摘要
This study presents a vertical dual-gap capacitive micromachined ultrasonic transducer (CMUT) that achieves dual-frequency operation in a single element. The device utilizes a two-degree-of-freedom (2-DOF) structure, operating in In-phase (IP) and Out-of-phase (OOP) modes to generate distinct frequency responses. Fabricated using a standard 0.18 \mu \mathrm{m} CMOS process followed by a five-step maskless post-process, the device features two vertically stacked gaps: a 400 nm titanium nitride composite (TiN-C) gap and a 125 nm metal-insulator-metal (MIM) gap. Experimental characterization confirms that the TiN-C gap transduction generates acoustic output in both low-frequency (LF) ( 3-6 \text{MHz} ) and high-frequency (HF) bands. In contrast, the MIM gap transduction provides a clean HF response ( 11-12.5 \text{MHz} ). This functional separation enables dual-frequency capability, eliminating the need for varied membrane sizes or complex T/R switching. The proposed architecture significantly improves area efficiency and simplifies system design for ultrasound applications such as contrast-enhanced ultrasound (CEUS) and multiscale imaging.