High-Q, large-stopband-rejection integrated CMOS-MEMS oxide resonators with embedded metal electrodes

A novel metal wet etching process to release CMOS-MEMS resonators has been developed and demonstrated for the first time to enable SiO₂-rich resonator structure and embedded metal electrodes with well-defined anchors for Q enhancement. In virtue of exceptional selectivity of metal wet etchant to SiO₂ among CMOS layers, the use of release holes needed for most of isotropic etching processes could be eliminated, hence substantially preserve the integrity of resonator structures and their anchors. With such a maskless metal release process, capacitively-transduced oxide resonators monolithically integrated with readout circuitry using standard CMOS 0.35 μm 2P4M process have been fabricated and tested, showing resonator Q's up to 4,400, stopband rejections from 40 to 80 dB, centered at 3 MHz with maximum breakdown voltage of 250 V and better temperature coefficient of frequency (TC_f) compared with that of mere-metal CMOS-MEMS counterparts due to “SiO₂-rich” resonator configuration.