Synthesis of c-axis oriented AlN thin films on metal layers: Al, Mo, Ti, TiN and Ni

Thin piezoelectric polycrystalline films such as AlN, ZnO, etc are of great interest for the fabrication of Thin Film Bulk/Surface Acoustic Resonators (TFBAR) or (TFSAR). It is well known that the degree of c-axis orientation of the thin films correlates directly with the electro-mechanical coupling. The texture of the piezoelectric films in turn is influenced by the structure of the substrate material. Thin AlN films, prepared in a magnetron sputtering system, have been deposited onto thin Al, Mo, Ni, Ti and TiN films. Such thin metal layers are used to form the bottom electrode of TFBAR as well as to define a short-circuiting plane in TFSAR devices. In both cases, they serve as a substrate for the growth of the piezoelectric film. It has been found that the texture of the bottom metal layer affects significantly the texture of the AlN films, and hence its electroacoustic properties. For this reason, the surface morphology and texture of the metal layers and their influence on the growth of AlN on them has been systematically studied. Thus, the texture and the electro-acoustic properties of the AlN films have been studied as a function of the texture and morphology of the underlying metal films. Subsequently, the deposition processes have been individually optimised with respect to obtaining high electromechanical coupling for all thin film combinations.