Time-frequency analysis assisted reconstruction of ruthenium optical constants in the sub-EUV spectral range 8nm-23.75nm

TL;DR

This paper presents a novel method combining time-frequency analysis and Bayesian inference to accurately reconstruct ruthenium's optical constants in the EUV spectral range, addressing uncertainties and sample surface effects.

Contribution

It introduces an integrated inverse-problem optimization strategy using time-frequency analysis, Bayesian inference, and surface modeling for optical constant reconstruction.

Findings

Reconstructed ruthenium optical constants with quantified uncertainties.

Validated the model's robustness against contamination and oxidation.

Compared results with previous datasets, demonstrating improved accuracy.

Abstract

The optical constants of ruthenium in the spectral range 8 nm to 23.75 nm with their corresponding uncertainties are derived from the reflectance of a sputtered ruthenium thin film in the Extreme Ultraviolet (EUV) spectral range measured using monochromatized synchrotron radiation. This work emphasizes the correlation between structure modelling and the reconstructed optical parameters in a detailed inverse-problem optimization strategy. Complementary X-ray Reflectivity (XRR) measurements are coupled with Markov chain Monte Carlo (MCMC) based Bayesian inferences and quasi-model-independent methods to create a model factoring the sample's oxidation, contamination, and surface roughness. The sensitivity of the modelling scheme is tested and verified against contamination and oxidation. A notable approach mitigating the high dimensionality of the reconstruction problem is elaborated with the results of this work compared to two previously published datasets. The presented dataset is of high interest for the continuing development of Extreme Ultraviolet Lithography (EUVL) and EUV astronomy optical systems.