Rayleigh waves on Love-wave substrates for touch-sensitive panels

Water droplets can produce false touch signals on conventional acoustic touchscreens which use Rayleigh waves on glass. Substituting horizontally polarized shear waves such as Love waves avoids this defect. At the 2000 Symposium, we reported on experiments with Love waves on a layered substrate consisting of glass microsheet bonded to a glass plate via a thin polymer layer. Even though these substrates are designed to carry Love waves, they still may be susceptible to parasitic signals due to propagation of Rayleigh-like modes. Here we report on an investigation of these Rayleigh-like modes. A general mathematical method has been created for computing Rayleigh-like surface modes on three-layer substrates. Boundary conditions, as a function of assumed phase velocity, are put in matrix form. When a mode exists with the assumed phase velocity, the matrix determinant is zero. The mode´s depth profile may be computed from the phase velocity and boundary conditions. The depth profile is related to the mode´s sensitivity to surface perturbations. The frequency dependence of phase velocity determines group velocity. Code has been developed for the numerical analysis of phase velocity, group velocity and sensitivity as a function of frequency and substrate, design parameters. Experimental data will be presented; they correlate well with predictions. Thus, a useful computational tool now exists for the analysis of Rayleigh-like waves on layered substrates