The effects of post-growth thermal annealing on the structural and electrical properties of RF-magnetron sputtered ZnO

The effects of two thermal annealing steps on the crystallographic orientation, film stress and crystal domain size in RF-magnetron sputtered ZnO thin films for ultrasonic transducers were investigated. As-grown films contained high levels of compressive stress introduced by the sputtering process, as well as low (002) crystal orientation selectivity. It was shown that residual stress could be eliminated by post-growth annealing at temperatures of 400°C or higher, however, that annealing process also lead to a reduction in the desired (002) crystal orientation selectivity. The use of the relatively low temperature (250°C) in-situ anneal in the sputtering chamber in an oxygen-rich environment was found essential for prompting grain growth and recrystallisation, resulting in (002) textured ZnO films that have high electrical resistivity.