THERMAL-STABILITY OF SPUTTER-DEPOSITED ZNO THIN-FILMS

This paper describes our investigation on the thermal stability of sputter-deposited, 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 degrees C) and the composition change of zinc and oxygen after high temperature annealing (up to 700 degrees 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 O-2) are thermally stable (up to 700 degrees C). Additionally, the out-diffusion of zinc atoms from the ZnO films at a high temperature (450 degrees C) is verified to be negligible using the XPS and C-V measurement techniques. The usage of a compound ZnO target, reactive environment with O-2, 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.