Platform Development for CMOS-MEMS Multi-Gap Capacitive Transducers

This work, for the first time, presents a platform for designing microelectromechanical system (MEMS) capacitive transducers featuring multiple transduction gaps implemented by a traditional
$0.18~\mu \text{m}$
1-Poly-6-Metal (1P6M) complementary metal-oxide-semiconductor (CMOS) technology. As a capacitive transducer, different sizes of the transduction gap will bring distinct pros and cons, and designers often need to make trade-off among various performance metrics. The proposed CMOS-MEMS platform provides three types of transduction gap spacing that cover from narrow to large in single chip with excellent circuit integration capability. Based on a partial etching technique applied on back-end-of-line (BEOL) metal layers while utilizing anti-reflective coating (titanium nitride, TiN) as electrodes, transduction gap spacing of 125 nm, 400 nm, and 930 nm can be fabricated simultaneously without damaging the integrated circuit. Among these three devices, the latest metal-insulator-metal (MIM) device, which features a 125 nm transduction gap and provides a record-low motional resistance, is introduced with detail design guideline and layout tutorial for the first time. The post-CMOS fabrication process of the multi-gap platform is validated using three simple clamped-clamped beam structures being verified as capacitive resonators with similar frequency for extracting their mechanical and electromechanical coupling characteristics. Finally, a dual-gap capacitive micromachined ultrasonic transducer (CMUT) array is illustrated as a feasible application of the proposed platform offering different transduction gaps for signal transmission and reception. [2023-0073]