Ultra-thin silicon oxynitride (SiOxNy) is the leading candidate to replace pure silicon oxide (SiO2) before high k dielectrics come into place because oxynitrides demonstrate several properties superior to those of the conventional gate oxides. The performance of the transistor was reported to depend on the N dose and its distribution in the gate oxide. Therefore, accurate characterization of SiOxNy is prerequisite to control the quality of the ultra-thin nitrided gate oxide. However, secondary ion mass spectrometry (SIMS) faces big challenges in analyzing ultra-thin gate oxide because of surface effect and matrix effect. In this work, MCs2+ (M stands for matrix element) was detected to reduce the matrix effect in depth pro. ling the ultrathin oxynitride. However, N pro. le was very close to the top surface if the oxynitirde was fabricated by decoupled plasma nitridation (DPN). With the conventional approach, the N dose was overestimated and the N pro. le was distorted near the top surface. To obtain a reliable N pro. le, the oxynitride was capped with a thin layer of oxide. The N pro. le of interest was hence in the region of sputtering equilibrium, i.e. the surface effect was minimized. As the results, reliable N dose and pro. le have been obtained. (C) 2008 Elsevier B. V. All rights reserved.