Depth profiling of SiO2/Si3N4/SiO2 structures was performed, using secondary ion mass spectrometry, in order to determine whether the N tail which was seen to go into the Si substrate was real or was an artefact of the ion bombardment. In order to determine this with certainty, samples were thinned from the reverse side to the SiO2/Si3N4/SiO2 layer and secondary ion mass spectroscopy depth profiling was carried out on the exposed underside of the SiO2/Si3N4/SiO2 layer. It was deduced that the layer of so-called nitride at the Si/oxide interface was an artefact of ion bombardment. Profiling from the reverse side showed there was no N tail in the Si substrate and that there was also no nitride at the oxide/Si interface. The interfacial nitride layer, and the apparent N tail which extended into the underlying Si, as detected by front-side profiling, were attributed to the effect of anomalous N diffusion during ion bombardment. It was suggested that N-O complexes were formed in the Si substrate as a result of nitridation, and that this adversely affected device performance. It was concluded that it was necessary to be cautious when interpreting secondary ion mass spectroscopic and Auger depth profiles from the front side.

Anomalous Diffusion of Nitrogen in SiO2 under Ion Bombardment. I.Banerjee, D.Kuzminov: Applied Physics Letters, 1993, 62[13], 1541-3