Multilayer X-Ray Mirrors; The State Of The Art.

Abstract

A review is given of the techniques to produce multilayer coatings for soft X -ray reflection. Current work of the improvements of the energy resolution of these systems is described. The state of the art in the production of imaging objectives is reviewed. I IntroductionArtificially layered structures have been of considerable interest during the last decades. The use in visible optics is widely known. Application of these stuctures in the nanometer range can be found in superconductivity30), magnetism33), neutron spectroscopy and optics in the regime from far UV to soft X -rays. Multilayer coatings can be applied to produce X -ray imaging optics as well as to perform spectroscopy in the soft X -ray range with a wavelength > 1A. Such coatings are basically stacks of thin films, composed of materials with alternately high and low scattering power or equivalently, optical density. The reflectivity of the structures is based on interference and therefore only occurs when the Bragg condition is satisfied. In principle all material combinations are possible, but in order to obtain stable structures, materials have to be chosen that do not interdiffuse. This problem was encountered in the first successful attempts to synthesize multilayer structures: combinations of Pb/Mg and Au/Mg, as produced by Dinklage and Frerichs ), turned out to degrade within days, while Fe/Mg structures were stable for at least one year. The dispersive quality of multilayer structures depends primarily on the optical properties of the material and the number of layers and is reduced by microroughness of the substrate and the interfaces. The absorption of the materials should be as low as possible in the wavelength range of interest. After the initiation of modem X -ray multilayer fabrication by Spiller6) and Barbee9) and coworkers, the interest in use and production of these superstructures is still growing. This is reflected by several conferences specially dedicated to this research object. Most of the present day fabrication is done by one of two basic deposition methods: Electron -beam evaporation and Sputter deposition. Both methods have their advantages and disadvantages for the various applications of multilayer coatings. Modeling of the properties of a coating can be done by using one of two approaches: An atomic and a macroscopic approach. In the atomic approach (see e.g. Underwood10)) each atom is considered to scatter or adsorb the radiation. The interaction takes place with the electrons and is strongly dependent on the wavelength of the incoming waves. The reflection is calculated by summing the partially scattered waves. In the macroscopic approach a complex refractive index is attributed to each layer14) and the reflectivity is calculated using Fresnel's equations. Because the two approaches describe the same phenomenon, a relation between the refractive index and the atomic scattering factors can be found13), Characterization of the properties of a coating can be done with a variety of methods (Thome24)). The most important ar...

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