Stress control of AlN thin film sputter-deposited using ECR plasma

AlN thin film is widely utilized for piezoelectric devices e.g. acoustic filters and sensors, since AlN has well-balanced properties such as high acoustic velocity, low loss at high frequency, moderate electro-mechanical coupling coefficient and moderate temperature coefficient, and can be deposited at relatively low temperature. The sputtering technology using ECR plasma can deposit a highly-oriented AlN thin film by the assist of low energy plasma flow. However, its residual stress is normally highly-compressive, and thus free-standing AlN MEMS are easily broken during the fabrication process. In this research, to solve this problem, we investigated the reduction of AlN compressive stress, and the full width at half maximum of the XRD rocking curve (FWHM_{XRD RC}) by changing various parameters such as substrate bias voltage, the gas flow ratio of Ar:N₂ and substrate temperature. It was experimentally confirmed that the positive substrate bias voltage and the substrate temperature can considerably reduce AlN compressive stress to nearly zero without remarkable deterioration of FWHM_XRD:RC.