Mechanisms of interdiffusion in Pd-Cu thin film diffusion couples

Interdiffusion in sputter-deposited polycrystalline Pd-Cu bilayers (thickness of each sublayer: 50 nm) was studied in the temperature range 175 oC-250 oC by sputter-depth profiling in combination with Auger electron spectroscopy. X-ray diffraction and transmission electron microscopy investigations revealed that the layers are polycrystalline, consisting of columnar grains separated by grain boundaries oriented more or less perpendicularly to the film surface. Considerable diffusional intermixing occurred in the studied temperature range, which was accompanied by the sequential formation of (ordered) phases Cu3Pd and CuPd. Volume interdiffusion coefficients were determined using the so-called 'centre-gradient' and 'plateau-rise' methods. Grain-boundary diffusion coefficients of Pd through Cu grain boundaries were determined by the Whipple-Le Claire method and grain-boundary diffusion coefficients of Cu through Pd grain boundaries were determined by the Hwang-Balluffi method. It was found that both volume and grain-boundary diffusion coefficients decreased roughly exponentially with annealing time. Activation energies were determined which pertain to the same (defect) microstructure at each temperature. The differences with literature results for macroscopic diffusion couples were discussed.