Incorporation of nitrogen yields better electrical properties to silicon oxynitrides as compared to oxides. But the amount and profile of nitrogen in these films need to be carefully monitored and controlled. This study focuses on a fundamental understanding of film properties uniformity in the silicon oxynitridation process along the furnace length. Gas-phase N2O decomposition is monitored using real-time mass spectrometry. Secondary ion mass spectrometry is used to obtain the O and N depth profiles in silicon oxynitrides fabricated at different loading positions in the furnace. Films are grown in N2O starting with both bare Si and a pre-grown oxide. It is found that at 900 degrees C and 1 atm, the amount and concentration profile of N in the oxynitride depend strongly upon gas-phase composition from the entrance to the exit of the furnace. Furthermore, at low N2O gas-phase decomposition, N incorporation is seen to be limited by the reaction at the substrate/oxynitride interface. At higher N2O gas-phase decomposition, the incorporation of N shifts to the solid-phase diffusion-limited regime. Effects of these findings on uniformity of N concentration profiles from wafer-to-wafer are of particular importance in designing furnaces for silicon oxynitridation and in determining wafer loading positions for uniform film properties throughout such systems.