Zinc-oxide thin-film surface-wave transducers

The piezoelectric film layer transducer represents the most efficient method for generating and detecting surface acoustic waves on nonpiezoelectric substrates. ZnO, which has a strong piezoelectric effect and can readily be sputtered as an oriented crystalline composite on a wide variety of substrates, is a natural choice as the piezoelectric film layer. This paper summarizes a body of knowledge which has been developed on the characteristics of transducer quality ZnO film layers, and focuses attention on those sputtering parameters and microstructural properties which characterize a superior surface-wave transducer film. Requisite sputtering conditions are high substrate temperatures (150-300°C), modest deposition rates (0.5-1.0 µm/h), low background vapor pressures (<5 µm Hg) and an ultraclean vacuum system. Transducer quality surface-wave films are characterized by their optical clarity, high density, smooth surface, small crystallite size, and well-oriented crystallite axes. Such ZnO films will play an important role in future surface acoustic wave device technology.