Observations of Dislocation Motion and Stress Inhomogeneities in a Thin Copper Film
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Observations of Dislocation Motion and Stress Inhomogeneities in a Thin Copper Film T. John Balk, Gerhard Dehm and Eduard Arzt Max-Planck-Institut für Metallforschung, Seestrasse 92, 70174 Stuttgart, Germany ABSTRACT In situ transmission electron microscopy has been utilized to study dislocation plasticity in a 200 nm thick copper film. The behavior of dislocations in a [111]-oriented grain was recorded during a thermal cycle. During cooling, it was observed that dislocations were emitted from a grain boundary triple junction in regular intervals of 30ºC to 40ºC. Subsequent glide occurred on a (111) plane parallel to the film surface, despite the expectation of zero resolved shear stress on such planes. The initial emitted dislocations remained close to the triple junction, avoiding contact with another [111] grain rotated by 17º. Glide into the opposite end of the grain was initiated only after the injection of several additional dislocations, which induced strong curvature in all dislocations near the active triple junction. Post mortem examination of dislocation curvature revealed that an inhomogeneous stress state existed within the grain. INTRODUCTION The mechanical behavior and reliability of thin metal films have been widely studied, but knowledge of the exact mechanisms that carry inelastic deformation is incomplete. Understanding the role of dislocations in thin film plasticity is critical for the testing and confirmation of existing models, e.g. the dependence of film strength on film thickness [1-3], and for the creation of new models. Transmission electron microscope (TEM) observations of dislocation behavior have been made for several face-centered cubic metals, including copper [4], and indicate that dislocations tangle during cooling of the film, leading to high dislocation densities. These observed densities do not appear to increase as a result of repeated thermal cycling, indicating that deformation microstructure is alternately healed and regenerated during heating and cooling of the film. Allen et al. [5] performed in situ thermal cycling experiments in the TEM, using 200 nm thick Al films deposited on single crystalline Si coated with amorphous SiNx. They observed a reversible and repeatable evolution (emission and recession) of dislocation loops, which alternately expanded and contracted within a grain during cooling and heating, respectively. Such observations agree with the thermomechanical behavior of thermally cycled thin films, which exhibit highly repeatable stress-temperature curves. It has been suggested that unpassivated metal films which do not form a native oxide may undergo constrained diffusional creep, whereby atoms diffuse from the grain boundaries to the free film surface in a film under tension at sufficiently high temperature [6]. The resulting diffusional "wedges" cause the film stresses to be partially relaxed at the grain boundaries, although the stress in the grain interior is not relaxed, assuming that the film does not slide relative to the substrate. This leads to an in
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