Correlation between microstructure and surface structure evolution in polycrystalline films.

Abstract

There is significant interest in the structure of metal thin films due to their wide application as critical components in electronic, magnetic and optical devices. The functionality of these devices is determined by the films physical structure that includes surface morphology and in the case of polycrystalline films, grains size distribution and crystallographic orientation. Many processing schemes have been developed to control film microstructure and surface roughness in practice, however, they usually have limited application related to specific material systems or growth conditions. A comprehensive model for structure development that incorporate deposition conditions and substrate material properties is still lacking. This thesis goal is to further the present understanding on polycrystalline film structure development with an emphasis on the correlation between surface morphology and underlying microstructure evolution. The growth of polycrystalline thin films at different deposition conditions has been explored through a combined characterization of surface structure and underlying microstructure evolution. Both early stage (before and after island coalescence) and later stage evolution (after film continuity) have been studied. Two distinct temporal regimes of film surface evolution: early surface smoothing regime followed by a roughening regime were explained based on the effects of grain growth process during film growth. The microstructure is characterized by development of a crystallographic texture in the initial regime, followed by continued grain growth and an asymptotically increase in the texture component in the later regime. The grain growth process has been identified as a combination of normal and abnormal grain growth mechanisms. Distinct surface morphologies develop during the later growth regime, indicative of different combination of mechanisms acting on specific length scales. The effect of the microstructure on surface morphology has been shown in a study of polycrystalline film evolution on different underlayers. Periodic ridges formed on vicinal Al(111) surfaces corresponding to near-(111) textured grains, and not on exact (111)-oriented grains. Alternatively, the effect of surface structure on the film microstructure has been shown in a study were nucleation of differently oriented grains at the film surface has been correlated with surface roughening during film growth.