In this paper we present the results of a study of the effect of high-temperature stress annealing in nitrogen on the index of refraction of SiO<sub>2</sub> layers in MOS (metal/oxide/semiconductor) devices. The dependence of mechanical stress in the Si-SiO<sub>2</sub> system on the annealing conditions has been experimentally characterized. Subsequently, we have correlated such properties with the dependence of the index of refraction on processing conditions and oxide thickness.
In this paper, spectroscopic ellipsometry (SE) has been used to investigate SiO<SUB>2</SUB>-Si structures, which were subjected to annealing under a high hydrostatic pressure at high temperature (HPHT) processes. Temperature and pressure varied from 450 degrees Celsius to 1280 degrees Celsius and from 10<SUP>5</SUP> Pa to 1.2 GPa, respectively. Investigations have been carried out by variable angle spectroscopic ellipsometer VASE of J.A. Woollam Co Inc. Using the standard optical model of the SiO<SUB>2</SUB>-Si structure, no satisfying fit of measured and calculated (Psi) and (Delta) ellipsometric parameters characteristics can be obtained. This inconsistency, expressed by the MSE parameter (Mean Squared Error), is proportional to temperature, pressure and time of the HPHT process. Elongated convexities on SiO<SUB>2</SUB> surface have been observed by scanning electron microscope (SEM). It was found that depolarization effect takes place as a result of non-uniformity of either chemical composition or SiO<SUB>2</SUB> film thickness. A model consisting of upper SiO<SUB>2</SUB> surface roughness, SiO<SUB>2</SUB> layer and SiO<SUB>2</SUB>-Si interface has been used for data analysis. Application of this model allowed determination of changes in thickness and refractive index of SiO<SUB>2</SUB> as a result of the HPHT process. An increase of refractive index and decrease of layer thickness can be ascribed to excessive stress in SiO<SUB>2</SUB> layer. This stress has been probably caused by densification of silicon dioxide.