Secondary ion mass spectrometry (SIMS) and electron spectroscopy for chemical analysis were used to study SiO2/Si3N4/SiO2 (ONO) structures. The purpose was to determine if the nitrogen tail seen going into the silicon substrate was real or an artifact of ion bombardment. To determine this without an element of doubt, samples were thinned from the backside to the ONO layer and SIMS depth profiling was carried out on the exposed underside of ONO. Profiling from the backside shows there is no nitrogen tail in the silicon substrate and there is no nitride at the oxide/Si interface, as seen from front side profiling. The effects seen during profiling from the front side are due to anomalous diffusion of nitrogen caused by ion bombardment. It is believed that nitrogen-oxygen complexes are formed in the silicon substrate as a result of nitridation, and this adversely affects device performance. Though this may still be true, one needs to be cautious in interpreting SIMS and Auger depth profiles from the front side in order to corroborate the electrical results.