A Fully Differential SOI-MEMS Thermal Piezoresistive Ring Oscillator in Liquid Environment Intended for Mass Sensing

In this paper, we present a silicon on insulator (SOI)-based MEMS oscillator that comprises a ring-shaped rotational mode resonator incorporating fully differential design for operation in aquatic environment using the thermal drive piezoresistive sense transduction. The differential scheme used in the resonator design enables high feedthrough cancellation, thus leading to floor value close to -62 dB with sufficient signal to feedthrough ratio (SFR) and allowing a later oscillator implementation. The proposed differential ring-shaped thermal piezoresistive resonator (DR-TPR) when made to operate in the rotational mode exhibits the special feature of minimizing the effect of viscous damping in liquid environment, attaining a quality factor value as high as 537 with a stopband rejection of 8.5 dB. The closed-loop measurement using the HF2LI lock-in amplifier with PLL from Zurich Instruments helps us to realize the differential ring-shaped thermal piezoresistive oscillator (DR-TPO) showing phase noise of -77.6 dBc/Hz at 1-kHz offset and -89.54 dBc/Hz at 10-kHz offset. The proposed DR-TPO in liquid environment can provide a frequency resolution of 6.95 Hz extracted from the measured Allan deviation, which corresponds to a minimum detectable mass of 4.11 pg, thus paving a way to both physical and biomedical detection in liquid.