Stresses in nanocrystalline materials

Thin film Ni-Cu diffusion couples (individual layer thicknesses: 50 nm) were prepared by direct-current magnetron sputtering on silicon substrates. Interdiffusion and solid-solution formation, the associated changes of microstructure and the evolution of mechanical stresses of the Ni-Cu diffusion couples during thermal treatments were investigated employing ex-situ X-ray diffraction phase and texture measurements and in-situ X-ray diffraction stress measurements, transmission electron microscopy and Auger-electron spectroscopy (sputter) depth-profiling. Annealing at relatively low temperatures (about 0.4 Tm) results in considerable diffusional intermixing, which is accompanied by the formation of the (Cu,Ni) solid solution. In addition to thermal stresses, tensile stress contributions arise in the sublayers during the diffusion anneals, which diffusion-induced stress contributions were identified by comparing the measured stress changes of the sublayers in the bilayer with those recorded from single layers of Cu and Ni annealed under identical conditions. The diffusion-induced stress changes of the sublayers could be quantitatively evaluated considering the measured concentration-depth profiles of the annealed Ni-Cu diffusion couples and were discussed in terms of possible mechanisms of stress generation. The dominant sources of stress buildup and relaxation were identified.