It was recalled that, although it was well known that thin metal films exhibited mechanical properties that were very different to those of the bulk counterparts, knowledge of the underlying mechanisms was 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 increased with decreasing film thickness, but exhibited a plateau for films 400 nm and thinner. It was also observed that a new type of dislocation motion became operative in this plateau region. The unexpected glide of dislocations on a (111) plane parallel to the film/substrate interface, which were termed parallel glide, completely replaces threading dislocation motion as the dominant mechanism in films 200nm and thinner. Parallel glide appears to be a consequence of constrained diffusional creep, which involved a diffusive exchange of atoms between the unpassivated film surface and the grain boundaries at high temperatures. This process was reversible during heating versus cooling, and was highly repeatable from one thermal cycle to the next. The observed populations of parallel glide dislocations fully accounted for the plastic strain measured in wafer curvature experiments.

Parallel Glide - Unexpected Dislocation Motion Parallel to the Substrate in Ultra-Thin Copper Films. T.J.Balk, G.Dehm, E.Arzt: Acta Materialia, 2003, 51[15], 4471-85