A Cavity-less Micromachined Capacitive Pressure Sensor for Wireless Operation in Liquid Ambient

This paper reports a micromachined capacitive pressure sensor that does not use the traditional cavity and diaphragm, and its use in aqueous environment. The device is fabricated with two micromachined plates of stainless steel and an intermediate polymer layer that is soft enough to deform in the target pressure range. A polyurethane room-temperature-vulcanizing liquid rubber of 38-μm thickness is used as the deformable material. For frequency-based interrogation of the capacitance, a passive inductor-capacitor tank is fabricated by combining the capacitive sensor with an inductive coil, which is formed using an 80-μm-diameter copper wire. Wireless sensing in liquid is demonstrated 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 estimate obtained by a bonded elastomer model. The geometrical impact on the frequency response is also evaluated.