Silicon oxynitride has been used as a shallow gate oxide material for microelectronics and its thickness has been reduced over the years to only a few tens of angstroms due to device size scaling. The nitride distribution and density characteristic in the gate oxide thus becomes imperative for the devices. The shallow depth profiling capability using time-of-flight secondary ion mass spectrometry (TOF-SIMS) has huge potential for the nitrogen characterization of the shallow gate oxide film. In this article, both positive and negative spectra of TOF-SIMS on silicon oxynitride have been extensively studied and it was found that the silicon nitride clusters SixN− (x = 1–4) are able to represent the nitrogen profiles because their ion yields are high enough, especially for the low-level nitride doping in the oxide, which is formed by the annealing of nitric oxide on SiO2/Si. The gate oxide thickness measured by the TOF-SIMS profiling method using 18O or CsO profile calibration was found to correlate very well with transmission electron microscope measurement. The nitrogen concentration in the gate oxide measured using the TOF-SIMS method was consistent with the results obtained using the dynamic SIMS method, which is currently applied to relatively thicker oxynitride films. Copyright © 2012 John Wiley & Sons, Ltd.