dc triode sputtered zinc oxide surface elastic wave transducers

Abstract

A piezoelectric film layer in proximity to an interdigital metal electrode provides a simple means for the excitation and detection of surface elastic waves on nonpiezoelectric substrates. These film transducer structures can be effectively used for the study of the surface elastic wave properties of solids and the development of monolithic acoustic and electronic circuit assemblies. Theory predicts that high coupling efficiencies can be obtained using piezoelectric film transducer structures. The properties of dc triode sputtered zinc oxide film transducers on fused quartz and oxidized silicon have been investigated at frequencies near 16 and above 200 MHz. Quantitative data on coupling factor, capacitance, and phase velocity have been obtained on plated and unplated structures for film‐thickness‐to‐acoustic‐wavelength ratios from 0.01 to 1.0. The experimental data follow the functional dependencies predicted by theory but differ in certain quantitative aspects. The maximum coupling constants (k²) measured on fused quartz were 0.0043 for low‐frequency plated transducers and 0.024 for unplated transducers above 200 MHz. Acoustic propagation losses as low as 1.5 dB/μsec at 215 MHz and 8.0 dB/μsec at 630 MHz were measured. One‐microsecond delay lines were fabricated at 213 MHz using zinc oxide on fused quartz which had a total of 12 dB and a 9‐MHz 3‐dB bandwidth. The measurements show that the surface‐wave electroacoustic properties of dc triode sputtered zinc oxide films approach those of single‐crystal ZnO layers.