Ellipsometric results may be in error for measuring the thickness of ultrathin oxide films on silicon because of the apparent refractive index changes with thickness. This problem have been assessed by comparing results on oxide thickness measured by ellipsometry with results of measurements by two independent techniques, such as medium energy ion scattering spectroscopy (MEIS) and high-resolution transmission electron microscopy (HRTEM), which should not be subject to error. Fitting the various ellipsometric models to the corresponding experimental spectroscopic ellipsometry data, we obtained the thicknesses and refractive indices of ultrathin $SiO_2$ films. The effective refractive index of the oxide film increased exponentially as the film thickness decreased from 7.5 nm to 1.5 nm. The oxide thicknesses are in a good agreement with each other, especially except for the results obtained by the bulk $SiO_2$ of ellipsometry and Si MEIS peaks.
Using variable-angle spectroscopic ellipsometry, we examined optical property of thermally grown $SiO_2$ layer on Si, with special focus on phase difference Δ and amplitude ratio tan φ for the s- and p-waves. The sensitivities for cos Δ and tan φ were shown to have maximum values at the same angle of incidence and photon energy in the s-wave antireflection condition. By fitting the variable-angle spectroscopic ellipsometry data measured in the s-wave antireflection condition, the thickness of the Si-$SiO_2$ interface was determined as about 0.77 nm for a 52-nm and 150-nm thick oxide sample, and the effective refractive index of the interface was about 2.02 at 546 nm for two samples. The interface structure of ultrathin $SiO_2$ films (less than 7.4 nm in film thickness) on c-Si substrate is obtained using the ellipsometric modeling of the O and Si depth profile by MEIS.
The interface structure of a graded $SiO_2/Si$ of 1.3 nm thickness based on the O and Si depth profile by MEIS shows nearly the same optical properties in Sellimeier approximation with the 0.6 nm interface and in two-film model with 0.66 nm interface obtained from the ellipsometric analysis by Bruggeman effective medium approximation (BEMA). This report shows that ellipsometric modelings can provide thickness information consistent with more detailed interface analysis by MEIS, which improves the reliability of ellipsometric analysis in nm range.
