Piezoelectric and elastic properties of Al0.60Sc0.40N thin films deposited on patterned metal electrodes

Sc-doped aluminum nitride (AlScN) allows for piezoelectric devices with large electromechanical coupling and the benefits increase with larger Sc doping in the film. However, with a larger Sc concentration, the process window narrows, and it is necessary to fine-tune the deposition parameters to achieve a good film. In this paper, we investigate depositions of highly doped AlScN (40% Sc) on unpatterned and patterned metal layers, to show how it is possible to maintain a good film quality on a metal electrode. We find how high-temperature deposition of the metal improves the AlScN film quality, how the gas mixture allows to reduce defects, and how film quality changes with thickness. We show that extreme care must be taken in the apparently trivial step of photoresist cleaning. Finally, we extract the mechanical, electrical, and piezoelectric properties of our optimized layer from a batch of fabricated resonators, obtaining a 5× improvement of piezoelectric coupling compared to undoped AlN and a 1.5× improvement from 32% doped AlScN.