MICROSTRUCTURAL EVOLUTION IN THIN-FILMS

This article reviews the topic of computer simulation of the various p rocesses that determine evolution of grain structure in polycrystallin e thin films. The evolution typically begins with the nucleation of in dividual 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 impingemen t, the grain boundaries so formed may migrate, which leads to grain gr owth. If the grain size is small compared with the film thickness, the n the grain structure and the grain growth process are three-dimension al. As the grains grow to become larger than the film thickness, so th at most grains traverse the entire thickness of the film, the microstr ucture may approach the conditions for a two-dimensional description t o apply. At this point, however, the surface energy associated with th e two free surfaces of the film becomes comparable with the surface en ergy of the grain boundaries, and the free surface may profoundly affe ct the grain growth. One effect is that grooves may develop along the lines where the grain boundaries meet the free surface, pinning the bo undaries against further migration and leading to grain-growth stagnat ion. Another possible effect is that differences in the free surface e nergy for grains with different crystallographic orientation may provi de a driving force for the migration of the boundaries that is additio nal to that provided by grain boundary capillarity. Grains with favora ble orientations will grow at the expense of grains with unfavorable o rientations, leading to abnormal or secondary grain growth in which a few grains grow very large by consuming the normal grains.