$c$-axis inclined ZnO films for shear-wave transducers deposited by reactive sputtering using an additional blind

This article reports on the growth and characterization of polycrystalline ZnO films having $c$ axis inclined up to 16° with respect to the substrate normal. These films allow the excitation of shear and longitudinal waves with comparable electromechanical coupling constants and are of significant interest for thin film bulk acoustic resonators (FBARs). The films are deposited on silicon substrates covered by $Al2O3$ and $SiO2$ buffer layers under low pressure using a modified reactive dc-pulsed magnetron sputtering system. A blind has been positioned between target and substrate, allowing oblique particle incidence without tilting the wafer. The study of structural properties of the deposited ZnO films by x-ray diffraction and scanning electron microscopy has permitted to show the presence of the inclined structure. Electromechanical coupling constants $K$ up to 13% have been extracted for shear-mode excitation using highly overmoded FBARs.