Reflectance methods for determining the optical constants of highly absorbing films: comparative analysis of the accuracy

Abstract

The accuracies of the most widely used methods for optical characterization of absorbing films with thicknesses greater than the penetration depth of light are compared quantitatively through estimation of the errors Δn and Δk in refractive index, n, and extinction coefficient, k. Among the great variety of possible combinations of reflectance, R, measured in both planes of polarization, Rp and Rs, reflectance ratios, Rp/Rs, and pseudo-Brewster angle, β, only four are selected as the most suitable methods: (R,Rp70), (Rpβ/R,β), (Rpβ,β) and (R,β). It is shown that the methods using β are more accurate than those with reflectance measurements only. Among the three methods with β, the least accurate is (Rpβ/R,β), while (R,β) and (Rpβ,β) have similar accuracies but slightly different application ranges. It is found that only samples with n, k≤3 can be characterized with methods including β if the largest incident angle, which can be measured, is 70°. Application of the error analysis predictions is illustrated through determination of the optical constants of 150 nm thick Au film.