A novel NH4F-last pregate clean process is proposed for high-k gate dielectric fabrication. Compared to conventional DHF-last pregate cleaning processes, the use of a novel NH4F-last pregate clean process improved Si interfacial thermal stability with HfO2 and enhanced channel carrier mobility. These improvements in the high-k gate stack could be attributed to the more ideal Si surface produced by the NH4F treatment. (c) 2005 The Electrochemical Society.