X-ray scattering of periodic and graded multilayers: comparison of experiments to simulations from surface microroughness characterization

TL;DR

This study compares experimental X-ray scattering data with simulations based on surface roughness characterization to validate a model for both periodic and graded multilayers, improving understanding of interface imperfections.

Contribution

The paper extends a surface roughness-based simulation method from periodic to graded multilayers, validating it with experimental data and defect analysis.

Findings

Good agreement between experiments and simulations for both multilayer types

Inclusion of AFM-observed defects improves model accuracy

Surface contamination and interfacial defects identified as scattering sources

Abstract

To enhance the reflectivity of X-ray mirrors beyond the critical angle, multilayer coatings are required. Interface imperfections in the multilayer growth process are known to cause non-specular scattering and degrade the mirror optical performance; therefore, it is important to predict the amount of X-ray scattering from the rough topography of the outer surface of the coating, which can be directly measured, e.g., with an Atomic Force Microscope (AFM). This kind of characterization, combined with X-ray reflectivity measurements to assess the deep multilayer stack structure, can be used to model the layer roughening during the growth process via a well-known roughness evolution model. In this work, X-ray scattering measurements are performed and compared with simulations obtained from the modeled interfacial Power Spectral Densities (PSDs) and the modeled Crossed Spectral Densities for all the couples of interfaces. We already used this approach in a previous work for periodic multilayers; we now show how this method can be extended to graded multilayers. The upgraded code is validated for both periodic and graded multilayers, with a good accord between experimental data and model findings. Doing this, different kind of defects observed in AFM scans are included in the PSD analysis. The subsequent data-model comparison enables us to recognize them as surface contamination or interfacial defects that contribute to the X-ray scattering of the multilayer.