Fully differential CMOS-MEMS square-plate oxide resonators with embedded poly-silicon electrodes

A fully differential CMOS-MEMS square-plate oxide (SiO₂) resonator with embedded polysilicon (PolySi) electrodes fabricated using a conventional 0.35 μm CMOS process has been demonstrated with Q > 4,000 and motional impedance R_m of 67.3 KΩ at 6.89 MHz. The key to attaining such performance involves leveraging high-Q materials such as SiO₂ and PolySi available from the CMOS back-end-of-line stacks. The composite SiO₂/PolySi structure also facilitates the electrical routing for a fully differential drive/sense configuration to suppress capacitive feedthroughs. In addition, the combination of large transduction areas and high-Q flexural-plate mode from the square-plate resonator significantly reduces R_m as compared to our previously developed CMOS-MEMS resonators. This technology paves a way toward monolithically integrated CMOS-MEMS oscillators and filters for future single-chip transceivers in wireless communications.