Design and characterization of the immersion-type capacitive ultrasonic sensors fabricated in a CMOS process

Abstract

This work presents the CMOS micromachined capacitive sensors for ultrasound detection in water. The sensing membranes are released by a post-CMOS metal etch and sealed by using either 2−μm silicon dioxide (type A) or parylene-D (type B). Nine membranes, each with an inner diameter of 60 μm, form a single detection unit with a capacitance value of 292.5 fF. An alternating voltage bias is applied to the membranes for stabilizing the sensed signals which would otherwise attenuate over time due to trapped charges between electrodes. Resonant frequencies of the type A and type B sensors in water are 8.8 and 6 MHz, with fractional bandwidths of 0.38 and 0.40, respectively. The measured sensitivities are 151.0 and 369.8 mV pp/MPa/V, and the associated capacitance changes are 7.55 and 18.5 fF/MPa. The equivalent noise pressures, based on the measured thermal noise, are 3.3 and 1.35 Pa/√Hz at a 1-V membrane bias.