Low-energy ion polishing of Si in W/Si soft X-ray multilayer structures

R V Medvedev,D Ijpes,A A Zameshin,I A Makhotkin,S N Yakunin,A E Yakshin,K V Nikolaev,F Bijkerk

doi:10.1063/1.5097378

R V Medvedev, D Ijpes + Show 6 more

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https://doi.org/10.1063/1.5097378

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Abstract

The effect of ion polishing in sputter deposited W/Si multilayer mirrors with a d-spacing of 2.5 nm was studied. 0.1 to 0.5 nm of Si were etched with 100 eV Ar+ ions. This process resulted in a pronounced reduction in diffused scattering, measured at wavelengths about 0.1 nm. However, CuKa X-ray specular reflectivity and AFM showed only a marginal reduction of the roughness amplitude in the systems. Furthermore, the soft X-ray reflectivity at 0.84 and 2.4 nm did not show any changes after the ion polishing as compared to the nonpolished structures. Grazing incidence X-ray reflectivity (GIXR) analysis revealed that there was no pure W present in the deposited multilayers, with WSi2 being formed instead. As a result, it was concluded that the initial roughness in W/Si multilayers grown by magnetron sputtering is not the major factor in the reflectivity deviation from the calculated value for an ideal system. Nevertheless, the grazing incidence small-angle X-ray scattering (GISAXS) analysis revealed that ion polishing reduces the vertical propagation of roughness from layer to layer by a factor of two, as well as favorably affecting the lateral correlation length and Hurst parameter. These improvements explain the reduction of diffused X-ray scattering at 0.1 nm by more than an order of magnitude, which is relevant for applications like high resolution XRD analysis.

Highlights

Multilayers with nanoscale thick layers are used in a large variety of applications, for the reason that they can be tuned to a specific wavelength and working angle required for a particular task
We investigated the effect of ion polishing of Si in W/Si multilayer mirrors with a period of 2.5 nm, deposited by magnetron sputtering
The lateral correlation length increased from 10 nm to 25 nm, and the Hurst parameter increased from 0.54 to 0.76 as a result of Si layer polishing

Summary

Introduction

Multilayers with nanoscale thick layers are used in a large variety of applications, for the reason that they can be tuned to a specific wavelength and working angle required for a particular task. One of the applications is X-ray fluorescence spectroscopy, where multilayers serve as analyzing crystals to resolve fluorescence emission lines from the investigated material. In this work, such analyzers are considered to resolve spectral lines from O Ka to Al Ka (525 eV–1486 eV) in order to produce a qualitative analysis of materials in this range. This corresponds to the range of wavelengths 0.84–2.4 nm. Intermixing of the elements can contribute to an additional interfacial roughness formation, which will further reduce reflectivity

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