ANOMALOUS DIFFUSION OF NITROGEN IN SIO2 UNDER ION-BOMBARDMENT

Depth profiling was done by secondary ion mass spectrometry (SIMS) on SiO2/Si3N4/SiO2 (ONO) structures 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 back side to the ONO layer and SIMS depth profiling was carried out on the exposed underside of ONO. It is determined that the layer of ''nitride'' at the Si/oxide interface is really an artifact of ion bombardment. Profiling from the back side shows there is no nitrogen tail in the silicon substrate and there is also no nitride at the oxide/Si interface. The interfacial nitride layer, and the apparent nitrogen tail extending into the underlying Si, seen from front-side profiling, are due to anomalous nitrogen diffusion during 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.