Grain-boundary diffusion of Ag through Cu films

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The interdiffusion of bimetallic couples consisting of 1850 Å Cu/1550 Å Ag on sapphire substrates has been studied using Rutherford backscattering for anneal temperatures ranging from 225 to 500 °C. The Ag-Cu couple was chosen because it represents a system in which only grain-boundary diffusion as opposed to lattice diffusion should occur. No Ag-Cu intermetallic compounds have been reported and the equilibrium solid solubility of Ag in Cu and Cu in Ag is under 3 at.% for the temperature range of interest. A series of in situ isothermal vacuum anneals established the following mechanism for interdiffusion in these couples. Initially, the Cu grain boundaries saturate with Ag. Then Ag atoms in the Cu grain boundaries diffuse to the Cu surface and spread out. Finally, the diffusion process ceases when about one monolayer of Ag covers the Cu surface. Also, at temperatures exceeding 350 °C, grain growth at the Ag-Cu interface roughens that interface and makes Rutherford-backscattering analysis more difficult. Grain-boundary diffusivity has been calculated employing the model of Hwang and Balluffi. Agreement with published values obtained by radioactive-tracer measurements on foil samples is quite good. A discrepancy was observed between grain size of Cu as estimated by the saturated concentration of Ag in Cu grain boundaries and that determined by TEM measurements. This can be accounted for by assuming that some grain boundaries are blocked by light impurity atoms or that the grain-boundary segregation coefficient of Ag in Cu is not unity. In either case, it is shown that analysis of the Rutherford-backscattering spectra with the Hwang and Balluffi model can take account of these factors.