Determination of the optical function n(λ) of vitreous silica by spectroscopic ellipsometry with an achromatic compensator.

Studies of dielectric materials by rotating-element spectroscopic ellipsometry (SE) are beset with a number of problems such as (1) low reflectance and hence low signal-to-noise ratio and (2) an almost zero (or 1800) change in the ellipsometric parameter A on reflection from the sample, which leads to significant errors in the measured parameters. These difficulties were overcome (1) by developing suitable procedures for correcting nonlinearity in the detection system and the deleterious effects of ambient light and (2) by incorporating an achromatic quarter-wave compensator in the SE system, respectively. A new rapid method of aligning and calibrating the compensator has also been developed. Test measurements with such an SE system on a vitreous silica sample revealed that the accuracy of measurements of Δ and Ψ are 0.03° and 0.015°, respectively, over the spectral range of 300-700 nm. The SE data were then analyzed by standard procedures with linear regression analysis to determine the optical function n(λ) (i.e., refractive index and its dispersion with wavelength) of vitreous silica and at the same time to characterize the microroughness of the surface layer of the sample. The refractive index of vitreous silica determined by this technique is within ±0.0004 of the best reported values in the literature over the spectral range of 300-700 nm. The SE data obtained on the vitreous silica sample revealed the presence of a 1.0-nm-thick microrough layer on the surface of the sample.