CMOS-MEMS thermal-piezoresistive oscillators with high transduction efficiency for mass sensing applications

In this work, a high-performance mass sensor utilizing CMOS-MEMS thermal-piezoresistive resonator (TPR) sustained by an instrumental Lock-in and PLL circuit for oscillation is demonstrated. Under a low dc power consumption of the device with only 1.75 mW, the motional transconductance (g
_m
) of the proposed TPR reaches record-high values both in vacuum (118.4 μA/V) and air (16.96 μA/V) among all reported CMOS-MEMS TPRs [1] and even on par with single crystal silicon (SCS) TPRs [2]. The unique design of a butterfly-shaped TPR with its low thermal capacitance (C
_th
) actuator beams is the key to improve the transduction efficiency and sensor sensitivity. The mass resolution of the proposed thermal-piezoresistive oscillator (TPO) attains 29.8 fg, which is extracted from the measured Allan deviation of 89 ppb. To verify the mass sensing capability, a pico-liter ink jet printing setup was used to demonstrate the real time response and frequency shifts corresponding to a number of droplets printed onto the proof-masses of the TPO with a high sensitivity of 1.946 Hz/pg, well suited for future aerosol detection.