Differentially Piezoresistive Sensing for CMOS-MEMS Resonators

A foundry-oriented capacitively driven CMOS-MEMS resonator using differentially piezoresistive sensing is successfully demonstrated to enable effective feedthrough cancellation with more than 20-dB feedthrough floor reduction as compared to its capacitive readout. The resonator is mainly formed by high-Q SiO₂ structure utilizing metal wet etching and XeF₂ release processes, while the polysilicon layer (originally CMOS gate poly material) embedded inside the resonator structure serves as a piezoresistor for vibratory detection. In addition, such a composite structure enabling electrical isolation realizes decoupling of the capacitive and piezoresistive transductions, allowing the selection (or switching) of the preferred readout scheme using the same resonator device. The proposed resonator consists of only one single capacitor for driving and a simple beam structure for both vibration and detection, therefore greatly simplifying the device design and facilitating future CMOS-MEMS implementation. This paper achieves resonator , more than 28-dB signal-to-feedthrough ratio, and two-times smaller motional impedance than that of the single-ended piezoresistive detection using the same device and driving condition. Furthermore, the piezoresistive operation offers a simple temperature compensation scheme for CMOS-MEMS resonators via the adjustment of the dc current through the piezoresistor, therefore showing 1.4-times improvement on thermal stability as compared to their capacitive readout.