Despite their wide use in microelectronics, the fatigue behavior of submicrometer metallic thin films is not yet fully understood. Unlike bulk materials, which typically exhibit persistent slip bands and extrusions, thin films experience surface roughening and grain boundary cracking due to their stronger dimensional constraints. Most of the previous studies were conducted on supported films, with the substrates influencing the fatigue behavior. For a better understanding of the fatigue lifetime of metallic thin films, freestanding 150-nm thick gold specimens were cyclically loaded in a custom bulge setup. Temperatures between 23 °C and 100 °C and nominal stress ranges of 100 to 300 MPa were applied in high cycle fatigue (HCF) experiments. A Woehler plot was generated showing a decreasing lifetime with increasing temperature and stress amplitude. Small failure strains of 1.5 % or below were recorded. As the samples fail catastrophically, the deformation mechanisms cannot be reconstructed from post-mortem observations. Because cyclic creep is prominent in the strain data at all stresses and temperatures, a steady-state strain rate was determined. Using the stress exponent n and activation energy Q – which are characteristic creep parameters – we compared the cyclic testing data with bulge creep experiments by Merle [1]. It was found that fatigue is creep-controlled at 100 °C whereas at lower temperatures other deformation mechanisms come into play. Activation energy results indicate interface diffusion which is in line with TEM observations on cyclic tested samples showing little to no dislocation activity.

[1] B. Merle, Journal of Materials Research 2019, vol. 34, no. 1, pp. 69–77. doi: 10.1557/jmr.2018.287