We provide an analysis of contemporary multilayer optics for extreme ultraviolet (EUV) solar astronomy in the wavelength ranges: λ=12.9-13.3 nm, λ=17-21 nm, λ=28-33 nm, and λ=58.4 nm. We found new material pairs, which will make new spaceborne experiments possible due to the high reflection efficiencies, spectral resolution, and long-term stabilities of the proposed multilayer coatings. In the spectral range λ=13 nm, Mo/Be multilayer mirrors were shown to demonstrate a better ratio of reflection efficiency and spectral resolution compared with the commonly used Mo/Si. In the spectral range λ=17-21 nm, a new multilayer structure Al/Si was proposed, which had higher spectral resolution along with comparable reflection efficiency compared with the commonly used Al/Zr multilayer structures. In the spectral range λ=30 nm, the Si/B4C/Mg/Cr multilayer structure turned out to best obey reflection efficiency and long-term stability. The B4C and Cr layers prevented mutual diffusion of the Si and Mg layers. For the spectral range λ=58 nm, a new multilayer Mo/Mg-based structure was developed; its reflection efficiency and long-term stability have been analyzed. We also investigated intrinsic stresses inherent for most of the multilayer structures and proposed possibilities for stress elimination.
The effect of Be layers on the reflection coefficients of Mo/Be/Si multilayer mirrors in the extreme ultraviolet (EUV) region is reported. Samples were studied using laboratory and synchrotron based reflectometry, and high-resolution transmission electron microscopy. The samples under study have reflection coefficients above 71% at 13.5 nm and more than 72% at 12.9 nm in a near normal incidence mode. Calculations show that by optimizing the thickness of the Be layer it should be possible to increase the reflection coefficient by another 0.5-1%. These results are of considerable interest for EUV lithography.
We have studied the surface treatment of polished fused silica by neutralized Ar ions with energy of 500-1500 eV and incidence angles of 0-90°. We found the following regularities: for samples that passed the standard procedure of deep polishing (initial effective roughness σ(eff)∼0.5 nm), the effective roughness decreases to the ultrasmooth level (i.e., σ(eff)∼0.25 nm in the range of spatial frequencies q∈[4.9×10(-2)-63] μm(-1)). The effect begins to be noticeable at the material removal of 150 nm and reaches saturation at depths of removal greater than 1 μm. For supersmooth samples (σ(eff)<0.3 nm), the effective roughness keeps the initial level at material removal down to tens of micrometers. The optimal ion energy range is 800-1300 eV (maximum smoothing effect); at higher energy some surface roughness degradation is observed. All the smoothing effects are observed at the incidence angle range θ(in)=0-35°. Increasing the ion energy above 1300 eV increases the etching rate by up to 4 μm per hour (J(ion)=0.8 mA/cm2), which allows for deep aspherization of sized substrates. The technique allows for manufacturing the optical elements for extreme ultraviolet and soft x-ray wavelength ranges with a numerical aperture of up to 0.6.
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