Effect of film thickness and grain size on fatigue-induced dislocation structures in Cu thin films

This letter presents systematic experimental observations of fatigue damage and corresponding dislocation structures in thin Cu films as a function of film thickness made using transmission electron microscopy and focused-ion-beam microscopy. It is found that, in thick films and grains of at least 3.0 μm diameter, coarse surface extrusions and dislocation wall and cell structures occur whereas, in thin films or in small-diameter grains, finer extrusions occur but no clearly defined dislocation structures are present. This minimum required dimension of 3.0 μm for fatigue damage formation may be caused by constrained dislocation motion in small dimensions.