Interdiffusion and stress development in Cu-Pd thin film diffusion couples

Thin film Cu-Pd diffusion couples (individual sublayer thicknesses: 50 nm) were prepared by direct current magnetron sputtering on silicon substrates coated with amorphous diffusion/reaction barriers (Si3N4 on top of SiO2). The microstructural variations, phase formation and stress evolution during diffusion annealing were investigated employing Auger-electron spectroscopy in combination with sputter-depth profiling, transmission electron microscopy and in-situ X-ray diffraction. Upon annealing at relatively low temperatures (185 °C) for durations up to 17 h, considerable diffusional intermixing occurs. Interdiffusion is accompanied by the formation of a compound phase (ordered Cu3Pd (α′)). The stress changes in the bilayer system during heating and isothermal annealing were investigated employing in-situ X-ray diffraction stress measurements and were compared to corresponding results obtained for single layers of the components in the Cu-Pd bilayer system, produced under conditions identical to those employed for the sublayers of the diffusion couple. The specific residual stresses that emerge due to diffusion (and reactions) between the (sub)layers in the bilayer could then be identified by comparing the stress evolutions upon annealing of the single layers with those recorded for the sublayers of the bilayer.