A System Based on Capacitive Interfacing of CMOS With Post-Processed Thin-Film MEMS Resonators Employing Synchronous Readout for Parasitic Nulling

Abstract

Thin-film MEMS resonators fabricated at low temperatures can be processed on CMOS ICs, forming high-sensitivity transducers within complete sensing systems. A key focus for the MEMS devices is increasing the resonant frequency, enabling, among other benefits, operation at atmospheric pressure. However, at increased frequencies, parasitics associated with both the MEMS-CMOS interfaces and the MEMS device itself can severely degrade the detectability of the resonant peak. This work attempts to overcome these parasitics while providing isolation of the CMOS IC from potentially damaging sensing environments. To achieve this, an interfacing approach is proposed based on capacitive coupling across the CMOS IC passivation, and a detection approach is proposed based on synchronous readout. Results are presented from a prototype system, integrating a custom CMOS IC with MEMS bridge resonators. With the MEMS resonators fabricated in-house at 175°C on a separate substrate, readout results with multiple different resonators are obtained. In all cases, the IC enables detection with >20 dB SNR of resonant peaks that are only weakly detectable or undetectable directly using a vector-network analyzer (VNA).