THERMAL NITRIDATION OF SIO2-FILMS IN AMMONIA - ISOTOPIC TRACING OF NITROGEN AND OXYGEN IN FURTHER STAGES AND IN REOXIDATION

We investigated the transport of oxygenic and nitrogenic species durin g thermal nitridation of silicon dioxide films in ammonia in a resista nce heated furnace, as well as during subsequent reoxidation of the re sulting oxynitride films. Isotopic tracing of oxygen and nitrogen was used, showing that the thermal nitridation of SiO2 films during long t ime intervals (around 1 h) is driven by essentially the same mechanism s as in the initial stages: the transport of NH3 by interstitial diffu sion toward the oxynitride silicon interface reacting with the silicon network as well as toward the surface where it desorbs. There is a ge neral enhancement of the effects previously observed in the initial st ages of nitridation, namely, the amount of nitrogen accumulated in the SiO2/Si interface is greatly enhanced. Thermal reoxidation in dry O-2 of the oxynitride films implies en increase of the film thickness. Th e isotopic tracing of oxygen and nitrogen during thermal reoxidation s hows the incorporation of freshly arriving oxygen mainly in the surfac e and in the oxynitride/silicon interface regions. Nitrogen atoms near the surface and near the interface are exchanged for oxygen atoms int roduced during the reoxidation. A general discussion of the mechanisms of thermal nitridation of SiO2 films in NH3 and the symmetry between the mechanisms of thermal nitridation and of thermal reoxidation in dr y O-2 of the resulting oxynitride films is presented.