Multilayer X-ray Mirrors Research Articles

Reactive diffusion in Sc/Si multilayer X-ray mirrors with CrB2 barrier layers

Processes undergoing in Sc/Si multilayer X-ray mirrors (MXMs) with periods of ∼27 nm and barrier layers of CrB20.3- and 0.7-nm thick within the temperature range of 420–780 K were studied by methods of small-angle X-ray reflectivity (λ=0.154 nm) and cross-sectional transmission electron microscopy. All layers with the exception of Sc ones are amorphous. Barrier layers are stable at least up to a temperature of 625 K and double the activation energy of diffusional intermixing at moderate temperatures. Introduction of barriers improves the thermal stability of Sc/Si MXMs at least by 80 degrees. Diffusion of Si atoms through barrier layers into Sc layers with formation of silicides was shown to be the main degradation mechanism of MXMs. A comparison of the stability for Sc/Si MXMs with different barriers published in the literature is conducted. The ways of further improvement of barrier properties are discussed.

X-ray optics at the Russian academy of Sciences’ institute of high-purity materials

We examine the current state, prospects for development, and the application of X-ray optics at the IMT-IHPM: X-ray diffraction optics based on X-ray multilayer mirrors and crystals, X-ray acousto-optics based on X-ray diffraction by acoustically modulated crystals, and the application of X-ray methods of diagnostics for the investigation of piezoelectric crystals.

Design of X-ray multilayer mirrors with maximal integral efficiency

We describe several techniques to design X-ray multilayer mirrors with enhanced bandwidth and, more especially, bandwidth with a shape that optimizes its matching with a given source angular distribution. A Gaussian angular distribution source emitting at the characteristic Mo- K α radiation ( λ=0.071 nm), a radiation regularly used in X-ray analytical applications, is used for the demonstration. Three approaches are developed and compared to design multilayer structures with maximum efficiency, where efficiency is defined here as the convolution of the source and reflectance angular profiles. The first (most general) approach, which can be applied for arbitrary source profiles, is based on the exact Euler–Lagrange equation and a general expression for the multilayer mirror reflectivity of arbitrary wideband. With this method, the depth-distribution of the multilayer bi-layer thickness was found to be smooth. Numerical refinements of the obtained analytical solution were not useful, giving insignificant efficiency increases despite significant complication of the thickness depth-distribution of the multilayer stack. The efficiency of some of these depth-graded structures exceeds that of a periodic mirror by a factor of two in our case. The main factor limiting the efficiency was absorption of the radiation by the materials. The second approach is based on designing a simpler depth-graded multilayer mirror with a linear variation of bi-layer thickness and a constant thickness ratio. Such a mirror proved to have an efficiency close to the maximum possible one. The same conclusion is also true for the simplest design approach, where several periodic substructures are stacked on top of each other, our third described approach. We found that about only four substructures are necessary. In this case the optimization procedure is based on simply three fitting parameters, independently of the number of periodic substructures. We show that the efficiency decreases quickly with increasing interface width. Therefore, multilayer structures with abrupt interfaces, e.g., Mo 2C/Si multilayers instead of the more conventional Mo/Si multilayers, are preferable in practice. The efficiency also depends strongly on the mean grazing angle incident on the mirror and the angular width of the source (for a fixed number of bi-layers). This must be taken into consideration when designing wide-band multilayer coated X-ray collimators or concentrators.

Development of a high-brilliance rotating-anode dual-wavelength X-ray generator and multilayer mirror for dual wavelength

Development of a high-brilliance rotating-anode dual-wavelength X-ray generator and multilayer mirror for dual wavelength

Multilayer X-ray mirrors based on La/B4C and La/B9C

Multilayer X-ray mirrors that are based on La/B4C and La/B9C and intended for the reflection of X-ray radiation in the spectral region near λ = 6.7 nm are prepared and studied. Reflection coefficients at a level of 40–60% are achieved for mirrors with various periods. The difference in the interlayer roughnesses reconstructed from the data measured in the hard and soft X-ray spectral regions is explained using a structural model with an asymmetric permittivity profile in a mirror period. A proximate technique is developed to estimate the permittivity profile in a multilayer-structure period using reflectometry data. The effect of antidiffusion Sn, Cr, and Mo barriers on the reflection coefficient of multilayer La/B4C structures is studied experimentally

Componentry of reflection optics for application in the tesis X-ray astrophysics experiment

Reflection characteristics of multilayer structures for the CORONAS-FOTON optical schemes are described. Mo/Si, Al/Zr, Cr/Mg, Si/Mg, and Mg/B4C material combinations were studied. Multilayer X-ray mirrors based on these combinations and used at wavelengths 13.2, 17.1, and 30.4 nm were designed.

Evaluation of Ion Milled Soft X-ray Multilayer Mirrors for Reflection Wavefront Correction

Surface roughness and soft x-ray reflectance of soft x-ray Mo/Si multilayer mirrors ion-milled for physical optical reflection wavefront correction with 0.1 nm accuracy have been measured. The multilayer mirror was milled by Ar ion beam accelerated at 500 V using area-selected mask template with 10×10 mm2 opening. Milled surface at peripheral was rougher than that at center of milling area. Measured maximum roughness of 0.47 nm RMS would cause only under 0.01% reduction of reflectance by theoretical estimation. 44% peak reflectance of the ion-milled multilayer at λ=14.3 nm is practical enough for composing soft x-ray objectives such as Schwarzschild objective. These results prove that ion milling method is effective and gentle milling for precise figure error correction of soft x-ray multilayer mirrors.

Characterization of Mo/Si soft X-ray multilayer mirrors by grazing-incidence small-angle X-ray scattering

The morphology of buried interfaces plays a key role in high performing Mo/Si soft X-ray mirrors. We show that grazing-incidence small-angle X-ray scattering is a highly effective and non-destructive diagnostic technique for analysis of buried interfaces. The parameters of average interface autocorrelation function can be determined unambiguously. Additionally period thickness, roughness of interfaces and an effective number of vertically correlated periods can be extracted. The multilayer mirrors were prepared by e-beam evaporation on heated and unheated substrates, ion beam assisted e-beam evaporation, ion beam sputtering and RF magnetron sputtering. The latter three techniques produce multilayer mirrors with comparable interface roughness. The differences in lateral correlation length and Hurst parameter are found.

Effect of defects on the reflectivity of Cr/C multilayer soft X-ray mirror at 4.48 nm

Cr/C is a promising material combination for multilayer mirror in the “near water window region” (4.4–6.7 nm). In the present paper, the effect of defects on the reflectivity of Cr/C soft X-ray multilayer mirror deposited by magnetron sputtering was studied. Formation of thin interlayer due to the interdiffusion, rough interface due to the non-sharp layer and contamination of O happened during the deposition process were found by a method combined by XPS, soft X-ray reflectivity at 4.48 nm and grazing incidence hard X-ray reflectivity at 0.154 nm. The XPS results show that both interlayers (Cr-on-C and C-on-Cr) are mixture composed of C sp2, C sp3, C O, C O, Cr Cr and Cr O bondings. No chromium carbide was found at the interlayer probably due to the blocking of oxides’ formation. Through the analysis of X-ray reflectivity, we obtained the multilayer structure parameters (thickness and roughness) and optical constants of each layer at 4.48 nm. Based on those results, a further calculation was carried out. The result shows that the formation of the thin interlayer contributes little to the decrease of the reflectivity, the rough interface decreases the reflectivity most and the contaminant (O) not only decreases the reflectivity but also shifts the position of the peak.

Evaluation of polarization characteristics of multilayer mirror for hard X-ray observation of astrophysical objects

Polarization measurements in the hard X-ray band can provide crucial information about high energy emission at astrophysical objects. Such measurements require an instrument with high signal-to-noise ratio and high photon throughput for which the multilayer hard X-ray mirror is ideally suited. Future hard X-ray missions equipped with the multilayer mirror including ASTRO-H (Formerly NeXT) and NuStar are expected to detect hard X-ray sources in the flux range of 10 μ Crab . We expect a polarimeter mission using such mirrors to follow. Reflection off the mirror surface can introduce small but finite artificial polarization at the focal plane. We have conducted an experiment to set an upper limit for such polarization using a polarized hard X-ray beam at SPring-8 in Japan. In the experiment, reflectance of a multilayer surface has been measured as a function of the angle between the scattering plane and the hard X-ray polarization plane. The angular dependence of the measured reflectance can be translated to the artificial polarization. We have derived from the experimental results that the artificial polarization is no more than ± 0.8 % at the scattering angle 0.51°. This upper limit ensures that we can measure polarization down to ∼ 1 % using the multilayer X-ray mirror.

Effects of O and N impurities on the nanostructural evolution during growth of Cr/Sc multilayers

Transition metal multilayers are prime candidates for high reflectivity soft x-ray multilayer mirrors. In particular, Cr/Sc multilayers in the amorphous state have proven to give the highest reflectivity in the water window. We have investigated the influence of impurities N and O as residual gas elements on the growth, structure, and optical performance of Cr/Sc multilayers deposited in high vacuum conditions by a dual cathode direct current magnetron sputter deposition. Multilayer structures with the modulation periods in the range of 0.9–4.5 nm and Cr layer to bilayer thickness ratios in the range of 0.17–0.83 were deposited with an intentionally raised base pressure (pB), ranging from 2 × 10−7 to 2 × 10−5 Torr. Compositional depth profiles were obtained by elastic recoil detection analysis and Rutherford backscattering spectroscopy, while the structural investigations of the multilayers were carried out using hard x-ray reflectivity and transmission electron microscopy. By investigating stacked multilayers, i.e., several multilayers with different designs of the modulation periods, stacked on top of each other in the samples, we have been able to conclude that both N and O are incorporated preferentially in the interior of the Sc layers. At pB ≤ 2 × 10−6 Torr, typically <3 at.% of N and <1.5 at.% of O was found, which did not influence the amorphous nanostructure of the layers. Multilayers deposited with a high pB ∼2 × 10−5 Torr, a N content as high as ∼37 at.% was measured by elastic recoil detection analysis. These multilayers mainly consist of understoichiometric face-centered cubic CrNx/ScNy nanocrystalline layers, which could be grown as thin at 0.3 nm and is explained by a stabilizing effect on the ScNy layers during growth. It is also shown that by adding a background pressure of as little as 5 × 10−6 Torr of pure N2 the soft x-ray reflectivity (λ = 3.11 nm) can be enhanced by more than 100% by N incorporation into the multilayer structures, whereas pure O2 at the same background pressure had no effect.

Reflectivity and structural evolution of Cr/Sc and nitrogen containing Cr/Sc multilayers during thermal annealing

It is shown that the thermal stability in vacuum of Cr/Sc multilayer thin films used as reflective optical components in soft x-ray instrumentation has substantial dependence on incorporation of N. The thermal stability is increased by incorporating 34 at.% of N in Cr/Sc multilayers. A pure Cr/Sc multilayer x-ray mirror starts a continuous degradation already at ∼100 °C with a complete destruction of the multilayer at 500 °C. The resulting structure is a mixture of Cr and Sc nanocrystallites. The degradation can be described by linear diffusion theory and is suggested to be due to the formation of uniformly distributed phase-separated nanocrystallites followed by an Ostwald ripening process with an apparent activation energy of 0.5 eV. At the multilayer-substrate interface, a 7 nm thin Sc–Si layer is formed which effectively hinders indiffusion of Si and outdiffusion of Cr and Sc. A nitrided multilayer, initially consisting of crystalline fcc CrNx and fcc ScNy layers (x and y<1), is observed to improve in structural quality up to ∼250 °C where it is stable for more than 12 h. At ∼330 °C, the multilayer separates into regions with two multilayer periods, differing by less than 0.04 nm, which are stable at 420 °C over an extended period of time >40 h. It is proposed that the separation into the different multilayer periods is a consequence of redistribution of N within the Cr layers. Sc is observed to be stabilized in the ScN layers, which, in turn, inhibit the formation of a Sc–Si barrier layer at the substrate leading to a strong exchange of Si and Cr across the film substrate. This leads to a Cr–Si/ScN layered structure close to the substrate and chromium silicide crystallites inside the substrate. Close to the top of the multilayer, a CrN/ScN multilayer appears to be retained.

Ternary systems and the effect of parity on the multilayer mirror reflectance

The possibility of increasing the maximum reflectance of multilayer x-ray mirrors based on ternary systems is demonstrated. The effect is due to separate profiling of the real and imaginary components of the dielectric permittivity, which is impossible in binary systems.

Characterization of ion beam sputter deposited W and Si films and W/Si interfaces by grazing incidence X-ray reflectivity, atomic force microscopy and spectroscopic ellipsometry

An ion beam sputtering system, which uses a commercial ECR microwave based plasma ion source, has been designed and fabricated in-house for deposition of soft X-ray multilayer mirrors. To begin with, in the ion beam sputtering system W, Si thin films, W/Si bi-layer and W/Si/W tri-layer samples have been deposited on c-Si substrates as precursors to W/Si multilayer stack. The samples have been characterized by grazing incidence X-ray reflectivity (GIXR), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE) techniques. By analyzing the results, density, thickness, surface roughness of the single layer samples and interface width of the bi-layer and tri-layer samples have been estimated.

Interface smoothing of soft x-ray Mo/Y multilayer mirror by thermal treatment

A systematic study of structure and thermal stability of a [(Mo/Y/) x 5] soft x-ray multilayer (ML) mirror prepared by electron beam evaporation system has been performed by means of x-ray reflectivity (XRR) and grazing incidence x-ray diffraction (GIXRD). The GIXRD patterns show that the Mo and Y layers are polycrystalline in nature. The obtained XRR patterns confirm the very good quality of the ML stack. Isochronal thermal annealing up to the temperature of 400 °C leads to a decrease in interface roughness which in turn results in sharpening of the ML interfaces. Further elevating the annealing temperature above 400 °C oxidizes the Y layers and the interface roughness increases. Thus increase in interface roughness beyond this temperature can be understood in terms of evolution of yttrium oxide, which significantly affects the ML structural properties. The modulation structure is well maintained even after annealing at 600 °C, which confirms very good thermal stability of the Mo/Y ML mirror. Our results demonstrate that low-temperature annealing (up to 400 °C) is an effective method to improve the quality of an as deposited Mo/Y soft x-ray ML mirror. Diffuse scattering measurements show very good correlation with the XRR fitting parameters.

Multilayer x-ray mirrors based on W/B 4 C with ultrashort (d = 0.7–1.5 nm) periods

A method using resonance diffuse x-ray scattering has been developed to investigate the properties of multilayer structures composed of ultrathin films. Multilayer structures W/B 4 C with ultra-short periods (d=0.7–1.5 nm) have been studied using specular and diffuse x-ray scattering. The contributions of the interface roughness and the mixing of the layer materials to the total width of the transition region between the layers have been separated for these structures. Quasi-Bragg peaks in the diffuse scattering spectra have been observed experimentally, and their analysis has shown that the interface roughnesses in multilayer W/B 4 C structures are correlated well.

A multilayer x-ray mirror in the form of an ellipsoid of revolution

Laboratory technology for the preparation of multilayer mirrors in the form of an ellipsoid of revolution for the hard x-ray wavelength range (λ = 0.1 nm) has been developed on the basis of the replica method. A prototype mirror in the form of a sector of an ellipsoid of revolution is prepared. Its shape and reflection characteristics are studied.

Design, fabrication, and study of wideband multilayer X-ray mirrors

Multilayer depth-graded (aperiodic) W/Si and W/B4C mirrors with a period changing over depth have been designed, fabricated, and investigated. The mirrors have almost constant reflectance (from 25 to 35% for different mirrors) at a wavelength of 0.154 nm in a wide range of grazing angles (the reflection bandwidth ranges from 0.25° to 0.4°). It is shown that the main reason for the distortions observed on the plateau of the reflection curve is incorrect consideration of the interlayers formed at the interface of neighboring films during fabrication of the structures. The mirrors with sharp interfaces (for example, WSi2/Si) are preferential for practical purposes.

Aperiodic x-ray multilayer mirrors and their application in plasma spectroscopy

We discuss the potentialities of aperiodic multilayer structures for the reflection of XUV radiation, including attooptical applications. Pilot samples of aperiodic normal-incidence broadband focusing multilayer mirrors are employed in a stigmatic transmission grating spectrograph with an operating range of 12.5–30 nm.This spectrograph was used to study (i) the XUV spectra arising from the charge exchange of laser-produced plasma ions with rare-gas (Xe, Kr, He) jet atoms, (ii) a debris-free Xe-jet plasma XUV source driven by Nd-laser pulses.

Characterization of r.f. sputtered thin Mo, W and Si films as precursors to multilayer X-ray mirrors

Single layers of Mo, W and Si thin films have been deposited by r.f. sputtering on float glass and c-Si substrates kept at room temperature. The films have been characterised by grazing incidence X-ray reflectometry (GIXR), X-ray transmission (XRT), Rutherford backscattering spectrometry (RBS), atomic force microscopy (AFM) and phase modulated spectroscopic ellipsometry (SE) studies. The thickness values obtained from the GIXR measurements have been used to calibrate the in situ thickness monitors. The surface roughness of the thin layers have also been determined from the GIXR measurements. The atomic mass density in the films have been obtained from the RBS measurements while X-ray absorption has been estimated from the XRT measurements. The surface morphology of the films has been investigated by the AFM micrographs. The Si thin films have also been characterized by the SE technique. The characterization of the samples by these complementary techniques have been very useful in optimizing the process parameters to obtain good quality layers as precursors to the fabrication of the multilayer X-ray mirrors based on Mo/Si and W/Si structures.