Be/Si/Al multilayer mirrors as the most promising optical elements for spectroscopy and imaging in the spectral region of 17-32†nm.

The reflective and structural parameters of Be/Si/Al multilayer mirrors have been studied. The extent of stability of their X-ray optical characteristics has been demonstrated during storage in air for 4 years and during vacuum annealing at temperatures up to 100°C. A high reflectance of 62.5% was obtained, together with a spectral selectivity of λ/Δλ≈59 at a wavelength of 17.14 nm and 34%, with λ/Δλ ≈ 31 at a wavelength of 31.3 nm. It was shown that Si interlayers reduce the interlayer roughness from 0.45 to 0.20 nm.

Influence of Mo interlayers on the microstructure of layers and reflective characteristics of Ru/Be multilayer mirrors.

The influence of Mo interlayers on the microstructure of films and boundaries, and the reflective characteristics of Ru/Be multilayer mirrors (MLM) were studied by X-ray reflectometry and diffractometry, and secondary ion mass spectrometry (SIMS). An increase in the reflection coefficients of MLM at a wavelength of 11.4 nm to record values of R = 72.2% and FWHM to Δλ1/2 = 0.38 nm is shown. The effect of interlayers on the structural and reflective characteristics of MLM is explained by the barrier properties of the Mo layers, which prevent the mutual mixing of the Ru and Be layers, which leads to the formation of beryllides and a decrease in the X-ray optical contrast at the boundaries.

High-aperture EUV microscope using multilayer mirrors and a 3D reconstruction algorithm based on z-tomography.

The article is devoted to the development of an EUV microscope using a wavelength of 13.84 nm. Due to the use of a mirror lens with a large numerical aperture, NA = 0.27, and a short depth of focus, it has been possible to carry out z-tomography of bio-samples for the first time with this type of microscope. A 3D image was reconstructed, and a pixel resolution of 140 nm was obtained. A new simple algorithm for the 3D reconstruction of absorption images from z-tomography data has been proposed that takes into account lens aberrations and a point spread function. The algorithm reduces the inverse absorption task to the corresponding well-studied task of fluorescence microscopy, with an error of 10% for cells up to 10 µm thick.

Highly reflective Ru/Y multilayer mirrors for the spectral range of 9-11†nm.

The results of the investigation of the reflective characteristics of multilayer mirrors based on Ru/Y are presented. Reflection coefficients at the level of 38.5% at an operating wavelength of 9.4 nm. It is shown that the deposition of B4C barrier layers onto Y layers makes it possible to significantly increase the reflection coefficient compared to structures without barrier layers. A reflectance of 54% was obtained for mirrors optimized for 11.4 nm, which is close to the theoretical limit for these materials.

Highly reflective Ru/Sr multilayer mirrors for wavelengths 9-12†nm.

The results of investigations of Ru/Sr multilayer coatings optimized for the spectral range of 9-12 nm are presented in this Letter. Such mirrors are promising optical elements for solar astronomy and for the development of beyond extreme ultraviolet (BEUV) lithography. A near-normal incidence reflectivity of up to 62.3% (λ = 11.4 nm) right after the synthesis is measured. The reflection coefficient decreases to 56.8% after five days of storage in air with a subsequent stabilization of its value. At a wavelength of λ = 9.34 nm, the reflection coefficient is 48.6% after two months of storage in air. To date, to the best of our knowledge, this is the highest reflectivity measured in this spectral range. The possibility of further increasing the reflectivity is discussed.

High-resolution laboratory reflectometer for the study of x-ray optical elements in the soft and extreme ultraviolet wavelength ranges.

A high-resolution laboratory reflectometer designed for operation in the soft x-ray (SXR) and extreme ultraviolet (EUV) ranges is described. High spectral resolution, up to 0.028 nm, in a wide spectral range is achieved due to the Czerny-Turner monochromator. A laser plasma generated by irradiating a solid-state target with a focused laser beam (wavelength 1.06 µm, pulse energy 0.5 J, duration 4 ns, and pulse repetition rate 10 Hz) is used as a source of SXR and EUV radiation. The goniometer allows the study of curved optical elements with an aperture up to NA = 0.5 and a diameter of up to 500 mm. The methods providing high efficiency of the optical system and spectral resolution in a wide range of wavelengths are described in detail. The problem of taking into account high orders in the recorded spectra of a laser plasma is discussed. A comparison of the measurement results with the described reflectometer and the optics beamline at the BESSY-II synchrotron is given.

Single-channel method for measuring the reflectance spectra of grazing incidence mirrors in the extreme ultraviolet range.

This Letter presents a one-channel method of measurements of a grazing incidence reflection coefficient. The idea is to use the quasi-flat field extreme ultraviolet spectrometer to produce direct and reflected from the sample images of the spectral lines simultaneously at the same detector matrix. The analysis of a set of spectral lines in the working spectral range of the spectrometer gives the spectral dependence of the reflection coefficient. The results of the refection coefficient measurements for a plane ruthenium mirror for grazing angles of 6° and 10° in the spectral range 15-40 nm are presented.

Ultrasmooth beryllium substrates for solar astronomy in extreme ultraviolet wavelengths.

The paper describes a multistage method of forming ultrasmooth substrates based on bulk beryllium. Such substrates are suggested to be used for multilayer extreme ultraviolet mirrors of spacecraft missions on solar corona investigations in the spectral range 17.1-58.4 nm. The technique for chemical nickel plating of the sample surface is described. The process parameters that provide the formation of an amorphous film with a thickness of about 100 microns are presented. The results of mechanical polishing are shown. The effective roughness of 1.3 nm is obtained, which is twice lower than one achievable for a nickel-free beryllium surface. The applicability of the ion beam figuring technique is demonstrated: the initial surface roughness of a nickel film after etching with Ar ions (Eion=200-800 eV) to a depth of 250 nm does not deteriorate. The amorphous silicon film deposition followed by ion polishing made it possible to reduce the microroughness (atomic force microscope frame 2×2 µm) to σ2×2=0.15 nm from the initial σ2×2=0.46 nm. The reflectivity of multilayer mirrors deposited on these substrates turned out to be close to the values obtained on "witnesses" (supersmooth silicon substrates). Moreover, for the Mg/MoSi2 mirror optimized for the wavelength λ=58.4 nm the values of the reflection coefficients of structures on the beryllium substrate and on the silicon "witness" were identical (about 28%).

Influence of barrier interlayers on the performance of Mo/Be multilayer mirrors for next-generation EUV lithography.

A comparative study was carried out of the structure and reflection performance of four types of multilayer mirror for extreme ultraviolet lithography at 11.2 nm; these were a pure Mo/Be structure and three Mo/Be-based structures with thin B4C, C and Si interlayers. It was demonstrated that Mo/Be mirrors show maximum reflectance at normal incidence, while maximum structural perfection is shown by Mo/Be/Si mirrors. The introduction of B4C and C layers into the structure increases the interlayer roughness and reduces the sharpness of the interfaces, adversely affecting the target coating characteristics. Results are presented for studies using four techniques: X-ray reflectometry, small-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy.

Reflective Schmidt-Cassegrain system for large-aperture telescopes.

A reflective modification of the Schmidt-Cassegrain system was built and tested. Ultraviolet (UV) and soft x-ray applications are discussed. The system consists of a planoid mirror with an aspheric profile and prime concave and secondary convex spherical mirrors. Spherical aberration in a wide field of view and astigmatism are compensated by the aspheric profile of the planoid. The main parameters of the scheme are as follows: an entrance aperture of 180 mm, a focal ratio F/3.2, an angular resolution better than 3'' (corresponding to a pixel size of a back-side illuminated CCD), a field of view of ±1.5° (2ω=3°) and a flat image field with a diameter of 30.4 mm. Due to the absence of chromatic aberrations and wide field of view, the scheme is of considerable interest for hyperspectral instruments. In particular, the operating range of the instruments can be expanded into vacuum UV and UV regions.

Advanced materials for multilayer mirrors for extreme ultraviolet solar astronomy.

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.

Ion-beam polishing of fused silica substrates for imaging soft x-ray and extreme ultraviolet optics.

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.

Problems in the application of a null lens for precise measurements of aspheric mirrors.

Problems in the application of a null lens for surface shape measurements of aspherical mirrors are discussed using the example of manufacturing an aspherical concave mirror for the beyond extreme ultraviolet nanolithographer. A method for allowing measurement of the surface shape of a sample under study and the aberration of a null lens simultaneously, and for evaluating measurement accuracy, is described. Using this method, we made a mirror with an aspheric surface of the 6th order (i.e., the maximum deviation from the best-fit sphere is 6.6 µm) with the parameters of the deviations from the designed surface PV=5.3 nm and RMS=0.8 nm. An approximation of the surface shape was carried out using Zernike polynomials {Z(n)(m)(r,φ),m+n≤36}. The physical limitations of this technique are analyzed. It is shown that for aspheric measurements to an Angstrom accuracy, one needs to have a null lens with errors of less than 1 nm. For accurate measurements, it is necessary to establish compliance with the coordinates on the sample and on the interferogram.