A Generic TiN-C Process for CMOS FEOL/BEOL-Embedded Vertically-Coupled Capacitive and Piezoresistive Resonators

An extended titanium nitride composite (TiN-C) CMOS-MEMS platform for high-resolution oscillating sensors is presented in this work that attempts to simultaneously validate the (i) vertically-coupled resonator (VCR) pair structure and (ii) embedded piezoresistive (PZR) transduction mechanism in a single chip. With a vertically-coupled design concept, it is beneficial to significantly increase the linearity through the high mechanical stiffness couplers between VCR pair while minimizing the device footprint. On the other hand, the PZR sensing technique is further employed to yield a better signal-to-noise ratio (SNR) with a high gauge factor from non-silicided polysilicon resistor. To accomplish such a design concept in standard CMOS, the enhanced TiN-C platform with "substrate-etching-first" approach is proposed to prevent the tungsten vias (W-VIAs) being etched during the structure release step. As a result, we have successfully demonstrated a 3-array VCR with more than 3× power handling capability in comparison to the conventional planar 9-array counterparts. Moreover, the sub-mW capacitive driving/poly-2 sensing scheme offers a 7× reduction on the background floor than purely capacitive operation based on a single FEOL-embedded VCR with Q > 4,000.