High- $Q$ Integrated CMOS-MEMS Resonators With Deep-Submicrometer Gaps and Quasi-Linear Frequency Tuning

Integrated CMOS-MEMS free-free beam resonators using pull-in mechanism to enable deep-submicrometer electrode- to-resonator gap spacing without interference in their mechanical boundary conditions (BCs) have been demonstrated simultaneously with low motional impedance and high Q. The key to attaining high Q relies on a decoupling design between pull-in frames for gap reduction and mechanical BCs of resonators. In addition, the use of metal-SiO₂ composite structures has been proved to greatly benefit the thermal stability of CMOS-MEMS resonators. Furthermore, tuning electrodes underneath pull-in frames were designed to offer “quasi-linear” frequency tuning capability where linear relationship between tuning voltage and frequency was achieved. In this paper, CMOS-MEMS free-free beam resonators with gap spacings of 110, 210, and 275 nm, respectively, were tested under direct one-port measurement in vacuum, demonstrating a resonator Q greater than 2000 and a motional impedance as low as 112 kΩ and, at the same time, allowing quasi-linear frequency tuning to achieve a total tuning range of 5000 ppm and a sensitivity of 83.3 ppm/V at 11.5 MHz with zero dc power consumption. Such a resonator monolithically integrated with a CMOS amplifier, totally occupying a die area of only 300 μm × 130 μm, was also tested with enhanced performance, benefiting future timing reference and RF synthesizing applications.