The effects of tight capacitive coupling on phase noise performance: A Lamé-mode MEMS oscillator study

In this work, we realize low-polarization-voltage (V_P) capacitive MEMS oscillators implementing vacuum packaged 50 nm-gap Lamé-mode silicon micromechanical resonators and investigate their phase noise (PN) performance. Two different oscillator configurations are studied; these are here referred to as differential-in-differential-out (DIDO) and single-in-differential-out (SIDO). Clear disparities in their respective PN profiles could be observed. The DIDO outperforms the SIDO with an improved close-to-carrier PN by more than 25 dB. This 17.6 MHz oscillator achieves a PN of -127 dBc/Hz at 1 kHz offset and -132 dBc/Hz far-from-carrier PN performance, which is competitive with state-of-the-art capacitive MEMS oscillators but with lowest V_P in this work. The measured PN is found to be dependent on V_P for our oscillators, which is probably due to the nonlinear effects augmented by the tight electromechanical coupling.