Effect of length scale on fatigue life and damage formation in thin Cu films

Fatigue behavior of thin Cu films with thicknesses between 50 nm and 3.0 μm on polyimide substrates has been studied using cyclic tensile testing. The results show a clear length scale effect on both fatigue damage morphology and fatigue life. Extrusions decrease in size and number while the number of cracks increases with decreasing film thickness. The change in fatigue damage with film thickness suggests a fatigue mechanism transition from dislocation mediated extrusion formation to crack formation controlled behavior. The very clear increase in fatigue life with decreasing film thickness at constant applied strain is attributed to an increase in yield stress.