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Humidity Sensitivity of Lamb Waves on Composite Polyimide/ZnO/Si3N4 Structure

The propagation characteristics and sensitivity to humidity of Lamb waves in a composite polyimide/ZnO/silicon nitride structure have been theoretically and experimentally studied. Measured values of phase velocity and the electromechanical coupling constant almost correspond to theoretical values. If we assume that the humidity dependence of polyimide stiffness almost coincides with that of polymethyl methacrylate, it is conceivable that in the low-humidity range, the change in phase velocity would be mainly due to change in polyimide density, and in the intermediate humidity range, due to change in polymide stiffness. When we normalize the sensitivity of phase velocity to humidity with the phase velocity and polyimide thickness, the sensitivity of Lamb waves is about six times that of Rayleigh waves.