The paper deals with the recent progress in fabrication of the graded multilayer mirrors to be used in a 21X Schwarzschild objective operating at the wavelengths about 4.5 nm ("carbon window" region). The graded Co/C reflective multilayer coatings were fabricated using DC-magnetron sputtering. Mask-assisted deposition was used to
create the required radial variation of the multilayer period. Accuracy of the multilayer's parameter measurements and quality of
nm-scale layer deposition were improved significantly with application of a number of new methods and approaches. The soft X-ray measurements were conducted at the ALS 6.3.2 beamline to quantify the graded periods on concave and convex mirrors of the Schwarzschild objective. They demonstrated that the reflectivity curves were adjusted with the accuracy of about 0.008 nm (0.3%) over the entire mirror surfaces. The total throughput of the objective with
full working aperture (NA ~ 0.2) is estimated to be as high as 0.25%.
This paper presents novel approaches and techniques in development and application of multi-mirror X-ray optical
systems operating in the spectral region of "carbon window" (λ ~ 4.5 nm). Two approaches for fabrication of the graded
Co/C multilayer mirrors for Schwarzschild objective are presented. A pair of the spherical mirrors with Co/C multilayer
coatings was tested in combination with scandium/carbon filters and laser produced plasma X-ray source for
stereoimaging of low density materials at λ ~ 4.5 nm.
Multilayer mirrors for the extreme ultraviolet (EUV) are key elements for numerous applications of coherent EUV
sources such as new tabletop lasers and free-electron lasers. However the field of applications is limited by the radiation
and thermal stability of the multilayers. Taking into account the growing power of EUV sources the stability of the optics
becomes crucial. To overcome this problem it is necessary to study the degradation of multilayers and try to increase
their temporal and thermal stability. In this paper we report the results of detailed study of structural changes in Sc/Si
multilayers when exposed to intense EUV laser pulses. Various types of surface damage such as melting, boiling, shock
wave creation and ablation were observed as irradiation fluencies increase. Cross-sectional TEM study revealed that the
layer structure was completely destroyed in the upper part of multilayer, but still survived below. The layers adjacent to
the substrate remained intact even through the multilayer surface melted down, though the structure of the layers beneath
the molten zone was noticeably changed. The layer structure in this thermally affected zone is similar to that of
isothermally annealed samples. All stages of scandium silicide formation such as interdiffusion, solid-state
amorphization, silicide crystallization etc., are present in the thermally affected zone. It indicates a thermal nature of the
damage mechanism. The tungsten diffusion barriers were applied to the scandium/silicon interfaces. It was shown that
the barriers inhibited interdiffusion and increased the thermal stability of Sc/Si mirrors.
The extreme ultraviolet (EUV) optical constants δ(ω) and β(ω) of amorphous carbon were determined on the basis of
transmission measurements at hω=18-450 eV, the first-principles calculation of the dielectric tensor at hω<25 eV, and
the Kramers-Kronig calculation of δ(ω). Our optical constants generally agree with the CXRO data, excluding the
vicinity of the K-edge. First-principles analysis shows that two thresholds of absorption (at 284 eV and 291 eV) found in
the present study are caused, respectively, by the π- and σ- bonds. Their weights are controlled by an orientation of
graphene sheets in a-carbon.
A novel technique, utilizing thin films with protective capping layers deposited onto silicon photodiode substrates, has been developed to accurately determine the optical constants of reactive elements such as the rare earths and transition metals. Depositing protected layers on photodiode substrates has three primary advantages over the study of the transmittance of free-standing films and the angle-dependent reflectance of coatings on mirror substrates. First, it is easy to deposit a thin protective capping layer that prevents oxidation or contamination of the underlying reactive layer. Second, very thin layers of materials that have intrinsically low transmittance can be studied. Third, the optical constants are determined from the bulk properties of the protected layer and are not influenced by reflectance from the top surface that can be affected by oxidation or contamination. These and other benefits of this technique will be discussed, and results for La and Tb will be presented. The determined optical constants are significantly different from the CXRO and other tabulated values. The rare earth (lanthanide) elements with atomic numbers 57-71 have 5d or 4f open shells, and this open shell structure results in transmission windows in the extreme ultraviolet wavelength range >45 nm where materials typically have low transmittance. These transmission windows make possible the fabrication of a new class of multilayer interference coatings, based on rare earth elements, with relatively high peak reflectances and narrow reflectance profiles, both important factors for the imaging of solar and laboratory radiation sources with multilayer telescopes.
The optical constants (n, k) of the wavelength-dependent index of refraction N = n+ik = 1-δ+ik of Nd (Neodymium)
and Gd (Gadolinium) are determined in the wavelength range of 3 nm to 100 nm by the transmittance method using
synchrotron radiation. Nd and Gd films with thicknesses ranging from 5 nm to 180 nm were fabricated on Si
photodiodes (which served as the coating substrates as well as the detectors) and capped with Si layers to protect these
reactive rare earth elements from oxidation. The imaginary part (k) obtained directly from the transmittance
measurement is used in the derivation of the real part (δ) of the complex index of refraction N through the Kramers-
Kronig integral. The measured optical constants are used in the design of currently developed Nd- and Gd-based
multilayers for solar imaging applications. Our results on Nd and Gd optical constants and the reflectance of some Nd-
and Gd-based multilayers are presented.
The determination of EUV optical constants in rare-earth metals is much hampered by the high reactivity and easy air contamination of these materials. The most difficult regions are a long wavelengths part of the EUV interval and the vicinity of absorption edges. In this study the optical constants of La and Tb are determined in a wide energy interval 14-400 eV. The study is performed by our recent method, which is suitable for reactive materials and for intervals around the absorption edges of elements. The samples are identically protected films with thickness ~ 10 nm or ~ 100 nm, which are deposited on silicon photodiodes. Mathematical treatment separates the contributions of a capping layer and a rare-earth metal and provides data which are free from the effects of air contamination. The precision and behavior of optical constants, as well as the parameters of the O2,3 and N4,5 edges in La and Tb are analyzed.
The paper deals with the recent results of the experiments on soft X-ray imaging of various carbon-containing objects: biological samples, artificial carbon fibres, graphite slices, etc. The working wavelength was chosen to be 4.5 nm due to high penetration ability of these soft X-rays in the carbon materials. The experimental set-up included: laser plasma X-ray source (generated with the 2nd harmonics of Nd:YAG laser), scandium-based thin-film filter and highly reflective spherical multilayer mirror. The Co/C multilayer's reflectivity was measured to be about 15 % at normal incidence that was high enough to produce soft X-ray images using one nanosecond-long exposure. The work demonstrates a possibility to produce high contrast images outside "water window" region for study of relatively thick (tens of microns) samples that may lead to new fields of applications of the soft x-ray microscopy.
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.
Processes going on at elevated temperatures between Sc and Si layers in Sc/Si coatings are studied by X-ray scattering and cross-sectional transmission electron microscopy. It is shown that the W layers of 0.5-0.8 nm placed at Sc-Si interfaces form effective barriers preventing the penetration of Si into Sc. The effects of Si-Sc diffusion and W-barriers on the reflectivity of coatings are calculated in good agreement with experimental results. Presented measurements show that the Sc/W/Si/W multilayers with the period of 20.5 nm fabricated by dc-magnetron sputtering possess thermal stability up to 250 C and the normal incidence reflectivity of 24% at wavelengths about 40 nm.
We report the first demonstration of plasma imaging and interferometry using a tabletop soft x-ray laser. A very compact capillary discharge Ne-like Ar laser operating at 46.9 nm was used as a backlighter to study the evolution of the plasma of a micro-capillary discharge. The resulting series of shadowgrams show the plasma created by ablation of the walls of the evacuated micro-capillary rapidly evolves from a non-uniform initial state into a symmetric column with minimum density on axis. In a second experiment we took advantage of the good spatial coherence of the capillary discharge laser to perform interferometry measurements in the plasma of a pinch discharge. In this experiment the laser was used in combination with a wavefront division interferometer based on Lloyd's mirror to map the electron density distribution in the cathode region of the discharge.
We report the measurement of the optical constants of Si, GaP, InP, GaAs, GaAsP and Ir at a wavelength of 46.9 nm (26.5 eV). The optical constants were obtained from the measurement of the variation of the reflectivity as a function of angle utilizing, as an illumination source, a discharge pumped 46.9 nm table-top laser operated at a repetition rate of 1 Hz. These measurements constitute the first application of an ultrashort wavelength laser to materials research.
In view of potential applications to the normal incidence optics we calculated the optical constants of metals Sc, Ti, V, Cr and Fe in the interval $HBAR(omega) less than or equal to 70 eV. The calculation was carried out within frames of the band structure approach with the use of the linear muffin-tin orbitals (LMTO) method and taking into account electron excitation of the nearest core levels. Obtained dielectric function shows good agreement with experimental spectra of the absorption (mu) ((omega) ), the reflectivity R((omega) ) and the electron energy loss spectroscopy (EELS) L((omega) ). The trends in optical spectra of metals along the 3d- series and mutual correlation of features of (mu) ((omega) ), R((omega) ) and L((omega) ) are considered and discussed.
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