A theoretical and experimental study of a conductivity-based surface acoustic wave gas microsensor

The operation of surface acoustic wave (SAW) gas sensors which depend upon the change in sheet conductivity of gas sensitive films has been examined both theoretically and experimentally. The sensor configuration consists of a SAW substrate coated with a thin film which has properties that vary with the presence of the target gas. Since the acoustic wave velocity and attenuation of the layered structure depends on the properties of the film and the substrate, the SAW velocity and attenuation become a function of gas concentration. A theoretical model has been developed which describes the variation of SAW velocity in a piezoelectric substrate when coated with a layer of variable conductivity. The structure has been examined using two different methods. The first method involves an iterative computer solution which solves the differential equations linking particle motion, stress, electric potential and electric displacement in the two media under the boundary conditions imposed by the physical structure. The second method consists of using a perturbation theory to develop a closed form equation to describe the change in SAW velocity and attenuation of the substrate when the film is present. While the first method is more accurate, the second method is more easily applied to a variable situation. The two methods have been applied to various layered structures, and the results of the perturbation approximation closely match those of the more accurate computer solution.; The theoretical model has been applied to a SAW sensor which uses a tungsten trioxide (WO{dollar}sb3{dollar}) film to detect hydrogen sulfide (H{dollar}sb2{dollar}S). Simultaneous measurements of the sheet conductivity of the film and the phase delay and attenuation of the SAW sensor were performed and compared with the theoretical predictions. The results of the experiments agreed with theory except for the case of velocity variations when the substrate was YZ-cut LiNbO{dollar}sb3{dollar}. The error in this case was probably due to inaccurate conductivity measurements of the WO{dollar}sb3{dollar} film. In addition the theoretical expression has been used to predict the operation of the SAW sensor under various conditions of operating temperature and exposure to H{dollar}sb2{dollar}S.