The redistribution of O and N during the final, thermal oxidation in dryO2step in the formation of ultrathin silicon oxide/nitride/oxide dielectric films (ONO) was investigated using isotopic tracing and depth profiling with nanometer resolution. The results show that the final oxidation step induces atomic transport of O and N species in the system, such that the formed ONO structures are not stacked layer structures, but rather a silicon oxynitride ultrathin film, having moderate concentrations of N in the near-surface and near-interface regions, and a high N concentration in the bulk. ©1998 American Institute of Physics.