The effect of rf power on reactively sputtered zinc oxide

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Abstract
Zinc oxide is a piezoelectric material which, in thin‐film form, is finding wide application in surface acoustic wave devices. In order to obtain good electromechanical coupling, the polycrystalline film must simulate single‐crystal properties. In this study, we report the effect of rf power on the crystallographic orientation and grain structure of reactively sputtered ZnO. Films were deposited at forward power levels ranging from 300 to 640 W on both uncoated and Al‐coated Si substrates by sputtering a ZnO target using a 75% Ar/25% O2 gas mixture. The results of this study show that both film microstructure and crystallography are dependent upon the rf power level, with an increase in the amount of structural disorder occurring with increasing power. The magnitude of this effect is dependent upon the substrate. The results are discussed in terms of the average deposition rate, the relative number of Zn to ZnO ions in the plasma, monitored by glow discharge mass spectrometry, and bombardment of the growing film by secondary electrons emitted from the target.