In this article, we report the results of a study of the effects of high-temperature stress annealing in nitrogen on the index of refraction of SiO 2 layers in metal/oxide/semiconductor (MOS) devices. In this study, we have experimentally characterized the dependence of mechanical stress in the Si-SiO 2 system on the oxidation and annealing conditions and correlated such properties with the dependence of the index of refraction on processing conditions and oxide thickness. We consider the contributions of the thermal-relaxation and nitrogen-incorporation processes in determining changes in the index of refraction with annealing time. This description is consistent with other annealing studies carried out in argon where only the thermal-relaxation process is present. Correlations of these experimental observations with the electrical properties of the same MOS devices are presented in a companion article. © 2002 American Institute of Physics.