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Design and characterization of an air-coupled capacitive ultrasonic sensor fabricated in a CMOS process

This work presents a CMOS micromachined capacitive sensor for the detection of acoustic pressure transmitted through the air. The microstructure has a suspended plate of 65 µm in diameter that produces a sensing capacitance of 35 fF and a resonant frequency of 1.2 MHz. The post-CMOS fabrication can effectively reduce the parasitic capacitance to enhance the signal-to-noise ratio. The measured input-referred circuit noise is 0.35 µV Hz1/2 at 40 kHz. The measured sensor output is 3.5 µV under a dc bias of 10 V, equivalent to a capacitance change of 1.7 × 10−2 aF. The corresponding electrode displacement and acoustic pressure are 4.1 × 10−2 Å and 1.3 Pa, respectively.