A CMOS-MEMS Thermal-Piezoresistive Oscillator for Mass Sensing Applications

We present a thermally driven piezoresistively sensed resonator primarily consisting of backend of line (BEOL) CMOS material for mass sensing applications. A detailed analysis of the operation mechanism has been performed to develop a mathematical and an electrical equivalent model. The thermal-piezoresistive resonator (TPR) is composed predominantly of low loss materials like silicon dioxide (SiO2) and polysilicon for drive and sense. A differential configuration with drive and sense isolation was employed to significantly reduce the feedthrough level. Theoretical and simulated values are validated with experimental data. The proposed design has a resonance frequency of 5.13 MHz with a transconductance (gm) of 7.8 μS in the vacuum (-3torr) for 1.2-mW bias while gm of 5 μS in the atmospheric pressure for 2.7-mW bias power at 5.09 MHz. One of the highest reported quality factors (Q) of 2600 for CMOS-MEMS was achieved in air, while the same in the vacuum was >10000. The thermal-piezoresistive oscillator (TPO) has Allan Deviation of 80 ppb and 20 ppm in ambient pressure using a lock-in amplifier with a phase-locked loop (PLL) and interface circuit using commercial amplifiers on printed circuit board (PCB), respectively. Using silver nanoparticles, the mass sensitivity of 24.96 kHz/ng was measured. The extracted mass resolution in air was 16.3 fg, thus having great potential to serve as an aerosol sensor.