Post-deposition annealing of RF-sputtered zinc-oxide films

Abstract Thin films of ZnO are currently used in both bulk and surface acoustic wave devices and have potential for use in integrated optical devices. Sensor applications are emerging as well. Sputtered ZnO films can develop large internal stresses which are dependent upon deposition parameters and substrate material. It is for the first time observed that stress relaxation by furnace annealing in air at 400°C for a few hours is accompanied by a reduction in optical attenuation especially for thin films (0.25 μm). In this study ZnO films have been grown on top of oxidized silicon substrates by reactive sputtering a zinc target in an oxygen plasma. Optical attenuations lower than 1 dB/cm (film thickness 1 μm) were obtained despite the damage induced by the inevitable electron bombardment in the standard RF sputter technique. The best optical films were produced for substrate temperatures between 250 and 300°C, at a sputter rate of 0.8 μm/h. Similar film quality could only be obtained by applying the magnetron sputtering technique, which operates at higher substrate temperatures and produces a reduced homogeneity of the film.