Atom-probe study of the initial stage of silicide formation

The initial stage of silicide formation was investigated utilizing the high mass resolution atom probe. Silicon-deposited Ni and W specimen tips were heated to various temperatures in a vacuum of 10^-7 Pa. A highly ordered image, which agreed well with the computer-simulated image of WSi₂ with the C11b structure, was observed after heating the tip to 950 K. Ni formed a mixed layer with Si even at 80 K. The depth profile of the silicide composition changed with temperature due to a high diffusibility of Ni. The silicide formed above 900 K consistently showed a well-ordered image corresponding to a cubic structure, while its composition varied from Ni₃Si₂ to NiSi with depth. The computer-simulated image suggests that the observed ordered structure could be a modified Cl structure. The evaporation field of the ordered silicide was found to be significantly higher than those of pure Ni and Si, possibly due to a strong binding between the constituent atoms.