Optimization of thin AlN sputtered films for X-band BAW resonators

We investigate the sputter growth of very thin aluminum nitride (AlN) films on iridium electrodes for high frequency filtering applications. The structure and piezoelectric activity of AlN films are assessed through XRD, FTIR, stress and frequency response measurements. A combination of a pre-deposition RF plasma treatment of the Ir bottom electrode followed by a two-step ac reactive sputtering of the AlN film allows to optimize the crystal quality and residual stress of AlN films with thicknesses as low as 160 nm. BAW resonators tuned around 8 GHz are built on top of polished Bragg reflectors composed of porous SiO₂ and Ir layers. Material coupling factors κ_mat² of 6.7% and quality factors Q up to 900 are achieved. The films obtained are competitive for X-band filter fabrication.