Grain boundary diffusivity of Ni in Au thin films and the associated degradation in electrical contact resistance due to surface oxide film formation

The low temperature diffusion from a nickel strike layer through varying thickness films of gold, a common electrical contact overlayer, was characterized and correlated to changes in electrical contact resistance (ECR). The diffusivity of Ni in Au was determined by the surface accumulation method (type C kinetics) for Au film thicknesses of 280 and 994 nm at an annealing temperature of 150 °C over a 32 day period. X-ray photoelectron spectroscopy (XPS) was used to determine the rate of Ni surface accumulation. The average product of grain boundary width and diffusivity for Ni in polycrystalline Au was calculated to be δbDb≅2.0 × 10−22cm3s using the Hwang-Balluffi model, and δbDb≅2.5×10−22cm3s using the Ma-Balluffi model. ECR measurements were made in parallel to surface accumulation measurements, revealing a correlation between ECR and Ni-O surface concentration; ECR values for both film thicknesses increased by 1 to 2 orders of magnitude at saturation coverage. Film cross-sections were extracted using focused ion beam milling and analyzed via scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy after 32 days of annealing, enabling a direct observation of the composition and thickness of the accumulated Ni-O layer, which was approximately 3 nm thick.