Thermal stability of sputter-deposited ZnO thin films

This paper describes our investigation on the thermal stability of sputterdeposited, piezoelectric, ZnO thin films, using x-ray photoelectron spectroscopy (XPS), capacitance-voltage (C-V) measurements of metal-insulator-semiconductor structures, and electron microprobe. We focus on out-diffusion of Zn from ZnO thin films at a high temperature (450°C) and the composition change of zinc and oxygen after high temperature annealing (up to 700°C), since these factors are related to reliability and integrated circuits-process-compatibility of the ZnO films which are being used increasingly more in microtransducers and acoustic devices. Our experiments with electron microprobe show that ZnO thin films sputter-deposited from a ZnO target in a reactive environment (i.e., with O₂) are thermally stable (up to 700°C). Additionally, the out-diffusion of zinc atoms from the ZnO films at a high temperature (450°C) is verified to be negligible using the XPS and C-V measurement techniques. The usage of a compound ZnO target, reactive environment with O₂ and optimized deposition parameters (including gas ratio and pressure, substrate temperature, target-substrate distance and rf power, etc.) is critical to deposit thermally stable, high quality ZnO films.