Fatigue behavior of thin cu films : Film thickness and interface effects

Abstract

The mechanical properties of thin metal films are strongly influenced by size through the films thickness and grain size. The best known example of a length scale effect in film mechanical properties is the increase in yield stress with decreasing film thickness or grain size. This effect is usually attributed to the inhibition of dislocation motion and nucleation in small volumes. In addition, previous work indicates a length scale effect on fatigue behavior in thin Cu films. Therefore, this study will present a systematic investigation of the effect of length scale on fatigue life and damage formation in thin Cu films at both room temperature and 200°C and with and without surrounding Ta layers. Fatigue testing of Cu films with thicknesses between 50 nm and 3.0 μm on Kapton substrate has been performed and the fatigue damage has been investigated using scanning electron and ion beam microscopy. It is observed that the extrusions decrease in number and size with decreasing film thickness or grain size, while the cracks increase in number. The fatigue life is also clearly influenced by size in that a higher strain range or more cycles are required in thinner films to form damage and cause failure. This transition in damage morphology and fatigue life with length scale is explained by a transition in mechanism from dislocation controlled plasticity in the thicker films to cracking along interfaces and boundaries in the thinner films. In order to gain better insight into the deformation mechanisms, synchrotron diffraction studies were performed on the thin films during cyclic loading. The fatigue damage of the Cu films loaded at 200°C is similar to that at room temperature, except that the extrusions are more rounded and the grain boundary grooves are larger. This indicates that diffusion processes play an important role during fatigue damage formation and likely account for the clearly reduced fatigue life at 200°C. The presence of surrounding Ta layers has a clear influence on the fatigue behavior of the Cu films. A Ta under-layer does not change much in the Cu film fatigue behavior, but a Ta over-layer leads to a dramatic improvement in fatigue life in the thicker Cu films. This is likely due to the observed inhibition of extrusion formation by the presence of the over-layer. In contrast, the Ta over-layer has little effect on the fatigue life of the thinner Cu films. This is presumably an indication that the Ta over-layer does not influence crack formation and is consistent with the transition to failure by crack growth in the thinner films.