SAW Pressure Sensor for Vacuum Control Applications

A new kind of surface acoustic wave (SAW) sensor has been developed in order to measure sub-atmospheric pressure below 100 mTorr with accuracy better than 0.1 mTorr. It provides an efficient measuring solution in a pressure range inaccessible in past by conventional diaphragm-based SAW sensors. Indeed, due to the small bending force in low pressure and limited sensitivity, diaphragm-based SAW sensors are only suited to monitor relatively high pressure with a precision hardly better than 0.5 Torr. In order to reach precision level better than I mTorr at sub-atmospheric pressure for vacuum technology applications, a radically different SAW-based solution is desired. Our device aims to measure sub-atmospheric pressure less than 100 mTorr with a threshold resolution better than 0.1 mTorr. The concept is similar to the one used by Pirani pressure gauges. However, it is claimed that a heated and suspended SAW device may have better sensitivity. A theoretical model based on the basic concepts of gas kinetic theory and thermodynamics is presented. The validity of the model is checked by comparison between theoretical and experimental results.