Extreme ultraviolet optical constants for the design and fabrication of multilayer-coated gratings

Abstract

The computational design of multilayer-coated diffraction gratings for the extreme ultraviolet (EUV) wavelength region and the experimental performance of the coated gratings depend on the optical constants of the layer materials. While accurate optical constants are available for many commonly used materials, the EUV optical constants can in practice differ significantly from the tabulated values. This is generally true near absorption edges, for reactive materials that may be subject to oxidation or contamination, and for the longer EUV wavelengths (>30 nm) where molecular effects can be important. Normal-incidence gratings with Mo/Si coatings operating in the 17-21 nm and 25-29 nm wavelength ranges were successfully designed and fabricated for the Extreme Ultraviolet Imaging Spectrometer (EIS) on the Solar-B mission, the first satellite instrument to carry a multilayer grating. Examples of multilayer gratings designed and fabricated for wavelengths <12 nm and >40 nm, using materials other than Mo/Si, will be given that have in many cases required the experimental determination of the optical constants owing to inaccuracies in the tabulated values.