THERMAL NITRIDATION OF SIO2-FILMS IN AMMONIA - ISOTOPIC TRACING OF NITROGEN AND OXYGEN IN THE INITIAL-STAGES

We investigated the transport of oxygenic and nitrogenic species durin g the initial stages of thermal nitridation of silicon dioxide films i n ammonia by means of isotopic tracing of oxygen and nitrogen. The tot al amounts of the different isotopes incorporated during oxidations an d nitridations under O-18(2), O-16(2), (NH3)-N-15, or (NH3)-N-14 were determined by nuclear reaction analyses, which are highly sensitive an d selective. The depth profiles of the different isotopes were determi ned using methods based on very narrow resonances existing in the nucl ear reaction cross sections at low energies. These methods lead to hig h depth resolution, especially near the surface, where most of the oth er available methods fail in providing acceptable depth resolutions. T he results here reported show that during the initial stages of therma l nitridation, both oxygenic and nitrogenic species are transported to ward the oxynitride/silicon interface as well as toward the surface. O xygen atoms are lost from the oxynitride film surface, exchanged for t he incorporated nitrogen atoms. A strong exchange between the nitrogen atoms incorporated into the oxynitride film and those from the ammoni a gas is also observed. The dependence of the nitrogen profiles on the ammonia pressure and on the nitridation time in the initial stages in dicates that the overall nitridation process is governed by diffusion mechanisms.