Interdiffusion between Films of Gold and Silver

Abstract

A knowledge of diffusion rates in thin films is often necessary in order to predict the useful operating temperature range of thin film components. The few reported measurements available generally indicate that film diffusion rates at low temperatures (25°–300 °C) are substantially higher than would be inferred from extrapolations of bulk diffusion data to these temperatures. This is not surprising since high defect densities are common in films, and defects are known to act as high diffusivity paths. In this paper, measurements of diffusion rates are reported for films containing a range of defect densities. Films were prepared by evaporation and getter sputtering onto glass substrates. For each set of preparation conditions, films were characterized in terms of topography and defect density, by electron microscopy techniques. Diffusion rate measurements on Au/Ag thin film couples were made by an optical reflectivity technique. Values of the diffusion coefficient for evaporated films typically lay in the range 10−15 to 10−17 for diffusion at temperatures between 100° and 200° C. For sputtered films values were about two orders of magnitude lower. Extrapolation of bulk D measurements to this temperature interval gives values of 10−21 and 10−27. A model for thin film diffusion is presented which relates the observed diffusivity with the defect content of the film.