The effects of oxygen and annealing temperature on the grain growth behavior of polycrystalline Ag thin film have been investigated. Thin films are deposited on glass by RF magnetron sputtering method. Annealing treatment is carried out at 250C and 600C. Specimens are annealed under vacuum and oxygen atmosphere, respectively. The grain growth behavior has been investigated primarily using scanning electron microscope.
When specimen is annealed at 250C in vacuum, grain growth proceeds very slowly. Whereas, three dimensional abnormal grain growth(AGG) is observed in specimen annealed under oxygen atmosphere. The role of oxygen on Ag is to increase the anistoropy of surface and grain boundary energy and, therefore, make the surface and grain boundary faceted. Oxygen atoms adsorb preferentially on the low index planes such as {111}, {100} and {110} and reduce the surface energy. Original surfaces break up into low index planes to reduce the total surface energy. In other experiments on Ag, grain boundary faceting has been observed frequently when annealed under oxygen atmosphere. At 600, the tendency of agglomeration is increased. Because thin films have a high suface-to-volume ratio, on heating, a thin, solid, continuous film may develop holes and may eventually agglomerate into shapes with a lower surface-to-volume ratio. In specimen annealed at 600C in vacuum, grain growth ceases to occur when the grain size is approximately 2-3 times of the film thickness. This phenomenon is known as the specimen thickness effect. Annealing under oxygen atmosphere is carried out using stagnant columnar structure which is annealed for 1 hr at 600C in vacuum as an initial structure. Further grain growth is observed and the grain growth behavior is normal. The effects of surface faceting as well as thermal grooving on grain growth behavior under the condition that surface and grain boundary energy is anisotropic are required to be considered.
The occurrence of the abnormal grain growth is very closely related to the existence of faceted grain boundary. It is possible that the grain boundary migration mechanism is varied with the grain boundary structure and the faceted boundaries have the non-linear mobility and anisotropic grain boundary energy. This may trigger abnormal grain growth behavior in thin films.
