It was found that applying a capping layer has an important impact on the work function (WF) of Mo gated metal-oxide semiconductor stacks. Before any postmanufacturing thermal treatment, uncapped Mo has a similar WF as one capped with a TaC layer. However, after forming gas anneal (FGA), the uncapped Mo gate exhibits a significantly higher WF than the TaC capped one does. This is understood as O incorporation during deposition, storage or FGA, and its subsequent piling up at the Mo/dielectric interface during FGA in the former case, which is an effect prevented by TaC capping. (C) 2008 American Institute of Physics.