Abstract
Pd/Y multilayer mirrors operating in the soft X-ray region are characterized by a high theoretical reflectance, reaching 65% at normal incidence in the 8-12 nm wavelength range. However, a severe intermixing of neighboring Pd and Y layers results in an almost total disappearance of the interfaces inside the multilayer structures fabricated by direct current magnetron sputtering and thus a dramatic reflectivity decrease. Based on grazing incidence X-ray reflectometry and X-ray photoelectron spectroscopy, we demonstrate that the stability of the interfaces in Pd/Y multilayer structures can be essentially improved by adding a small amount of nitrogen (4-8%) to the working gas (Ar). High resolution transmission electron microscopy shows that the interlayer width is only 0.9 nm and 0.6 nm for Y(N)-on-Pd(N) and Pd(N)-on-Y(N) interfaces, respectively. A well-defined crystalline texture of YN (200) is observed on the electron diffraction pattern. As a result, the measured reflectance of the Pd(N)/Y(N) multilayer achieves 30% at λ = 9.3 nm. The peak reflectivity value is limited by the remaining interlayers and the formation of the YN compound inside the yttrium layers, resulting in an increased absorption.
Highlights
In spite of impressive progress in the fabrication technology of multilayer mirrors for soft Xray (SXR) and extreme ultraviolet (EUV) radiation, these high-quality mirrors were developed for several spectral regions only
Pd/Y multilayer mirrors operating in the soft X-ray region are characterized by a high theoretical reflectance, reaching 65% at normal incidence in the 8-12 nm wavelength range
A severe intermixing of neighboring Pd and Y layers results in an almost total disappearance of the interfaces inside the multilayer structures fabricated by direct current magnetron sputtering and a dramatic reflectivity decrease
Summary
In spite of impressive progress in the fabrication technology of multilayer mirrors for soft Xray (SXR) and extreme ultraviolet (EUV) radiation, these high-quality mirrors were developed for several spectral regions only. In the case of the Pd/Y multilayer fabricated with pure Ar, the ratio of the atomic concentrations of Pd and Y is kept constant (~2.4) starting at a depth of approximately 10 nm This value is very close to the ratio (~2.3) of the deposited atoms of Pd to Y, assuming the bulk density of materials and the same thickness of Pd and Y layers prescribed when designing the structure with abrupt interfaces. Adding 4% nitrogen to the working gas resulted in the formation of a clearly observed periodic Pd(N)/Y(N) structure and an essential mitigation of interdiffusion between the Pd and Y layers. The concentration of yttrium reduced to zero on the surface Following [22] we can assume that interdiffusion of Pd and amorphous Y is suppressed due to incorporation of considerable amount of nitrogen into both materials and formation of strong chemical metalnitrogen bonds decreasing mobility of atoms
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