The growth process of thin film crystals directly affects the structure and performance of the thin film (Minbo in Thin film technology and thin film materials. Tsinghua University Press, Beijing, 2006). Figure 9.1 shows the motion state of atoms during thin film deposition and the growth process of the thin film. Atoms or molecules directed towards the substrate surface or thin film surface collide with the surface, some are reflected, and others remain on the surface. The atoms or molecules remaining on the surface, under the action of their own energy and the energy generated by the substrate temperature, undergo surface diffusion and surface migration, some evaporate again, leaving the surface, and some fall into the potential valley bottom, are adsorbed by the surface, that is, the condensation process occurs. Condensation is accompanied by the formation and growth of crystal nuclei, the formation, merging and growth of islands (i.e., uniform, small and movable atomic groups), and finally the formation of a continuous film layer.