Numerical study of the effects of film properties to the mass sensitivity of surface acoustic wave sensors

We present a computational study of the sensitivity of SAW chemical sensors. Such sensors detect chemical vapors by showing resonant frequency shifts caused by the mass variations of chemoselective films coated on the pass way of surface waves. The sensitivity of this kind of sensors is significantly influenced by the film properties. The optimal design requires the quantitative understanding of the effects of film geometry, stiffness and conductivity on the sensor sensitivity. Previous analytical models for the study of the effects of film properties on the sensitivity of SAW chemical sensors assume that the film thickness is negligible compared to the central wave length of surface waves. As the central frequency of a SAW chemical sensors increases, the central wavelength decreases and the effects of film thickness may become significant. We have studied these effects using a finite element model [Xu, 2001]. The model has been developed based on the Galerkin method by discretizing the equations of wave propagation in piezoelectric materials and can be used for direct simulation of electromechanical wave propagation in SAW chemical sensors. Our results show linear response of frequency shift to film density variation and indicate better sensitivity for thicker and softer films within the range of these parameters we have studied. The computational results correlate well with the experimental results obtained by I. D Armonv, et al [Avramov, 2002]. In addition, our simulation shows that films with relative low stiffness can significantly delay the surface waves, and an increase in density can prolong this delay. The implications of these findings on the optimal design of SAW chemical sensors are discussed