Interstitial injection during oxidation of very low energy nitrogen-implanted silicon

Abstract

Oxidation of nitrogen-implanted silicon is one of the methods developed in order to incorporate nitrogen within a growing thermal gate oxide in order to improve its reliability in the ultra thin (<4 nm) regime. The main advantage of the method is that it enables the formation of various oxide thicknesses across the silicon substrate by performing local nitrogen implantations of various dose and using one single oxidation step, which is very helpful especially for Systems On Chip fabrication. It is also known that oxidation of silicon causes an interstitial injection into the silicon substrate, which enhances the diffusivity of boron, which diffuses via an interstitial-mediated mechanism, in the transistor channel area causing fluctuations in the MOS (Metal-Oxide-Semiconductor) transistor threshold voltage. In this article we present a study of interstitial injection during oxidation of very low energy nitrogen-implanted silicon using boron δ-layers as interstitial monitors. No interstitial injection enhancement in comparison to common dry oxidation was observed. This result is different from N 2O oxynitridation, during which an enhancement of interstitial injection of the order of 20–25% was observed, revealing the influence of interfacial nitrogen on interstitial kinetics. One possible explanation, supported also by other experiments, is that implanted nitrogen acts as an interstitial sink. In particular, it seems that nitrogen “captures” the excess interstitials produced by implantation and oxidation during its non-Fickian diffusion towards the surface.