Differential measurement of piezoresistive transduction for silicon-based MEMS resonators

A differentially piezoresistive readout is implemented, for the first time, in single-crystal-silicon (SCS) bulk-mode resonators driven by electrostatic force, performing effective feedthrough cancellation using simple piezoresistors from their supports while maximizing driving areas of capacitive transduction. The SCS resonators were fabricated using a conventional SOI-MEMS technique together with a polysilicon refill process to enable 80nm gap spacing for attaining high electromechanical coupling coefficient. In the resonator design, the corner supporting beams of the resonator serve not only mechanical supports but piezoresistors for motional signal detection, hence greatly simplifying the overall resonator configuration without the need of additional doping (i.e., extra lithography step) for piezoresistive sensing. In addition, the fabricated resonators could be operated either in a capacitive sensing or piezoresistive readout under the same driving conditions. A comparison is also provided for these readouts, both of which exhibit relatively high feedthrough levels. To eliminate feedthrough signals from parasitics, a differential measurement technique of piezoresistive transduction is proposed in this work, demonstrating more than 25 dB improvement on the feedthrough level as compared to its single-ended piezoresistive counterpart as well as purely capacitive sensing readout. Furthermore, the thermal stability of the resonator with the piezoresistive readout was also greatly enhanced with 7× improvement as compared with its capacitive detection.