A Bipolar-Biased Differential CMOS-MEMS CMUT

This paper presents, for the first time, a monolithic differential capacitive micromachined ultrasound transducer (CMUT) receiver (RX) front-end based on CMOS-MEMS technology that exploits bipolar DC-biasing technique. By applying positive and negative (a.k.a. “bipolar”) dc bias voltages to distinct CMUTs in an array, the pressure-induced out-of-phase motional currents can be desirably formed and sourced out from individual membranes according to the corresponding bias polarity, hence providing a device-level differential operation. To enhance the electrostatic transduction efficiency, a Capacitor-Top-Metal (CTM) etching technique is developed for CMUT fabrication, yielding an effective capacitive transduction gap of only 470 nm. A fully-differential transimpedance amplifier (TIA) and a time-gain compensation (TGC) variable gain stage are also integrated on-chip to form an analog RX prototype and consume merely 0.54 mW. The proposed differential CMUT RX occupies a transducer area of 0.0625 mm2 and can be operated at a low bipolar-bias of ± 5V, showing a center frequency of 1.69 MHz, a 66% fractional bandwidth (FBW), and a sensitivity of 1.14 mV/kPa immersed in water. The proposed differential CMUT RX is implemented in TSMC 0.18 μm standard CMOS.