Substrate texturing for homogeneous deposition of tilted c-axis AlN films for shear mode operation

We propose two methods to improve the growth process of AlN films displaying the c-axis uniformly tilted, intended for the excitation of shear modes in acoustic resonators. The two methods aim at modifying the surface topography of the substrates in order to produce a family of parallel planes offering a suitable orientation for the subsequent growth of uniformly tilted AlN microcrystals. The first method is based on an ion milling process over rough substrates. Rough substrates, obtained by depositing a W layer on sputtered porous silicon oxide layers, are bombarded with a wide beam of Ar ions impinging on the sample surface at an angle of 65°. All treated surfaces lead to increased mean tilt angles of the subsequently grown AlN films, which in turn improves the coupling factor of the shear mode resonators. The second method consists in transferring the topography of a blazed diffraction grating to a photoresist layer spun over metal substrates. The photoresist pattern is then transferred to the substrates by a non- selective directional reactive ion etching process with SF
₆
.