We have investigated the reactions of thin Ni films on Ge-on-Si substrates with patterned SiO2 structures. For rapid thermal anneals (RTAs) hotter than similar to 300 degrees C, an undesirable growth mode is observed whereby voids form in the Ge next to the SiO2 and germanide grows over the SiO2. A model is proposed in which Ge is the dominant diffusing species during germanidation in the presence of topographies with Ge/SiO2 boundaries. This undesirable growth is suppressed by the use of a 2-RTA process, preferably involving an RTA1 at similar to 250 degrees C, followed by a selective etch of the unreacted Ni, and an RTA2 at similar to 330 degrees C. (c) 2007 The Electrochemical Society.