A capacitive absolute-pressure sensor with external pick-off electrodes

This paper describes a capacitive absolute-pressure sensor in which the sealed lead transfer is eliminated. The pick-off capacitance is between a flap or skirt-like extension of the flexible diaphragm that reaches past the sealed cavity, and an electrode patterned on the substrate directly below this extension. The sidewall of the cavity is relatively narrow and flexible. Finite-element analysis is used to explore the relevance of various dimensional parameters and to estimate the sensitivity and temperature coefficients of the device. The device is fabricated from p++ Si on a glass substrate using the dissolved wafer process with three masking steps. The measurements of fabricated devices with 1 mm radius, 7 μm cavity height, and 8 μm wall thickness show −84 ppm kPa−1 sensitivity at room temperature in touch-mode operation. In non-touch-mode operation the sensitivity is significantly higher. A reference device with similar dimensions shows a less than 22 ppm K−1 temperature coefficient of offset below 150 ◦C.