Influence of Film/Substrate Interface Structure on Plasticity in Metal Thin Films
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Influence of Film/Substrate Interface Structure on Plasticity in Metal Thin Films G. Dehm, B.J. Inkson*, T.J. Balk, T. Wagner, and E. Arzt Max-Planck-Institut für Metallforschung, Seestr. 92, 70174 Stuttgart, Germany *Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, U.K.
ABSTRACT In-situ transmission electron microscopy studies of metal thin films on substrates indicate that dislocation motion is influenced by the structure of the film/substrate interface. For Cu films grown on silicon substrates coated with an amorphous SiNx diffusion barrier, the transmission electron microscopy studies reveal that dislocations are pulled towards the interface, where their contrast finally disappears. However, in epitaxial Al films deposited on single-crystalline αAl2O3 substrates, threading dislocations advance through the layer and deposit dislocation segments adjacent to the interface. In this latter case, the interface is between two crystalline lattices. Stresses in epitaxial Al films and polycrystalline Cu films were determined by substratecurvature measurements. It was found that, unlike the polycrystalline Cu films, the flow stresses in the epitaxial Al films are in agreement with a dislocation-based model. INTRODUCTION The stresses that develop in metal thin films deposited on rigid substrates can significantly exceed the flow stresses in the corresponding bulk metal. This phenomenon has been attributed to geometrical constraints on the films, which alter the energetics of dislocation motion. Dislocation-based models for the flow stress assume that the film/substrate interface acts as an obstacle for dislocations moving in the metal film. A dislocation gliding on a plane inclined to the film/substrate interface creates a dislocation line at this interface [1,2]. A number of transmission electron microscopy (TEM) studies have been performed in order to investigate dislocation plasticity in metal thin films [3-13]. While some TEM studies report the formation of interfacial dislocations during straining of metal thin films [3-5], there are also TEM observations describing the fading of interfacial dislocation contrast due to disappearance of their strain field [5-7]. However, unstable interfacial dislocations contradict the concept of dislocation-based models for the flow stress. Furthermore, work hardening in thin films, which is explained by the interaction of gliding dislocations with interfacial dislocation segments, would have to be reconsidered. Recently, the high flow stresses in thin Ag films were ascribed to thermally activated dislocation motion [8]. TEM observations revealed smooth dislocation motion in Ag grains at elevated temperatures (low film stresses), but jerky dislocation motion at low temperatures (high film stresses), with pinning distances significantly smaller than the grain dimensions [8]. In the present study, we have investigated dislocation-interface interactions and dislocation motion in two metal thin film systems with different film/substrate interface structure. Poly
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