Biaxial alignment in sputter deposited thin films

Biaxially aligned thin films have not only a preferential crystallographic out-of-plane orientation, but also have an alignment along a certain reference direction parallel to the substrate plane. This type of film has been obtained by unbalanced reactive magnetron sputter deposition on both amorphous glass and randomly textured polycrystalline substrates tilted with respect to the incoming material flux. First, we focus on the development of microstructure and crystallographic out-of-plane orientation. The results are summarized in an extended structure zone model. Based on experimental results, a mechanism for the in-plane alignment is proposed which shows that an in-plane alignment can only be obtained when an overgrowth mechanism drives the microstructural evolution of the thin film. The quality of the in-plane alignment can be evaluated from X-ray diffraction pole figures. The influence of several deposition parameters (target–substrate distance, target–substrate angle, deposition pressure, and substrate bias) on the degree of in-plane alignment is discussed. The influence of these parameters can be traced to the influence of two main properties, i.e. the mobility of the adatoms at the growing surface and the angular spread of the incoming material flux. Finally, since impurities are hard to exclude during deposition, their influence on the microstructure, the preferential out-of-plane orientation, and the in-plane alignment is reported.