Theory of interdigital couplers on nonpiezoelectric substrates

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Abstract
This paper is concerned with the theory of excitation of surface acoustic waves on a nonpiezoelectric material, by using an interdigital electrode structure overlaid with a piezoelectric film. The impedance and capacity of such transducers have been worked out. The results are given in terms of Δv/v, the perturbation in acoustic wave velocity when a perfect conductor is placed in the position of the transducer. A computer program which calculates surface wave propagation on a layered medium consisting of a film on a substrate, with arbitrary elastic and piezoelectric properties for both media, has been used to determine Δv/v, and hence the coupling coefficients and radiation resistance of the interdigital transducer. As a function of piezoelectric film thickness, two peaks in coupling are observed. The results for the interdigital couplers are in quantitative agreement with experiments employing single‐crystal materials. Data for experiments using sputtered piezoelectric films also give qualitative agreement with the theory. If high‐quality crystalline layers are used, the theory suggests that interdigital couplers on nonpiezoelectric delay lines with properties comparable to or better than the present piezoelectric delay lines may be achievable.

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