A Low Impedance CMOS-MEMS Capacitive Resonator Based on Metal-Insulator-Metal (MIM) Capacitor Structure

This letter presents the first nano-gap microelectromechanical systems (MEMS) resonator based on a Metal-Insulator-Metal (MIM) capacitor structure in standard CMOS. We develop an improved mask-less post-CMOS process to chemically remove the thin aluminum copper (AlCu) of the Capacitor-Top-Metal (CTM) with high selectivity to the adjacent materials, yielding a well-defined capacitive transduction gap of only 160 nm. The proof-of-concept resonator is designed in a standard TSMC 180 nm RF CMOS process, with a center frequency of 5.9 MHz, a quality factor of 1,000, and a low motional impedance of 5.7 kΩ under a medium dc-bias of 25V. Upon further optimizations, nano-gap CMOS-MEMS resonators based on the proposed CTM-etching technique are promising candidates for fully integrated oscillator applications.