Synthesis of C-axis-oriented AlN thin films on high-conducting 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 (TFBARs or TFSARs). It is well-known that the degree of c-axis orientation of the thin films correlates directly with the electromechanical coupling. However, the degree of c-axis orientation of the piezoelectric film is, in turn, influenced by other parameters such as the structure of the substrate material, the matter of whether the c-axis is up or down (polarity), and the growth parameters used. The correlation of these three aspects with the electromechanical coupling of the AlN-thin films, is studied here. Thin AlN films, prepared in a magnetron sputtering system, have been deposited onto thin Al, Mo, Ni, Ti, and TiN films. Such thin high-conducting layers are used to form the bottom electrode of TFBAR devices 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 degree of orientation and the surface roughness of the bottom metal layer significantly affects 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. Finally, FBARs with both Al and Ti electrodes have been fabricated and evaluated electroacoustically.