Abstract
In this work, we realize the low-polarization-voltage (Vp) capacitive MEMS oscillators implementing vacuum packaged 50nm-gap Lamè-mode silicon micromechanical resonators and investigate their phase noise performance. Single-crystal-silicon square-plate microresonators were fabricated by a foundry-oriented SOI-MEMS plus a poly-Si refill process. The devices were hermetically encapsulated at wafer-level using a eutectic bonding technique, which reduces air damping and enables high Q. The frequency of the resonators are around 17.6MHz and the extracted Q is about 20,000 while the motional impedance is around 6.7kΩ (Vp = 3V). We also report the design and characterization of high gain-bandwidth TIA (transimpedance voltage amplifier) which is composed of two stages: the inverter-based I-to-V stage and voltage gain amplifier. The tunable-gain TIA circuit is fabricated using 2P4M 0.35µm CMOS technology and has been demonstrated with the maximum gain of 80dBΩ, 3-dB bandwidth of 134MHZ. The fully-differential oscillators based on 0.35µm TIA achieve a phase noise of –92dBc/Hz at 1kHz offset and -117dBc/Hz far-from-carrier phase noise performance. In order to reduce the phase noise, we study the different oscillator configurations which are composed of commercial IC; these are here referred to as differential-in-differential-out (DIDO) and single-in-differential-out (SIDO). Clear disparities in their respective phase noise profiles could be observed. The DIDO outperforms the SIDO with an improved close-to-carrier phase noise by more than -127dBc/Hz at 1kHz offset and -132dBc/Hz far-from-carrier phase noise performance, which is competitive with state-of-the-art capacitive MEMS oscillators but with lowest Vp in this work.