Oxynitrides of silicon have been attracting attention for over a decade now for use as dielectrics in the Ultra Large Scale Integration (ULSI) era. Since nitrogen gives an edge to oxynitrides as compared to oxides, its amount and concentration profile in the oxynitride film are of utmost importance. Because N prevents the penetration of B from the p~+ gate layer during subsequent high temperature processing, it is useful to have N close to the top of the dielectric surface. Also, N imparts other properties like improved hot carrier resistance and reduction of interface state generation; thus, it is required at the dielectric/substrate interface. In this study, we have explored several processing sequences that include oxynitridation in NO or N_2O followed by oxidation in O_2 and a second oxynitridation in NO or N_2O, all at l atm and 900 deg C. It is seen that to obtain a bimodal profile, less than a `critical' amount of N is to be incorporated during the first oxynitridation step. Possible explanations for such findings are discussed.
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