A procedure was found to obtain a reflectance increase with multilayers of absorbing materials. It started by selecting the set of materials with the most extreme optical constants at the desired wavelength (1, 2). If we ignore any imperfection in the bulk or at the interface of each layer, calculations predict a persistent reflectance increase by adding each material with extreme optical constants to the multilayer. Each material would contribute to the multilayer with a sub-quarterwave thickness, so that a period with a layer of each selected material would approach two quarterwaves, as with standard two-material multilayers.
The reflectance increase can be obtained comparing both with single layer coatings, such as in the long extreme UV (EUV) or with standard two-material multilayers, such as in the short EUV and soft x-rays. Examples of reflectance increase over standard multilayers were obtained at 50 nm, 30.4 nm, 11.3 nm, and 13.4 nm. Simple algorithms to design such multilayers were developed for multilayers both with and without barrier layers.
Plot of the intrinsic spectral bandwidth of ten-period, subquarterwave multilayers optimized for the highest reflectance at 50.0 nm at normal incidence. The optical constants of the different materials were set constant throughout the spectral region shown, equal to their value at 50.0 nm. The benefit of increasing the number of materials from two to three is observed. The top curve corresponds to the reflectance of a multilayer with all the materials in the outermost polygon