In this work, experimental results of complementary metal-oxide-semiconductor (CMOS)-based capacitive micromachined ultrasonic transducers (CMUTs) operated in the collapsed mode will be reported. Our CMUT is fabricated by TSMC 0.35μm two poly Si and four metal layer (2P4M) CMOS-MEMS standard process followed by a post customized wet etching. Conventionally, the applied DC bias is 80% of the collapse voltage during the CMUT operation. Compared to the conventional operation, the CMUT membrane will be snapped-down on the bottom electrode and this will change the center frequency of the transducers. The collapsed voltage of our CMUT is designed at approximately 40V. The center frequency (in immersion) of our CMUT in conventional mode operation is 2.89MHz while the frequency is shifted to 9.12MHz in collapsed-mode operation. The sensitivity is proved to be 4-times larger in the collapse-mode operation because the increased electrical-mechanical coupling efficiency.

The comparison of single-electrode and double-electrode design will be reported as well. With the double-electrode design, the collapse voltage is first applied to the center electrode while a separated DC bias is applied to the side electrode to actuate our CMUTs. The maximum displacement of the CMUT membrane will be appeared on the side electrode. It is proved that CMUTs with the double-electrode design can transmit a higher output pressure and receive signals with a higher sensitivity compared to CMUTs with the single-electrode design. It is believed that the our CMUTs with standard CMOS-based process will broaden the application spaces such as in biomedical imaging and nondestructive evaluation.