Phase formation and interfacial microstructure evolution of nickel germanides formed by rapid thermal annealing in a 15-nm Ni/Ge (100) system have been studied. Coexistence of a NiGe layer and Ni-rich germanide particles was detected at 250°C. Highly textured NiGe film with a smooth interface with Ge was observed. Annealing at higher temperatures resulted in grain growth and severe grooving of the NiGe film at the substrate side, followed by serious agglomeration above 500°C. Fairly low sheet resistance was achieved in 250-500°C where the NiGe film continuity was uninterrupted.