MICROSTRUCTURAL EVOLUTION IN THIN-FILMS

This article reviews the topic of computer simulation of the various processes that determine evolution of grain structure in polycrystalline thin films. The evolution typically begins with the nucleation of individual crystals, which then grow until they impinge upon each other and form a continuous film. The microstructures created depend on both the nucleation conditions and the growth conditions. After impingement, the grain boundaries so formed may migrate, which leads to grain growth. If the grain size is small compared with the film thickness, then the grain structure and the grain growth process are three-dimensional. As the grains grow to become larger than the film thickness, so that most grains traverse the entire thickness of the film, the microstructure may approach the conditions for a two-dimensional description to apply. At this point, however, the surface energy associated with the two free surfaces of the film becomes comparable with the surface energy of the grain boundaries, and the free surface may profoundly affect the grain growth. One effect is that grooves may develop along the lines where the grain boundaries meet the free surface, pinning the boundaries against further migration and leading to grain-growth stagnation. Another possible effect is that differences in the free surface energy for grains with different crystallographic orientation may provide a driving force for the migration of the boundaries that is additional to that provided by grain boundary capillarity. Grains with favorable orientations will grow at the expense of grains with unfavorable orientations, leading to abnormal or secondary grain growth in which a few grains grow very large by consuming the normal grains.