Reliable determination of wavelength dependence of thin film refractive index

Abstract

Depending on the choice of thin film models and measurement data used for the characterization analysis one can obtain essentially different characterization results. It is especially difficult to reliably determine refractive index wavelength dependencies in the case of low accuracy measurement data. We consider possible approaches aimed to improve a stability of refractive index determination. The ways of the verification of characterization results are also discussed. Practical examples used to illustrate the proposed approaches are connected with the most difficult case of the determination of the refractive indices of fluoride films in the VUV spectral region.

Oscillations in spectral behavior of total losses (1 − R − T) in thin dielectric films

“…This dependence is frequently called total losses (TL) in the investigated thin film sample [4,7]. Typically, in the spectral range where the substrate and the thin film are non-absorbing and non-scattering, one expects zero total losses in the sample.…”

Optical characterization and reverse engineering based on multiangle spectroscopy

“…8, noticeable deviations of ripples of model spectral characteristics from ripples of measured characteristics are still observed. According to the analysis described in [5], these deviations can be connected with random errors in layer thicknesses d 1 ; …; d m . We consider relative errors in layer thicknesses and find these errors by minimizing the discrepancy function estimating a closeness between model and measured R and T data:…”

“…In the course of the reverseengineering procedures, we follow the same two-step algorithm that was proposed in [5]. At the first step, we refine the refractive indices of HfO 2 and SiO 2 .…”

“…However, for optical characterization of dielectric films having thicknesses of a few tens of nanometers, another approach can be used. In [5], we proposed using reverse engineering of a quarter-wave mirror for the determination of refractive index wavelength dependencies of thin films forming this mirror.…”

“…Optical characterization based on normalincidence R and T measurements requires applying a special methodology for the reliable determination of thin film optical parameters [4,5]. In comparison with optical characterization of single thin films, reverse engineering of multilayer stacks is an even more complicated problem because, in general, more unknown parameters are to be found.…”