Effect of a surface layer on the stress relaxation of thin films

Abstract

The salient features of the deformation mechanisms that can be obtained from stress-temperature plots of thin films deposited on substrates are discussed in this article. It is shown that the slope of the cooling portion of these plots at low temperatures indicates when dislocation-glide-dominated mechanisms are important and whether these occur without changes in microstructure. Furthermore, the behavior of the films during and after isothermal holds gives important information about the relaxation processes. Some of these general principles have been demonstrated by experimental observations on the shape of the stress-temperature curves for 1-μm-thick copper, silver, and aluminum films deposited on silicon. Two specific observations can be made based on these results. First, for the materials and conditions examined, isothermal holds do not affect the residual stress state upon subsequent cooling. Second, while the behavior of the copper and silver films can be qualitatively described by constitutive models appropriate for bulk forms of those materials, the behavior of aluminum is anomalous. It is believed that the properties of the aluminum film are dictated by the presence of the native oxide which suppresses relaxation mechanisms associated with diffusion and dislocation climb. To support this contention, copper, silver, and aluminum films were passivated by SiO2 layers. The shape of the stress-temperature plots for copper and silver changed qualitatively from one consistent with bulk relaxation mechanisms to one consistent with dislocation-glide mechanisms. The behavior of the aluminum film was unaffected by passivation. © 1996 American Vacuum Society.