Thermal-Piezoresistive Resonators and Self-Sustained Oscillators for Gas Recognition and Pressure Sensing

This paper presents experimental and theoretical investigation of the response of microscale dual-plate thermal-piezoresistive resonators (TPRs) and self-sustained oscillators (TPOs) to different gases and pressures. It is demonstrated that the resonant frequency of such devices follow particular trends in response to the changes in the surrounding gas and its pressure. A mathematical model is derived to explain the damping dependent frequency shift characteristic of the TPO. The solution of the model indicates that the stiffness of the actuator beam decreases as the value of the damping coefficient drops at lower gas density caused by the change in the gas molecular mass or pressure. When operated in the TPR mode (linear operation), however, the frequency shift of the same silicon structure is mainly a function of gas thermal conductivity. The two different sensing mechanisms are confirmed by the measurement results showing opposite frequency shift for the TPR and TPO in helium-nitrogen mixtures. In pressure tests, frequency shifts as high as -2300 ppm are measured for a TPO by changing the air pressure from 84 to 43 kPa.