Multilayer X-ray Mirrors Research Articles

High-reflectance magnetron-sputtered scandium-based x-ray multilayer mirrors for the water window.

We present an experimental comparison of several Sc-based short period multilayer mirrors including Cr/Sc with B4C barrier layers and CrNx/Sc, and we propose a new material combination that provides high reflectance in the water window domain. Multilayer samples with period thickness in the range 1.5-1.7nm have been deposited by magnetron sputtering and characterized by x-ray reflectometry with a Cu-Kα source and with synchrotron radiation near the Sc-L2,3 edge. Best results are achieved by combining the nitridation of Cr layers and the addition of B4C barrier layers. Near normal incidence reflectance as high as 23% has been measured at photon energy of 397eV. A simulation model of the multilayer structure is proposed and it predicts that reflectance higher than 32% is achievable with CrNx/B4C/Sc mirrors.

Понижение степени межфазного перемешивания в многослойных рентгеновских зеркалах Sс/Si

Понижение степени межфазного перемешивания в многослойных рентгеновских зеркалах Sс/Si

Структурные и рентгено-оптические характеристики многослойных рентгеновских зеркал W/Si

Структурные и рентгено-оптические характеристики многослойных рентгеновских зеркал W/Si

Stress and structure properties of X-ray W/Si multilayer under low temperature annealing

The X-ray timing and polarization telescope proposed in China is for imaging spectroscopy in an energy range of 1-30 keV. To obtain the high energy spectrum response with a large effective area, W/Si multilayer mirrors each with a mirror thickness of only 0.3 mm are used. This makes the figure accuracy of the mirror and the distortion caused by the multilayer stress an important issue during the telescope development. W/Si multilayer mirror is an important component of X-ray telescope for astronomical observation. To reduce the effect of the multilayer stress and maintain a high reflectivity at the same time, the W/Si multilayers prepared by magnetron sputtering deposition are annealed at low temperatures of 150 ℃, 175 ℃ and 200 ℃, respectively, for 3 h. The stress of the multilayer is determined based on the surface figure measurements of each sample before and after annealing. The X-ray reflectance and layer structure of the multilayer are characterized by the grazing incidence X-ray reflectometry (GIXR) and the reflectance fitting curves. The first Bragg peak reflectivity of the as-deposited sample is 67% at 8.04 keV and the multilayer stress is around -260 MPa. After annealing at 150 ℃ for 3 h, the first Bragg peak reflectivity and the layer structure are almost the same as before annealing, while the stress reduces 27%. The fitting results display almost the same interface widths of the multilayer before and after annealing. As the temperature increases to 175 ℃, the first Bragg peak reflectivity drops by about 2%. The multilayer structure begins to deteriorate and the W/Si interface widths increase from 0.346 nm/0.351 nm to 0.356 nm/0.389 nm, according to the fitting results, while the stress reduces about 50%. After annealing at 200 ℃ for 3 h, the stress reduces 60% and the stress decreases down to about -86 MPa. However, the first Bragg peak reflectivity drops by 17%, and the layer structure undergoes significant change after annealing. The W/Si interface widths increase from 0.352 nm/0.364 nm to 0.364 nm/0.405 nm. The GIXR results also show that the d-spacing between the multilayers decreases after annealing, and a higher annealing temperature causes a larger decrease. The stress reduction should be mainly caused by the enhanced atomic diffusions at the interface and inside the layer structure during the annealing. The enlarged interface and the possible compound formation contribute to the decrease of X-ray reflectance and the layer compactness. These results provide important guidance for developing low-stress X-ray multilayer mirrors.

Phase measurement of soft x-ray multilayer mirrors.

We propose a new model enabling the extraction of the phase of a multilayer mirror from photocurrent measurements in the soft x rays. In this range, the effects of the mean free path of the electrons inside the stack can no longer be neglected, which prevents the phase reconstruction by conventional photocurrent measurements. The new model takes into account this phenomenon and thus extends up to the x rays the applicability range of the technique. This approach has been validated through a numerical and experimental study of chromium/scandium multilayers used near 360 eV. To our knowledge, this work constitutes the first measurement of the phase of a multilayer mirror in the soft x-ray range.

In situ x-ray reflectivity study of swift heavy ion induced interface modification in a W/Si multilayer x-ray mirror

Intermixing induced in W/Si multilayers by irradiation with 150 MeV Ag+ ions has been studied. In situ x-ray reflectivity measurements during heavy ion irradiation allows one to monitor the evolution of interface mixing as well as internal stresses as a function of time during irradiation. The technique of x-ray reflectivity is sensitive enough to measure diffusion length down to 0.1 nm. A finite intermixing occurs in the system, in spite of it having a positive heat of mixing. High accuracy of the technique allows one to discern subtle effects of factors like stress or film thickness, on swift heavy ion irradiation induced intermixing. The measurements provide clear evidence of an enhanced intermixing in the presence of tensile stress in the film. Mixing efficiency decreases with increasing film thickness. The film thickness dependence of mixing efficiency can be understood in terms of the thermal spike model by taking into consideration the changes in electron mobility due to scattering from surface and interfaces.

Investigation of the characteristics of EUV backward transition radiation generated by 5.7 MeV electrons in mono- and multilayer targets

This paper presents experimental results of a study of radiation in the vacuum ultraviolet and extreme ultraviolet spectral region, generated by electrons with an energy of 5.7 MeV in a multilayer X-ray mirror. The angular distribution of the radiation generated in the rear hemisphere from the input surface of the target was measured. The contribution to the radiation yield caused by the periodic structure of the multilayer target was experimentally investigated.

Tunable two-color hard x-ray multilayer Bragg mirrors.

A tunable two-color multilayer Bragg coating capable of simultaneously reflecting the fundamental and the third harmonic of an x-ray free-electron laser at the same angle and with high reflectance R>0.70 is presented. The novel coating will enable two-color x-ray pump/x-ray probe experiments. This mirror consists of a Si substrate that is coated with two different types of multilayer systems, Mo/B4C layers with a periodicity of d=3. nm directly on the substrate and Ni/B4C layers with a periodicity of d=11.85 nm on top. Fundamental radiation with photon energies between 3 and 9 keV is reflected by a Ni/B4C multilayer system while the third harmonic (9 keV<hν<27 keV) passes this system and is reflected by the Mo/B4C multilayers. The principle has successfully been proven at the beamline BM05 at ESRF.

Optimization of the design of a multilayer X-ray mirror for Cu-Kα energy

A method for optimizing the design of a multilayer X-ray mirror to obtain a high X-ray reflectivity at a specific angle and for a specific energy, based on quarter-wave layer thickness, is described. The quarter-wave design method is widely used for designing optical multilayers, and there is extensive experience in applying this method, which can be useful when designing multilayer X-ray mirrors. The purpose of this paper is to investigate if the quarter-wave design method can be adapted to designing X-ray multilayers. The method is demonstrated for the case of reflectivity from a multilayer structure, of the Cu Kα line (8.04 keV) at 5.5°. Theoretical reflectivity higher than 50% can be achieved with the proposed design by using 500 layers.

Optimized x-ray multilayer mirrors for single nanometer focusing

We present numerical simulations optimizing the layer shapes of curved focusing x-ray multilayer mirrors deployed at synchrotron radiation facilities using a wave-optical model. The confocal elliptical shapes of the inner layers are corrected for refraction based on the modified Bragg law. Simulated wave amplitudes are further propagated to the focal region, promising nanometer focusing.

Experimental setup on 5.7 MeV microtron for investigation of vacuum ultraviolet and ultrasoft X-ray radiations generated in multilayer mirror

The progress in development of an experimental setup for investigating the ultra-soft X-rays and vacuum ultraviolet radiation generated at incidence of relativistic electrons on the surface of multilayer X-ray mirror is described. The results of the first test experiments carried out for observing the contribution of parametric radiation in generated X-rays are presented.

Thermal stability studies of ion beam sputter deposited C/B4C X-ray multilayer mirror

We report the results of thermal stability study carried out on C/B4C multilayer structure. We have analyzed the structure of as-deposited and vacuum annealed C/B4C multilayer film by soft X-ray reflectivity measurements. We observed that multilayer period expansion continues till 600°C and slight contraction at higher annealing temperature. The results show that the multilayer structure is stable even after 700°C annealing. Raman spectroscopy indicates graphitization of carbon layer with increasing annealing temperature. Graphitization of carbon results in increases of layer thickness and decreases in density as also observed by soft X-ray reflectivity. We observed reduction in measured soft X-ray reflectivity at 6.56 and 4.39nm wavelengths after 800°C annealing. C/B4C multilayer structure has been tested over a period of one year to investigate its temporal stability.

Development of a multilayer mirror for high-intensity monochromatic x-ray using lab-based x-ray source

A parabolic, multilayer x-ray mirror, which can be used with a general lab-based x-ray source, was designed and fabricated. A glass substrate for the mirror was fabricated. Its surface was determined by following the rotation of a parabolic curve and was polished precisely. On the substrate surface, six W/Al bilayers were deposited to form the multilayer mirror. The effects of the mirror on x-ray images were investigated based on the calculated modulation transfer function (MTF) and image intensity values. Higher MTF and intensity values of an x-ray image were obtained using the mirror.

Wave-optical theory of nanofocusing x-ray multilayer mirrors

We have derived a wave-optical model of curved nanofocusing x-ray multilayer mirrors used at synchrotron radiation sources, using a Takagi-Taupin-like approach. In a first approximation, the individual layers are assumed to be confocal elliptical. This assumption leads to a convenient spatial description in elliptical coordinates. As a first optimization, we study a rotation-like modification and compare numerical simulations to established results for planar multilayers.

Fabrication and Evaluation of Large Area Mo/Si Soft X-Ray Multilayer Mirrors at Indus SR Facilities

Large area Mo/Si multilayer (ML) mirrors with high reflectivity are fabricated using magnetron sputtering deposition system. Thin film growth is optimized for film roughness, density, and interface quality by changing process parameters through fabrication of thin films. Mo/Si MLs are fabricated with varying thickness ratio, number of layer pairs, and periodicity from 0.3 to 0.45, 5 to 65, and 40 to 100 Å, respectively. The samples are characterized using hard X-ray reflectivity and transmission electron microscopy. Soft X-ray performance tests of MLs are done by soft X-ray reflectivity using Indus-1 synchrotron radiation. ML coating with thickness errors of ~0.03% per layer and interface roughness in the range of 2 to 5 Å has been realized. The lateral variation of the periodicity is controlled within 0.5 Å over the mm2 area of the plane substrate by using substrate motion and appropriate masking arrangement. Maximum variation of periodicity from run to run is less than 0.5 Å. Peak reflectivity of ~63% at wavelength of ~127 Å is achieved for incident angle of 71 degree.

Hard X-ray multilayer mirror round-robin on the wavefront preservation capabilities of W/B4C coatings

A round-robin between the multilayer deposition laboratories of the Advanced Photon Source, the European Synchrotron Radiation Facility and the National Synchrotron Light Source II has been initiated in order to study standard W/B4C multilayer mirrors produced by the different facilities. The use of such multilayer mirrors for hard X-ray monochromatisation represents an important alternative to crystal-based devices when greater photon flux density is desirable for, e.g., X-ray imaging applications and other photon-intensive techniques. Currently, knowledge about the potential degradation of the wavefront in terms of beam profile distortion and coherence properties due to reflection on a multilayer mirror is limited. In order to address this issue, the beam profile and coherence properties of a monochromatic synchrotron beam reflected by the individual mirrors were studied at the Advanced Photon Source insertion device beamline 32-ID. The results indicate that by using the same coating material, commercially available high quality substrates and a similar coating technique, mirrors with comparable performance can be produced with quite different multilayer deposition facilities. Furthermore, no wave-optical formalism is available at this time which relates the influence of a multilayer reflection on the wavefront to the structural quality of the mirror. Hence, the experimental studies presented are highly targeted in order to identify parameters which have a potential influence on the wavefront preservation properties of a multilayer.

Setup for study of radiation in VUV region generated by 5.7 MeV electrons in a multilayer mirror

The experimental setup created for investigating the radiation in the vacuum ultraviolet (VUV) region generated by 5.7 MeV electrons in a multilayer X-ray mirror at different angles of the electron beam incidence on the X-ray mirror surface is described. The method proposed for parametric X-rays separation from the generated radiation is analyzed. The method is based on the K-edge effect of photon absorption in fluorine at transmission of generated radiation through a thin Teflon filter. The results of the first test measurements are discussed.

Applications of wavelet analysis to study growth characteristics of x-ray multilayer mirrors

Transmission electron microscopy images and hard x-ray reflectivity curves are used to obtain information on the growth characteristics of x-ray multilayer mirrors. A multi-resolution approach based on wavelet analysis is used to decompose the interface profiles obtained from transmission electron microscopy images into a number of different spatial frequency ranges. The replication factor (vertical) and the characteristic length (lateral) of the interfacial roughness are determined in these spatial frequency ranges. By changing the upper or lower limits of the wavelet transform of the x-ray reflectivity curves, the structure uniformity (lateral) and evolution of the surface damage (vertical) are determined. These results are significant for the study of the growth characteristics of ultrathin x-ray multilayer mirrors and enable more effective methods to determine their imperfections.

Ion beam sputter deposited W/Si multilayers: Influence of re-sputtering on the interface structure and structure modification at ultra short periods

X-ray multilayer mirrors of period ranging from 9.6 to 1.7 nm, deposited using ion beam sputtering, have been examined using grazing incidence X-ray reflectivity (GIXRR) and grazing incidence X-ray diffraction. Detailed analysis of GIXRR data revealed that significant amount of re-sputtering of Si layer takes place while W deposition is underway. Re-sputtering is mainly due to bombardment of high-energy neutrals getting reflected from the W target. Due to re-sputtering interface of the multilayer becomes asymmetric. This puts a major hindrance in avoiding the intermixing and achieving sharp interfaces at shorter periods. Maximum thickness of Si which gets lost due to re-sputtering during deposition is ∼0.8 nm. The shortest period multilayer estimated, that could be deposited without intermixing, was 2.7 nm. These results are of significance for developing low period W/Si multilayers.

Breakthroughs in understanding elastic grain interaction and whisker formation made possible by advances in X-ray powder diffraction

Recent work on laboratory parallel-beam diffraction is summarized. Special attention is paid to instrumental aberrations and their corrections. Two types of primary beam optics are considered: X-ray lenses and X-ray multilayer mirrors. A high photon flux, high brilliance and minimal divergence of the incident beam make a recently pioneered setup, based on a combination of a microfocus rotating anode X-ray source with a single-reflection mirror, particularly suitable for grazing-incidence diffraction, and thus for the investigation of very thin films (yielding low diffracted intensities) and of stress and the anisotropy of the microstructure of materials (such as the crystallographic texture, requiring the acquisition of large measured data sets, corresponding to the variation of the orientation of the diffraction vector with respect to the specimen frame of reference). A comparative discussion of primary optics which can be used to realize parallel-beam geometry demonstrates the range of possible applications of parallel-beam diffractometers. Such advances in laboratory parallel-beam diffraction instrumentation enable highly accurate diffraction investigations of thin films and, in particular, diffraction measurement at constant, tuneable penetration depth. These novel possibilities thereby provide a pathway for fundamental investigations of thin films and nanocrystalline materials. In particular, breakthroughs have been realized in the fields of elastic grain interaction in ultrathin films and the understanding of whisker formation on Sn coatings.