Cyclic creep and fatigue testing of nanocrystalline copper thin films

This study performed microtensile testing to investigate the mechanical properties of cyclic fatigue in freestanding nanocrystalline copper thin films of sub-micrometer thickness. We observed the mechanical response associated with tension–tension fatigue under various stress amplitudes and mean stress conditions at cyclic loading frequencies of up to 10 Hz. Experiments were carried out using feedback to provide load control for sputter deposited 300, 500, and 700 nm Cu thin films. Cu films were deposited on the Si substrate and then separated from the substrate following the completion of processing. The feedback control maintains the stress within the film at a constant value, even when the stress threatens to drop due to plastic deformation during the course of the experiment. As anticipated, the number of loading cycles to failure exceeded 106 under low mean loads at low amplitudes with an increase in the load leading to a decrease in the number of cycles to failure. These results provide clear evidence of cyclic creep rate dependence and changes in the failure mechanism from crack formation to extended plasticity following a decrease in the mean load or load amplitude. Moreover, we observed how the fatigue and cyclic creep of the tested films depended on the length scale.