Dependence of the electromechanical coupling on the degree of orientation of c-textured thin AlN films

Highly c-oriented thin aluminum nitride (AlN) films have been grown at room temperature with reactive sputter deposition. Membrane film bulk (FBAR) thickness excited resonators have been subsequently fabricated by bulk micro machining of silicon (Si). The resonators were then electrically characterized with a network analyzer in a one-port configuration. Subsequently, the coupling coefficients and the Q factors of both the longitudinal and the shear mode were extracted from fitting the measured admittance with that of the equivalent circuit model at the resonance frequencies. The goal of this work is to study the variation of the electromechanical coupling and the quality factor of the resonators, for both the longitudinal and the shear modes as a function of the degree of film texture. It is observed that the films exhibit a mean tilt of the c-axis relative the surface normal. This tilt is found to depend on both the film texture and the distance from the wafer radius. It is also demonstrated that the textured films exhibit a behavior of the electromechanical coupling effectively identical to that of a single crystalline material of equivalent tilt. Thus, it is shown that the electromechanical coupling for the longitudinal mode decreases from 8% to 4%, and that for the shear mode increases from 0% up to 3% by varying the full width half maximum (FWHM) of the (002) rocking curve in the interval from 2degrees to 10degrees.