We have investigated the thermal stability of nickel monogermanide (NiGe) films formed by rapid thermal annealing on both single- and polycrystalline Ge substrates. We found that the NiGe phase is the only one present after nickel germanidation in the temperature range 400-700°C. A fairly uniform NiGe film formed on the single-crystalline Ge; it possessed excellent resistivity (15.6 μΩ cm) and was thermally stable up to 550°C, but it degraded rapidly at higher temperatures as a result of agglomeration. In contrast, the NiGe film formed on the polycrystalline Ge exhibited much poorer thermal stability, possibly because of polycrystalline Ge grain growth, which resulted in columnar NiGe grains interlaced with Ge grains that had a dramatically increased sheet resistance. As a result, we observed that the sheet resistances of NiGe lines subjected to annealing at 500°C depended strongly on the linewidth when this width was comparable with the grain size of the polycrystalline Ge.