Abstract
We present an analytical method for the design of narrow-band X-ray multilayer coatings having greatly reduced reflected side-lobe intensity, for the realization of X-ray mirrors that have improved spectral purity. The method uses a specific variation of the individual layer thicknesses as a function of depth in the multilayer stack, derived from Laplace transform analysis of the multilayer's reflectance profile. The design process and mathematical foundations are outlined. Pt/C multilayers with 5 nm d-spacing for hard X-rays are designed, fabricated and measured to demonstrate the validity and effectiveness of the method are presented. As an extrapolation, three additional side lobe suppressed multilayers for soft X-rays and EUVs are also designed and investigated: 1) Cr/Sc multilayer for soft X-rays (4.96 nm wavelength) at high grazing angle (30°), 2) Mo/Si multilayer for EUV (13.5 nm wavelength) at normal incidence angle and 3) SiC/Mg multilayer for EUV (30.4 nm wavelength) at normal incidence angle. The calculated reflectances demonstrate that the presented method is robust for the energy range from X-rays to EUVs.
Highlights
Nanometer-scale multilayer structures are widely used as X-ray reflective coatings for a variety of applications [1,2,3,4]
Periodic multilayer coatings in particular, which consist of a number of repeating, identical bi-layers, can act as highly efficient narrow-band X-ray filters operating in reflection
The side lobes that are present in multilayer reflectance profiles can degrade spectral purity
Summary
Nanometer-scale multilayer structures are widely used as X-ray reflective coatings for a variety of applications [1,2,3,4]. Side lobes in multilayer coatings designed for operation in the visible portion of the spectrum have been suppressed previously by precisely controlling the thickness and the optical constants of the materials comprising each layer in the film stack, using, e.g., chemical etch-leach or co-evaporation processes [9,10,11,12]. This approach cannot be used effectively for X-ray multilayers, due to the nature of optical constants of solids in the X-ray range, and incompatibility with the deposition techniques required for precise X-ray multilayer deposition [1]. Suppression, based on control of individual layer thicknesses following purely analytical methods
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