A RESONANT PRESSURE SENSOR-BASED ON A SQUEEZED FILM OF GAS

A new resonating sensor for gas pressure is described. The device meas ures the resonance frequency of a silicon microstructure that contains a thin film of gas trapped in the structure by the squeeze-film effec t. The gas is confined in the structure during an oscillation cycle on ly by its viscosity. It exerts a measurable influence on the resonance frequency at a pressure of 10(-2) mbar, and provides the dominant spr ing force at pressures near one atmosphere, where the short-term stabi lity of the sensor is of the order 1/30 mbar. We present data showing that the resonance frequency is species independent, which implies tha t the compressions are isothermal. Damping, however, depends on the ga s viscosity, and the measurement of the oscillator Q value allows spec ies to be identified. In the device, the oscillating diaphragm is not under a pressure load so that long-term creep is not a consideration. Further, at higher pressures, the device directly measures the spring constant of a defined volume of gas with only a small contribution fro m the silicon spring constant. It therefore has potential as a long-te rm reference of pressure.