The Generation and Propagation of Acoustic Surface Waves at Microwave Frequencies

The generation and propagation of acoustic surface waves is reviewed with particular emphasis on the microwave-frequency range. Theoretical work on optimizing the generation efficiency and the bandwidth of interdigital transducers is compared with recent experimental results. The minimum Iinewidth of 0.9 /spl mu/m which can be produced by optical photolithographic techniques places an upper limit of about 1 GHz on the maximum frequency that can be generated at the fundamental mode. Overtone operation has been used to generate 3 GHz surface waves on LiNbO/sub 3/ but this method has the disadvantage of reduced efficiency plus the complication of volume-wave generation. A better solution for generation above 1 GHz is the fabrication of interdigital transducers by means of electron beam exposure of the photoresist. The surface-wave propagation loss gives a significant contribution to the total insertion loss of delay lines operating at microwave frequencies. Losses of 1.1 dB//spl mu/s and 3.8 dB//spl mu/s at 0.9 GHz and 2 GHz, respectively, have been measured for propagation along the Z-direction of Y-cut LiNbO/sub 3/ by means of a laser deflection method. Larger losses have been observed for quartz. The additional complexities for surface-wave propagation due to the anisotropic single-crystal substrates which are necessary at microwave frequencies are also described.