 1). The base and top plates were cut from type-304 stainless-steel sheets with thickness of 200 µm and 50 µm, respectively, using cylindrical electrodes with 190-µm diameter (Figure 4a). The base plate was still connected to the original sheet through two tethers after the machining as shown in Figure 4a. A two-part polyurethane RTV liquid rubber (Poly 74-20, part A: polyurethane pre-polymer, part B: polyol, Polytek Development Co., PA, USA) with the softener (part C: plasticizer), which is vulcanized to very soft (<20 Shore A) and robust rubber, was used to form the intermediate polymer layer. The softness of the rubber can be adjusted by changing the proportion of the softener to be mixed. This effort used a formulation of part A:B:C=1:1:1. The mixed solution was applied to the upper surface of the base plate (step 2), and then the top plate was placed on it (step 3). In this step, the top plate is self-aligned to the base due to surface tension of the solution. After curing, the device was released as shown in Figure 4b by mechanically breaking the tethers (step 4). The measured thickness of the cured polyurethane layer was approximately 38 μm. The thickness of the layer can be adjusted by controlling the amount of the solution to be applied. (In large scale production, many of the kinds of parameters that are used to control the thickness of photoresist in photolithography – polymer viscosity, substrate spin speed, etc. – can be used in this context as well.) Finally, the device was coupled with an inductive coil: For the device in Figure 1b, the coil was formed by winding an enamel-coated copper wire (AWG 36, 40 turns) directly on the sensor and bonding the terminals on separate stainless-steel plates with conductive adhesive (step 5). The fabricated L-C tank shown in Figure 4c was measured to have nominal capacitance of 6.3 pF and inductance of 640 nH. Measured resonant frequency and quality factor of the tank, which were probed via test leads shown in Figure 4c, were 106 MHz and 1.9 respectively. The measured resonant frequency of the tank is close to the theoretical frequency of about 80 MHz that is obtained from the measured capacitance and inductance of the tank. The capacitive structure was also coupled with and centered in a larger circular coil (5-mm diameter, 5 turns) formed using AWG 40 (φ 80 μm) enamelled copper lead. This configuration was selected for preliminary wireless testing to enlarge the magnetic coupling coefficient [25] between the device and the external antenna/coil while reducing the negative impact of eddy current generated in the stainless-steel plates. The use of conducting adhesive between the stainless-steel plates (without surface preparation) and copper leads of the coil with a conductive adhesive provided high contact resistance between them. This caused the low quality factor mentioned earlier, which limits the

Sensors 2008. 8 2324 Figure 5. Pressure vs strain measured with a 3-mm-cubic polyurethane rubber sample 5000.o 267KPa 20 E=67KPa 0.2 0.3 4.2. Characterization of the L-C tanks and Wireless sensing Tests The fabricated devices were tested with both hard-wired and wireless set-ups. In the wired set-up the tank with the directly wound coil shown in Figure 4c was placed in a pressurized chamber with air, and the variation of its reactance peak with applied pressure was monitored by an HP4195 spectrum analyzer using the test leads transferred through the chamber wall. This reactance is an output that assumes a series capacitor-resistor model of the analyzer. This model exhibits the most distinct shift in e set-u Figure 6 illustrates the set-up used for the wireless sensing tests. The L-C tank device, which was coupled with the 5-mm-diameter coil, was placed within another sealed chamber with thin plastic walls, and magnetically coupled with an external coil (o-10 mm, 185 nh) through the chamber walls The resonant frequency of the tank was monitored by tracking the frequency of the characteristic peak, which was reflected by the resonance of the tank, in an S-parameter(sl1)of the external coil that was connected to a network-spectrum analyzer while changing pressure inside the chamber. The rF power fed from the analyzer to the external coil was 100 mW in this test. The chamber was filled with deionized (DI)water for this wireless experiment to demonstrate operation in liquid; the device provided a distinct resonant peak without packaging/coating for electrical protection. With the same set-up, the frequency dependence on temperature was also evaluated at atmosphere pressure Temperature of the chamber was controlled by changing the distance between the device and a source of heat located outside of the chamber as shown in Figure 6 Figure 7 shows the shifts of the reactance peaks measured with the wired set-up due to gauge pressure change in 69 KPa steps up to 345 KPa at room temperature(20C). The result is plotted in Figure 8a, indicating the response of 2.6-9.6 Hz/Pa and sensitivity of 1l-39 ppm/KPa in this pressure range. The nonlinear behavior observed in the plot is consistent with the measured response in the compression modulus of the polyurethane rubber, i.e., the layer becomes stiffer as it is squeezed resulting in the reduced response

Sensors 2008, 8 2324 Figure 5. Pressure vs. strain measured with a 3-mm-cubic polyurethane rubber sample. 4.2. Characterization of the L-C tanks and Wireless Sensing Tests The fabricated devices were tested with both hard-wired and wireless set-ups. In the wired set-up, the tank with the directly wound coil shown in Figure 4c was placed in a pressurized chamber with air, and the variation of its reactance peak with applied pressure was monitored by an HP4195 spectrum analyzer using the test leads transferred through the chamber wall. This reactance is an output that assumes a series capacitor-resistor model of the analyzer. This model exhibits the most distinct shift in the set-up. Figure 6 illustrates the set-up used for the wireless sensing tests. The L-C tank device, which was coupled with the 5-mm-diameter coil, was placed within another sealed chamber with thin plastic walls, and magnetically coupled with an external coil (φ ~10 mm, 185 nH) through the chamber walls. The resonant frequency of the tank was monitored by tracking the frequency of the characteristic peak, which was reflected by the resonance of the tank, in an s-parameter (s11) of the external coil that was connected to a network-spectrum analyzer while changing pressure inside the chamber. The RF power fed from the analyzer to the external coil was 100 mW in this test. The chamber was filled with deionized (DI) water for this wireless experiment to demonstrate operation in liquid; the device provided a distinct resonant peak without packaging/coating for electrical protection. With the same set-up, the frequency dependence on temperature was also evaluated at atmosphere pressure. Temperature of the chamber was controlled by changing the distance between the device and a source of heat located outside of the chamber as shown in Figure 6. Figure 7 shows the shifts of the reactance peaks measured with the wired set-up due to gauge pressure change in 69 KPa steps up to 345 KPa at room temperature (20 °C). The result is plotted in Figure 8a, indicating the response of 2.6-9.6 Hz/Pa and sensitivity of 11-39 ppm/KPa in this pressure range. The nonlinear behavior observed in the plot is consistent with the measured response in the compression modulus of the polyurethane rubber, i.e., the layer becomes stiffer as it is squeezed, resulting in the reduced response