Silicon oxynitride is one of the most researched silicon-based dielectric currently used in IC fabrication. Properties of the oxynitride films can be modified to achieve desirable functions by specific engineering of thickness and stoichiometry. In the case of gate stacks, considerable progress has been made in optimising the properties of oxynitride dielectrics in order to reduce boron penetration and dielectric leakage with respect to pure SiO2, allowing sub-2 nm dielectrics. In this study ultra low energy (ULE) secondary ion mass spectrometry (SIMS) has been used to assess a range of ultra thin (< 1.5 nm) silicon oxynitride films containing varied amounts of nitrogen (up to 18 at.%) and the results have been compared with shallow angle surface X-ray photoelectron spectroscopy (XPS). Despite the limited number of atomic layers available for probing, this investigation indicates that both techniques are capable of extracting useful and reliable chemical and physical information from these oxynitride films. Critical parameters such as stoichiometry and film thickness have been evaluated, whilst consistency between the two techniques is also addressed. (c) 2006 Elsevier B.V. All rights reserved.