Ultrafast sputtered ZnO thin films with high kT for acoustic wave device applications

The fabrication of high frequency acoustic wave devices requires the development of thin films of piezoelectric materials with improved morphological and electro-acoustical properties. In particular, the crystalline orientation of the films, surface morphology, film stress and electrical resistivity are key issues for the piezoelectric response. In the work reported here, ZnO thin films were deposited at high rates (>;50 nm/min) using a novel process known as the High Target Utilisation Sputtering (HiTUS). The films deposited possess excellent crystallographic orientation, high resistivity (>; 10⁹ Ωm), and exhibit surface roughness and film stress one order of magnitude lower than films grown with standard magnetron sputtering. The electromechanical coupling coefficient of the films, k_T, was precisely calculated by implementing the resonant spectrum method, and was found to be at least 6% higher than any previously reported k_T of magnetron sputtered films to the Authors´ knowledge. The low film stress of the film is deemed as one of the most important factors responsible for the high k_T value obtained.