Fatigue in thin films: lifetime and damage formation

Two new techniques, developed for studying the fatigue behavior of thin metal films on substrates, are presented: The first technique involves deposition of Cu films onto elastic polyimide substrates. During cyclic tensile testing of the film–substrate composite, the film is subjected to tension–compression cycles, since it is plastically deformed, while the substrate undergoes only elastic deformation. Using this technique, it was found that, for 3 μm thick Cu films, the number of cycles to failure follows the phenomenological Coffin–Manson relationship. For the other technique, thin films, here Ag films with thicknesses ranging from 0.2 to 1.5 μm, are deposited onto micromachined SiO2 cantilever beams. The beams are then deflected with a frequency of 45 Hz using a nanoindentation system. A detailed investigation of the damage formation in both fatigued Cu and Ag films revealed surface roughening prior to failure. Extrusions and cracks are formed inside large grains and between small grains, respectively.