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

This work presents the CMOS micromachined capacitive sensors for ultrasound detection in water. The sensing membranes with a 60 µm diameter are released through small etchant holes of 2 µm × 2 µm by a post-CMOS metal etch and sealed with the thinnest possible silicon dioxide (type A) or parylene-D film (type B). Nine membranes form a single detection unit with a capacitance value of 292.5 fF. Convenient routing, which is desired for making a large two-dimensional array, is allowed with the detection circuits being placed directly beneath the sensing membranes. An alternating voltage bias is applied to the sensing electrodes for stabilizing the sensed signals which would otherwise attenuate over time due to trapped charges between electrodes. Resonant frequencies of type-A and type-B sensors in water are 8.8 and 5.8 MHz, with fractional bandwidths of 0.43 and 0.55, respectively. The measured sensitivities are 151.0 and 369.8 mVpp MPa−1 V−1. The equivalent noise pressures, based on the measured thermal noise, are 3.3 and 1.35 Pa Hz−1/2 at a 1 V membrane bias.