Role of argon ion bombardment in sputtered AlN films for SAW devices

In this work we present a study of the effect of particle bombardment on the orientation and residual stress of AlN polycrystalline thin films obtained by planar RF sputtering. AlN films were deposited on silicon [100] substrates by sputtering an aluminum target in Ar and N₂ mixtures. The main deposition parameters were changed as follows: pressure (4-11 mTorr), N₂ content (20-80%), RF voltage (700-1900 V), and negative substrate bias (10-30 V). We observed that by controlling the substrate bias we were able to modify the preferred orientation of the films for a wide range of variation of the other process parameters. If a sufficiently high substrate bias was applied, (00.2)-oriented films were obtained for the full range of pressure, target voltage and N₂ content. This feature provided us with a large flexibility to optimize other essential parameters such as residual stress, independently of the intended preferred orientation. Films with stress values from -3 to +1 GPa have been obtained for different deposition conditions. Our results suggest that the energy of the argon ions colliding with the substrate during deposition is a key parameter to control the preferred orientation of the films, whereas the ion incidence angle is the main factor determining the amount of residual stress.