Self-sustained micromechanical resonant pressure sensors

This work presents a new gas pressure sensing technique based on self-sustained oscillations in micromechanical thermal-piezoresistive resonators. Electrothermal force generation in such structures can be coupled to the structural stress through the piezoresistive effect. This could lead to spontaneous mechanical vibrations in the resonant structures upon application of a large enough DC bias current. It has been demonstrated via measurements that resonant frequency of such oscillators is sharply dependent on the surrounding gas pressure. For a 3.46MHz thermal piezoresistive oscillator, frequency shift of -2300ppm was observed by changing the surrounding air pressure from 84kPa to 43kPa. In addition, the same structure was also operated in a forced excitation mode as a resonator actuated by a combination of DC and AC currents. Interestingly, the frequency shift in the self-sustained oscillation mode is far more significant than the frequency shift in forced resonance mode and is opposite in direction. In order to explain this observation, a mathematical model has been developed for the thermal-piezoresistive oscillators. The final solution from this model indicates that the dynamic stiffness of the spontaneously vibrating structures decreases as the value of the damping coefficient is reduced at lower gas pressures.