Microstructure of ZnO films used for acoustic surface-wave generation

Zinc oxide, vacuum deposited as a crystalline aggregate with preferential orientation of its fiber grains, has been used extensively as a film transducer for generating micro-acoustic waves. The orientation and size of the crystallite grains have a pronounced effect on the film’s electromechanical properties. The microstructure of dc-triode compound sputtered ZnO films was investigated using the analytical tools of scanning electron microscopy (SEM), reflection electron diffraction (RED), and x-ray diffraction (XRD). The ZnO films investigated had been deposited on fused quartz and oxidized silicon having surface interdigital electrode arrays from which the acoustic surface-wave properties of coupling efficiency, velocity, and propagation loss had been determined. Optically clear films having electromechanical and propagation properties approaching those of single-crystal zinc-oxide layers were found to have a smooth surface, a dense, amorphous-like vertical structure, an average crystallite size estimated at 110 Å, and a c-axis normal orientation of the crystallites to within ±5 °. Deviations from these microstructural conditions evident at the upper surface of the film layer reduced the coupling efficiency and increased the propagation loss. The major microstructural manifestation was a roughened surface giving the film a cloudy optical appearance. Only minor deviations in the average orientation and grain size of the crystallites were evident.