Shear mode coupling and tilted grain growth of A1N thin films in BAW resonators

Abstract

Polycrystalline AlN thin films were deposited by RF reactive magnetron sputtering on Pt(111)/Ti elec- trode films. The substrates were tilted by an angle rang- ing from 40 to 70 with respect to the target normal. A low deposition temperature and a high sputter gas pres- sure 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 tan- gent of 0.3%. Two shear modes as well as the longitudinal mode could be clearly identified. The coupling coefficient 2 of the fundamental thickness shear mode (TS0) was found to be about 0.5%, which is compatible with a c-axis tilt of about 6 .