Abstract
We report on the development of a hybrid mirror realized by integrating an EUV-reflecting multilayer coating with a lamellar grating substrate. This hybrid mirror acts as an efficient Bragg reflector for extreme ultraviolet (EUV) radiation at a given wavelength while simultaneously providing spectral-selective suppression of the specular reflectance for unwanted longer-wavelength radiation due to the grating phase-shift resonance. The test structures, designed to suppress infrared (IR) radiation, were fabricated by masked deposition of a Si grating substrate followed by coating of the grating with a Mo/Si multilayer. To give the proof of principle, we developed such a hybrid mirror for the specific case of reflecting 13.5 nm radiation while suppressing 10 μm light, resulting in 61% reflectance at the wavelength of 13.5 nm together with the 70 × suppression rate of the specular reflection at the wavelength of 10 μm, but the considered filtering principle can be used for a variety of applications that are based on utilization of broadband radiation sources.
Highlights
The use of diffractive optical elements plays an important role in advanced optical systems that are based on the utilization of radiation emitted by broadband sources
Magnetron sputtering deposition was employed, enabling layer growth with subnanometer roughness levels over the thickness of the spacer layer. This is a critical requirement in order to obtain high extreme ultraviolet (EUV) reflectance from the Mo/Si multilayer deposited onto the spacer layer [28]
The reflectance of a Mo/Si multilayer deposited on the spacer layer outside the masked area, is 68%, the 1% reduction of the reflectance probably caused by roughness of the spacer layer
Summary
The use of diffractive optical elements plays an important role in advanced optical systems that are based on the utilization of radiation emitted by broadband sources. In the past 20 years various combinations of grating structures with multilayer Bragg-reflector coatings were intensively explored as optical elements with improved spectral characteristics for the EUV and soft X-ray ranges of the electromagnetic spectrum [1,2,3,4,5,6,7] Such multilayer gratings are attractive for optical applications that require high reflection for radiation in the EUV wavelength range and simultaneously demand suppression of radiation outside this prime range. As an advantage application of multilayer grating mirrors for the radiation filtering allows straightforward cooling unlike free-standing grids and foils, and these ideally could replace a standard multilayer optical element that allows to avoid introduction of additional elements in the existing optical system These filter designs use a filtering principle based on the diffractive deflection of OoB radiation from the direction of EUV beam propagation. We applied an alternative manufacturing procedure allowing a significant reduction of the grating period with respect to the previously reported method [7] improving the angular separation of “useful” and “parasitic” radiation
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