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, 171Sensors 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 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 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]