The effect of the sputtered Ni layer thickness (7–14nm) on the silicide phase transition was studied at 260°C. In 7-nm-thick layers, the complete mixing of Ni and Si occurring during deposition produces stable Ni2Si transrotational structures which further evolve into similar NiSi domains within 50min of annealing. In 14-nm-thick layers, the residual unmixed Ni atoms diffuse towards the interface and speed up the transition from Ni2Si to NiSi (25min) by promoting the nucleation of polycrystaline NiSi grains. A competition between NiSi trans- and poly-structures occurs, and the resulting layer morphology depends on the reaction temperature.