Parallel glide: unexpected dislocation motion parallel to the substrate in ultrathin copper films

Although it is well known that thin metal films exhibit mechanical properties very different from those of their bulk counterparts, knowledge of the underlying mechanisms is incomplete. In this study of plasticity in unpassivated Cu thin films, thermal cycling experiments were performed using both wafer curvature equipment and in situ transmission electron microscopy. It was found that the room temperature flow stress increases with decreasing film thickness, but exhibits a plateau for films 400 nm and thinner. It was also observed that a new type of dislocation motion becomes operative in this plateau region. The unexpected glide of dislocations on a (1 1 1) plane parallel to the film/substrate interface, which we have termed parallel glide, completely replaces threading dislocation motion as the dominant mechanism in films 200 nm and thinner. Parallel glide appears to be a consequence of constrained diffusional creep, which involves a diffusive exchange of atoms between the unpassivated film surface and the grain boundaries at high temperatures. This process is reversible during heating versus cooling, and is highly repeatable from one thermal cycle to the next. The observed populations of parallel glide dislocations fully account for the plastic strain measured in wafer curvature experiments.