The impact of nitrogen plasma nitridation on the interfacial quality of ultrathin oxides (1.8 and 2.6 nm) have been investigated and compared with NO nitridation. It is found that plasma-nitrided oxides are more immune to nitridation-induced degradation of channel hole mobility, and have lower intrinsic interface-trap density as compared to NO-nitrided oxides. In addition, plasma-nitrided oxides can further suppress hole mobility degradation induced by boron penetration. The superior performance of nitrogen plasma nitridation is attributed to its capability of incorporating a high level of nitrogen at the top oxide surface, while keeping the Si-SiO2 interface intact. (C) 2002 The Electrochemical Society.