CMOS-based Capacitive Micromachined Ultrasonic Transducers operating without external DC bias

In this study, a high sensitivity CMOS-based Capacitive Micromachined Ultrasonic Transducer (CMUT) is developed. This CMUT device provides high performance for operations with and without DC bias. It is implemented based on the TSMC 0.35μm 2P4M CMOS-MEMS process. The hypothesis is that driven by a commercial pulser, charges were injected into charge traps in CMUT and they were temporarily stored providing an internal electric field comparable to that created by an external DC bias. Therefore, the external DC bias is not needed but high sensitivity can still be achieved. From measurements, the device can produce ultrasound with a center frequency of 7-8 MHz, a fractional bandwidth of 75-85%, a sensitivity of 2.9mV/kPa, and an acoustic filed of 760kPa. Moreover, an image of a coin was reconstructed to demonstrate its performance. Compared with previous CMOS-based CMUTs, our device has good sensitivity and a wide bandwidth. The zero-bias CMUT based on a commercial CMOS process was successfully demonstrated with several potential applications. Nonetheless, our hypothesis on the charge trapping was also tested with discrepancy that still needs further investigation.