Enhanced Reliability of Thin Silicon Dioxide Grown on Nitrogen-Implanted Silicon

ABSTRACTThin oxides (3-20 nm) have been grown on nitrogen-implanted silicon by regular thermal oxidation and by rapid thermal oxidation in dry oxygen. The implant dose ranged from 1×1013 to 1×1015 cm−2. Significant oxidation retardation has been observed for nitrogen doses above 1×1014 cm−2. Al-gate MOS capacitors were fabricated to characterize the thin oxides for dielectric breakdown strength and leakage. Gate oxides, grown by our standard baseline process, exhibited a decrease in their dielectric strength from ∼10 MV/cm for thickness > 18 nm to 3-4 MV/cm for < 8 nm thickness. The nitrided oxides maintained their integrity at ∼10 MV/cm as thickness decreased, unless a critical dose was exceeded, which resulted in poor performance. These electrical measurements indicate that a nitrogen implant, prior to gate oxide growth, is beneficial to oxide integrity.The structure of the SiO2/Si interface has been probed using X-ray photoelectron spectroscopy (XPS) and analyzing Si 2p core level spectra. The XPS analyses on as grown samples of nitrided and un-nitrided oxides of similar thickness (3 nm) do not show any significant suboxide peaks corresponding to Si1+, Si2+ or Si3+ states at the interface. However, on exposing the surface to argon ion sputtering at 3.5 kV for 30 seconds, prior to XPS analysis, the presence of suboxides at the SiO2/Si interface is detected. The SiO suboxide (Si2+) density in oxides grown on nitrogen-implanted silicon is much less than that in the oxides grown on unimplanted silicon. This is a direct evidence of sputter damage resistance of nitrided thin oxides. The beam-induced damage in the oxide is also found to be less in nitrided oxides. The suppression of suboxide formation at the interface due to the presence of nitrogen appears to be responsible for the enhanced reliability of nitrided oxides.