Shear mode coupling and tilted grain growth of AlN thin films in BAW resonators

Polycrystalline AlN thin films were deposited by RF reactive magnetron sputtering on Pt(111)/Ti electrode films. The substrates were tilted by an angle ranging from 40° to 70° with respect to the target normal. A low deposition temperature and a high sputter gas pressure were found ideal for tilted growth. The resulting grain tilt angle amounts to about half the substrate tilt angle. For coupling evaluation, 5 GHz solidly mounted resonator structures have been realized. The tilted grain AlN films exhibited a permittivity in the 9.5-10.5 range and loss tangent of 0.3%. Two shear modes as well as the longitudinal mode could be clearly identified. The coupling coefficient k²_eff, of the fundamental thickness shear mode (TSO) was found to be about 0.5%, which is compatible with a c-axis tilt of about 6°.