Surface acoustic wave substrates for gas sensing applications

The bulk of the previous research on surface acoustic wave (SAW) gas sensors has focused on the sensing film itself and on gas-film interactions. Very little has been done to optimize the SAW substrate for gas sensing applications. The fact that many films are known to be more sensitive to target gases at certain temperatures other than room temperature has not been factored into substrate selection. In order to identify temperature stable substrates, a systematic search of various orientations of quartz has been made. Theoretical calculations show that the rotated Y-cut of quartz offers temperature compensated cuts over a broad range of temperatures. Each orientation in this cut has a parabolic temperature variation in Δτ/τ, where τ is the delay time from the input to the output of the SAW device, with a unique turnover temperature (the temperature where Δτ/τ is constant). These turnover temperatures range from -35°C to well over 500°C. Experimental data on selected rotated Y-cuts in quartz have exhibited satisfactory agreement with the theory. Finally, an H₂ S sensor with an optimum operating temperature of 200°C has been fabricated using 27° rotated Y-cut quartz. The stability of this sensor is shown to be superior to SAW gas sensors using YZ-LiNbO ₃ substrates