Research On Micromachined Resonant Beam Pressure Sensors

A microstructure resonant pressure sensor measures applied pressure by detecting change of resonant frequency of its mechanical resonator. The sensor is characterized by its very small size, high accuracy, excellent stability and convenient interfacing with digital electronics, showing broad application prospects. Electrostatic, electromagnetic and optical thermal excitations have been used successfully in resonant pressure sensors, however, the fabrication and detection of these sensors are relatively difficult to realize. This paper presents the design, modeling, fabrication and test of electrothermally excited resonant beam pressure sensors. A design of using LPCVD silicon-rich silicon nitride of low residual stress as the resonant beam is proposed based on technology of sacrificial porous silicon and a new type peninsula structure is also proposed for high pressure sensitivity. Using commercial FEA software, computer simulation is carried on the characteristics of the resonator and the pressure sensitivity for several silicon/silicon nitride beam structures. Resonant pressure sensors are fabricated by IC process and MEMS technology and bonded to a stress isolating mechanical structure and sealed into an evacuated package. Both an open-loop and a close-loop measuring system are established for detecting vibration of the resonant beams and the response of the resonant frequency at different applied pressure loads, based on special lockin amplification technology. The relationships of the resonant frequencies with excitation conditions, applied pressures and environmental temperatures for several structures are measured by these systems.Silicon and silicon nitride beam resonant pressure sensors with complete packages have been made successfully. The quality factor Q values are about 2000 in air and rise to ~40000 in vacuum. Both beams show a little difference in Q values. The full scale of the pressure sensor is between 100~600kPa, the sensitivities are from 5 to 200Hz/kPa for different structure. The new type peninsula design exhibits sensitivity at least 10 times higher than the rectangular cavity design. There is a satisfactory agreement between simulation and experimental results.