Mechanical Stresses During Solid State Amorphization of Zr/Co Multilayers

Thin film structures of crystalline Zr and Co, deposited in UHV, are investigated by bending beam technique and by X-ray diffraction. During isothermal annealing and interdiffusion reaction of the thin film double layer and multilayer packages large compressive stresses are generated while an amorphous ZrCo-phase is formed. This can, at first hand, be understood in terms of the Kirkendall effect where Co atoms, as the main moving species, lead to a volume increase of the film beyond the Co interface. The observed change in Zr lattice spacing in accordance with the evolution of mechanical stress indicates that the compressive stress is built up particularly within the Zr layer due to the solution of Co in Zr grains during the initial amorphization reaction. Film structures, having Co already present in the crystalline Zr layer after film deposition, show a decrease in reaction kinetics combined with a lower stress level, indicating that the interdiffusion reaction is depending on the stress state in the Zr grains. At late stages of annealing in high vacuum a sudden increase of additional compressive stress is observed, which could be attributed to the oxidation of Zr, very likely due to the formation of diffusion paths for oxygen through the Co (cap-) layer (Kirkendall voids). Such oxidation behavior was not observed with samples measured in situ in UHV directly after film deposition.