We use spatially resolved spectroscopy in a scanning transmission electron microscope to study the thermal stability of the HfOxNy/Si(001) system with and without an in situ capping layer of silicon. The films were deposited by metallorganic chemical vapor deposition using the amide precursor tetrakis(diethylamido)hafnium with NO as the oxidant. A SiOxNy interfacial layer (similar to 1.8 nm) is observed at the HfOxNy/substrate interface for films directly exposed to air. In addition, the N loss in the HfOxNy film for the uncapped sample is significant. In contrast, in situ capping is found to reduce the thickness of the interfacial layer and to keep the N content in the final dielectric film high. The capped HfOxNy films are quasi-amorphous, whereas uncapped films are polycrystalline following exposure to air. Oxinitridation at both interfaces is observed following a rapid thermal annealing process (900 degrees C in N-2 for 30 s) of the capped HfOxNy film. However, the interfacial layers remain thin (similar to 1 nm) and a significant amount of N is present in the HfOxNy film. The rapid thermal annealing leads to the partial crystallization of the HfOxNy film and the Si capping layer. No Hf silicate is detected on a scale of similar to 6 angstrom the electron energy loss spectroscopy analysis. (c) 2005 The Electrochemical Society. All rights reserved.