Determining the diffusion coefficient of Ni in Cu from a Ni/Cu thin film using linear heating, scanning Auger microscopy and a numerical solution of Fick's second law

Abstract

In previous studies, linear temperature ramping was used to determine diffusion coefficients from bulk-to-surface segregation experiments of a low concentration solute. Thin film diffusion studies usually employ a classical heating regime, where a sample's annealing time is taken as the time between insertion and removal from a furnace. The aforementioned study type assumes that the time it takes to heat up a sample after insertion is instantaneous, while the sample cools down instantaneously after removal from the furnace. This assumption is incorrect, as it does not compensate for the various mechanisms that govern heat transfer. In order to eliminate the uncertainty, a linear ramping regime is used and samples were annealed inside an ultra high vacuum (UHV) environment with a programmed linear heating scheme. After each anneal, a depth profile was obtained by simultaneously bombarding the sample with Ar + ions and monitoring the exposed surface with an electron beam which excites Auger electrons, among others. The depth profiles were normalised and the time scale converted to depth. In order to compare the diffusion profiles obtained from classical annealing studies to the linearly ramped studies, the diffusion coefficient obtained for a classical study of Ni diffusing in Cu was inserted into a numerical solution of Fick's second law, modified to include linear temperature ramping. A diffusion coefficient was also obtained from linearly ramped samples. Comparison of the diffusion profiles calculated with the diffusion coefficients obtained from classical heating and linear heating showed a large discrepancy between the profiles.

Triple Junction Transport and the Impact of Grain Boundary Width in Nanocrystalline Cu

“…Indeed, the GB diffusion coefficients in our case are orders of magnitude smaller than the tracer ones. Other measurements on interdiffusion of Ni/ Cu thin films 40,41 also indicate that chemical interdiffusion via GBs is orders of magnitude slower than expected tracer diffusion if realistic grain sizes are considered. Obviously, the present knowledge of the concentration dependence of GB transport, in particular for NiCu but also in general, is not satisfying and requires further dedicated investigations.…”