Optimized tilted c-axis AlN films for improved operation of shear mode resonators

Biosensors based on AlN bulk acoustic wave resonators require shear mode operation to avoid acoustic losses upon analysing liquid samples. To generate shear modes, the microcrystals of the AlN thin film must be uniformly tilted with respect to the normal, yet keeping good piezoelectric properties. In this paper, we demonstrate AlN solidly mounted resonators displaying simultaneously material electromechanical coupling of 3.5% and quality factor (QS) of 250 for the shear mode in air. Upon testing the devices in liquid, QS factor drops to 150, which is still large enough for biosensing applications. We deposit the AlN film with tilted microcrystals using a two-stage off-axis sputtering process that involves the use of a seed layer and an active piezoelectric layer. We have studied the parameters involved in each stage of the deposition and identified the factors that are key to achieve reproducible high-quality films. By comparing this method to off-axis deposition of AlN on randomly rough substrates, we conclude that the latter produces hardly reproducible and lower quality AlN thin films.