High-resolution investigation of atomic interdiffusion during Co/Ni/Si phase transition

We studied the phase transition of Co/Ni thin films on amorphous silicon using an approach based on in situ sheet resistance analyses during isothermal annealing processes. Compared to conventional Co/Si systems, Co/Ni/Si reaction produces a double peak in the resistance curve versus time. This behavior has been extensively studied by Rutherford backscattering spectroscopy, x-ray diffraction, energy filtered transmission electron microscopy, and selected area diffraction analyses. It has been found that the kinetic evolution of the Co/Ni/Si structure occurs through two consecutive transition stages which are strictly related to each of the resistance peaks. At the beginning, cobalt atoms remain confined at the surface while nickel reacts with silicon hence producing the occurrence of the first resistance peak. The second peak is instead related to the cobalt atoms diffusing through the grain boundaries of the underlying ${\mathrm{Ni}}_2\mathrm{Si}$ layer, converting ${\mathrm{Ni}}_2\mathrm{Si}$ in a continuos CoSi film and forming pipelines through the underlying NiSi down the substrate. As a result, a ternary compound nucleates in contact with silicon. The final structure at the plateau of the resistance curve is a multilayer of $\mathrm{CoSi}/\mathrm{NiSi}/\mathrm{Co}(\mathrm{Ni}){\mathrm{Si}}_2,$ different from the case of Co/Ni thick-layer reaction.