Low-Temperature Formation of NiSi2 Phase in Ni/Si System

The silicide phase formation is examined in the Ni/Si system. The multiphase of NiSi and NiSi2 is confirmed in a specimen, in which a 30-mm-thick Ni film is sputter-deposited on Si(100) at 350 degrees C and subsequently annealed at 400 degrees C for 1 h. This is interpreted that the NiSi and NiSi2 phases nucleate from the amorphous alloys, as a super-cooled melt, at a composition corresponding to those of eutectic points in different composition, which appear in the intermixed Ni-Si alloy layer with a Ni concentration gradient owing to intermixing between Ni and Si at the interface. To confirm this idea, we performed Ni deposition on a 350 degrees C-heated Si substrate with a thin SiO2 layer. The results show the direct formation of NiSi2 in a nonuniform fashion. This is because the thin SiO2 layer suppresses Ni diffusion into Si, resulting in the formation of a Ni-Si alloy with a Si-rich composition, from which NiSi2 nucleates at a low temperature. We can demonstrate that the high-temperature phase of NiSi2 nucleates under kinetic constraints from an amorphous alloy with a suitable composition.