An etched multilayer as a dispersive element in a curved‐crystal spectrometer: implementation and performance

Etched multilayers obtained by forming a laminar grating pattern within interferential multilayer mirrors are used in the soft X‐ray range to improve the spectral resolution of wavelength dispersive spectrometers equipped with periodic multilayers. We describe the fabrication process of such an etched multilayer dispersive element, its characterization through reflectivity measurement and simulations, and its implementation in a high‐resolution Johann‐type spectrometer. The specially designed patterning of a Mo/B₄C multilayer is found fruitful in the range of the C K emission as the diffraction pattern narrows by a factor 4 with respect to the non‐etched structure. This dispersive element with an improved spectral resolution was successfully implemented for electronic structure study with an improved spectral resolution by X‐ray emission spectroscopy. As first results, we present the distinction between the chemical states of carbon atoms in various compounds, such as graphite, SiC and B₄C, by the different shape of their C K emission band. Copyright © 2012 John Wiley & Sons, Ltd.     

High‐resolution x‐ray analysis with multilayer gratings

Periodic multilayers are nowadays widely used to perform x‐ray analysis in the soft x‐ray range (photon energy lower than 1 keV). However, they do not permit to obtain high‐resolution spectra such as natural or synthetic crystals. Thus, multilayers cannot resolve interferences between close x‐ray lines. It has been shown and demonstrated experimentally that patterning a grating profile within a multilayer structure leads to a diffractive optics with improved resolving power. We illustrate the use of an Mo/B₄C multilayer grating in the Fe L and C K spectral ranges, around 700 and 280 eV, respectively. First, in the Fe L range, the improved spectral resolution enables us to distinguish the Fe Lα and Lβ emissions (separated by 13 eV). In addition, using a sample made of a mix of LiF and an iron ore, we show that it is possible to easily resolve the F K and Fe L emissions. These examples demonstrate that an improved x‐ray analysis can be obtained with multilayer gratings when there is the need to study samples having elements giving rise to close emission lines. Second, in the C K range, by comparing C Kα spectra from B₄C and cellulose, we show that the shape of the emission band is sensitive to the chemical state of the carbon atom. Copyright © 2009 John Wiley & Sons, Ltd.     

Interface characterization of B4C‐based multilayers by X‐ray grazing‐incidence reflectivity and diffuse scattering

B₄C‐based multilayers have important applications for soft to hard X‐rays. In this paper, X‐ray grazing‐incidence reflectivity and diffuse scattering, combining various analysis methods, were used to characterize the structure of B₄C‐based multilayers including layer thickness, density, interfacial roughness, interdiffusion, correlation length, etc. Quantitative results for W/B₄C, Mo/B₄C and La/B₄C multilayers were compared. W/B₄C multilayers show the sharpest interfaces and most stable structures. The roughness replications of La/B₄C and Mo/B₄C multilayers are not strong, and oxidations and structure expansions are found in the aging process. This work provides guidance for future fabrication and characterization of B₄C‐based multilayers.     

Development of Al-based multilayer optics for EUV

We report on the development of multilayer optics for the extreme ultra-violet (EUV) range. The optical performance of Al-based multilayer mirrors is discussed with regard to promising reflectivity and selectivity characteristics and the problems of the interfacial roughness for this type of multilayers. We demonstrate a possibility to reduce the average roughness by introducing additional metal layer (W or Mo) rather than depositing a buffer layer at each interface. We have prepared and tested Al/SiC, Al/W/SiC and Al/Mo/SiC multilayers of various periods for the spectral range from 15 to 40 nm, which is the range of increasing interest for high-order harmonic generation, synchrotron radiation and astrophysics. The structure of the three-component systems has been optimized in order to obtain the best reflectivity for each wavelength within the spectral range. We have shown that introduction of refractory metal in Al-based periodic stack can improve the optical performance of multilayer reflecting coatings designed for the EUV applications.     

Introduction of Zr in nanometric periodic Mg/Co multilayers

We study the introduction of a third material, namely Zr, within a nanometric periodic Mg/Co structure designed to work as optical component in the extreme UV (EUV) spectral range. Mg/Co, Mg/Zr/Co, Mg/Co/Zr and Mg/Zr/Co/Zr multilayers are designed, and then characterized in terms of structural quality and optical performances through X-ray and EUV reflectometry measurements, respectively. For the Mg/Co/Zr structure, the reflectance value is equal to 50% at 25.1?nm and 45° of grazing incidence and reaches 51.3% upon annealing at 200°C. Measured EUV reflectivity values of tri-layered systems are discussed in terms of material order within a period and compared to the predictions of the theoretical model of Larruquert. Possible applications are pointed out.     

XMCD studies of magnetic polarization at Mo atoms in CoMo alloy and magnetically coupled Co/Mo multilayers

Magnetic polarization of Mo atoms in Co₉₆Mo₄ alloy film and Co/Mo multilayered structures has been studied by X‐ray magnetic circular dichroism. Samples with Mo spacers of two different thicknesses (0.9 nm and 1.8 nm) were investigated. Mo atoms receive a magnetic moment of ?0.21μ_B in the alloy. In the multilayer with the thinner Mo spacer (d_Mo = 0.9 nm) the magnetic moment is much smaller (?0.03μ_B). In both cases the measured induced moment at the Mo site is oriented antiparallel to the moment at the Co atoms. The presence of the induced moment in the Mo spacer coincides with antiferromagnetic coupling between the Co component slabs. In contrast, neither measurable induced moment at the Mo site nor interlayer coupling between the Co layers has been found for the multilayer with the thicker Mo spacer. Possible mechanisms of the coupling associated with the induced moment are discussed in detail.     

Terbium-based extreme ultraviolet multilayers

We have fabricated periodic multilayers that comprise either Si/Tb or SiC/Tb bilayers, designed to operate as narrowband reflective coatings near 60 nm wavelength in the extreme ultraviolet (EUV). We find peak reflectance values in excess of 20% near normal incidence. The spectral bandpass of the best Si/Tb multilayer was measured to be 6.5 nm full width at half-maximum (FWHM), while SiC/Tb multilayers have a more broad response, of order 9.4 nm FWHM. Transmission electron microscopy analysis of Si/Tb multilayers reveals polycrystalline Tb layers, amorphous Si layers, and relatively large asymmetric amorphous interlayers. Thermal annealing experiments indicate excellent stability to 100 degrees C (1 h) for Si/Tb. These new multilayer coatings have the potential for use in normal incidence instrumentation in a region of the EUV where efficient narrowband multilayers have not been available until now. In particular, reflective Si/Tb multilayers can be used for solar physics applications where the coatings can be tuned to important emission lines such as O V near 63.0 nm and Mg X near 61.0 nm.     

月基极紫外相机多层膜反射镜

月基极紫外相机用于月球表面对地球等离子体层辐射出的30.4 nm谱线进行成像观测,多层膜反射镜是月基极紫外相机的重要光学元件。根据月基极紫外相机技术参数,选择了B₄C/Mg,B₄C/Mg₂Si,B₄C/Al,B₄C/Si,Mo/Si等材料,对其周期厚度、材料比例、周期数等参数进行优化。计算了以上材料组合在30.4 nm的反射率曲线。考虑到月球环境的特殊性和材料的物理化学性质,从中选择出Mo/Si和B₄C/Si两种组合,利用磁控溅射进行镀制。Mo/Si和B₄C/Si多层膜在30.4 nm反射率分别达到15.3%和22.8%。     

La/B4C small period multilayer interferential mirror for the analysis of boron

An La/B₄C multilayer interferential mirror with small period d (4.8 nm) was produced by diode sputtering for the detection of the boron K emission by wavelength‐dispersive x‐ray spectrometry at a large Bragg angle (close to 45°). The structure of the mirror was characterized by grazing incidence x‐ray reflectometry and its performance at the energy of the boron K emission (183 eV) was evaluated by means of polarized synchrotron radiation. Spectrometric measurements showed that the La/B₄C mirror improved the detection limit of boron using by a factor of 2 with respect to similar Mo/B₄C mirrors and by a factor of 4 with respect to a lead stearate crystal. Copyright © 2005 John Wiley & Sons, Ltd.     

The thermal stability of Al(1%wtSi)/Zr EUV mirrors

Six Al(1%wtSi)/Zr multilayers are deposited on Si substrates by using the direct-current magnetron sputtering system, and annealed from 100?°C to 500?°C temperature in a vacuum furnace for 1?h. To evaluate the thermal stability of Al(1%wtSi)/Zr multilayers, the multilayers were characterized by grazing incidence X-ray reflectance, X-ray diffraction, X-ray emission spectroscopy, and near-normal incident extreme ultraviolet (EUV) reflection. The symmetric and asymmetric interlayer models are used to present the interfacial structure before and after 300?°C. The Al(1%wtSi)/Zr multilayer annealed up to 200?°C maintains the initial symmetric multilayer structure, and keeps almost the similar EUV reflectivity as the nonannealed sample. From 300?°C, interdiffusion is much greater at the Zr/Al interface compared with the Al/Zr interface. And the interfacial phases of Al-Zr alloy transform from amorphous to polycrystalline, which induces the deterioration of multilayer structure and the decrease of EUV reflectivity. However, up to 500?°C, the polycrystalline Al-Zr compound does not destroy the multilayer completely.     

低原子序数材料窄带多层膜的研制

为研制极紫外波段窄带多层膜反射镜,采用低原子序数材料组合设计了30.4 nm波长处Mg/SiC,Si/SiC,Si/B4C和Si/C多层膜反射镜,并与极紫外波段传统的Mo/Si多层膜反射镜进行对比。采用直流磁控溅射技术制备了这些多层膜,在国家同步辐射实验室辐射与计量光束线完成了多层膜反射率测量,测量结果表明:Mg/SiC多层膜的带宽最小,为1.44 nm,且反射率最高,为44%;而Mo/Si多层膜的反射率仅为24%,带宽为3.11 nm。实验结果证明了采用低原子序数材料组成的多层膜的带宽要比常规多层膜窄,该方法可以应用于极紫外波段高分辨研究。     

Interface characteristics of Mo/Si and B4C/Mo/Si multilayers using non-destructive X-ray techniques

A methodology combining non-destructive X-ray techniques is proposed to study the interfacial zones of periodic multilayers. The used X-ray techniques are X-ray emission spectroscopy induced by electrons and X-ray reflectivity in the hard and soft X-ray ranges. The first technique evidences the presence of compounds at the interfaces and gives an estimation of the thickness of the interfacial zone. These informations are used to constrain the fit of the X-ray reflectivity curves that enables to determine the thickness and roughness of the various layers of the stacks. The results are validated in the soft X-ray range where the reflectivity curves are very sensitive to the chemical state of the elements present in the stack. The methodology is applied to characterize Mo/Si (1-4 nm/2 nm) and B₄C/Mo/Si (1 nm/2 nm/2 nm) multilayers. It is shown that the two interfacial zones of the Mo/Si multilayers are composed of the silicides MoSi₂ and Mo₅Si₃. It is found that the interface thickness is about to be 0.4-0.8 nm depending on the samples. The molybdenum silicides are also evidenced at the interfaces of the B₄C/Mo/Si multilayers. However, their interface thickness is 0.2 nm thinner than that of the same stack without the B₄C layers, these layers being at the Mo-on-Si side or at the Si-on-Mo side. Thus, the B₄C layers do not stop but only reduce the interdiffusion between the Mo and Si layers.     

Characterization of Pd/Y multilayers with B4C barrier layers using GIXR and X‐ray standing wave enhanced HAXPES

Pd/Y multilayers are high‐reflectance mirrors designed to work in the 7.5–11 nm wavelength range. Samples, prepared by magnetron sputtering, are deposited with or without B₄C barrier layers located at the interfaces of the Pd and Y layers to reduce interdiffusion, which is expected from calculating the mixing enthalpy of Pd and Y. Grazing‐incident X‐ray reflectometry is used to characterize these multilayers. B₄C barrier layers are found to be effective in reducing Pd–Y interdiffusion. Details of the composition of the multilayers are revealed by hard X‐ray photoemission spectroscopy with X‐ray standing wave effects. This consists of measuring the photoemission intensity from the samples by performing an angular scan in the region corresponding to the multilayer period and an incident photon energy according to Bragg's law. The experimental results indicate that Pd does not chemically react with B nor C at the Pd–B₄C interface while Y does react at the Y–B₄C interface. The formation of Y–B or Y–C chemical compounds could be the reason why the interfaces are stabilized. By comparing the experimentally obtained angular variation of the characteristic photoemission with theoretical calculations, the depth distribution of each component element can be interpreted.     

提高极紫外光谱纯度的多层膜设计及制备

极紫外光刻是实现22nm技术节点的候选技术。极紫外光刻使用的是波长为13.5nm的极紫外光,但在160～240nm波段,极紫外光刻中的激光等离子体光源光谱强度、光刻胶敏感度以及多层膜的反射率均比较高,光刻胶在此波段的曝光会降低光刻系统的光刻质量。从理论和实验两方面验证了在传统Mo/Si多层膜上镀制SiC单层膜可对极紫外光刻中的带外波段进行有效抑制。通过使用X射线衍射仪、椭偏仪以及真空紫外(VUV)分光光度计来确定薄膜厚度、薄膜的光学常数以及多层膜的反射率,设计并制备了[Mo/Si]40SiC多层膜。结果表明,在极紫外波段的反射率减少5%的前提下,带外波段的反射率减少到原来的1/5。     

Comparative study of multilayers used in monochromators for synchrotron‐based coherent hard X‐ray imaging

A systematic study is presented in which multilayers of different composition (W/Si, Mo/Si, Pd/B₄C), periodicity (from 2.5 to 5.5 nm) and number of layers have been characterized. In particular, the intrinsic quality (roughness and reflectivity) as well as the performance (homogeneity and coherence of the outgoing beam) as a monochromator for synchrotron radiation hard X‐ray micro‐imaging are investigated. The results indicate that the material composition is the dominating factor for the performance. By helping scientists and engineers specify the design parameters of multilayer monochromators, these results can contribute to a better exploitation of the advantages of multilayer monochromators over crystal‐based devices; i.e. larger spectral bandwidth and high photon flux density, which are particularly useful for synchrotron‐based micro‐radiography and ‐tomography.     

X-ray properties and interface study of B4C/Mo and B4C/Mo2C periodic multilayers

We present a comparative study of B₄C/Mo and B₄C/Mo₂C periodic multilayer structures deposited by magnetron sputtering. The characterization was performed by grazing incidence X-ray reflectometry at two different energies and high resolution transmission electron microscopy. The experimental results indicate the existence of an interdiffusion layer at the B₄C-on-Mo interface in the B₄C/Mo system. Thus, the B₄C/Mo multilayers were modeled by an asymmetric structure with three layers in each period. The thickness of B₄C-on-Mo interfacial layer was estimated about 1.1 nm. The B₄C/Mo₂C multilayers present less interdiffusion and are well modeled by a symmetric structure without interfacial layers. This study shows that B₄C/Mo₂C structure is an interesting alternative to B₄C/Mo multilayer for X-ray optic applications.     

Magnesium doping in InAlAs and InGaAs/Mg films lattice-matched to InP grown by MOVPE

Mg-doped InAlAs and InGaAs films were grown at 560 °C lattice matched to InP semi-insulting substrate by metalorganic vapor phase epitaxy (MOVPE) under various Cp₂Mg flow conditions. Hall effect, photoluminescence (PL), high-resolution X-ray diffraction (HR-XRD), and secondary ion mass (SIMS) were the tools used in this work. The crystalline quality and the n-p conversion of the InAlAs and InGaAs/Mg films are described and discussed in relation to the Cp₂Mg flow. Distinguishing triple emission peaks in PL spectra is observed and seems to be strongly dependent on the Cp₂Mg flow. SIMS is employed to analyze the elements in the epitaxial layers. The variation of indium and magnesium components indicates a decrease of magnesium incorporation during the growth of InAlAs layers leading to a contracted lattice. In addition, the magnesium incorporation in the InGaAs lattice during growth has been confirmed by SIMS.     

Effect of metal diffusion into polycrystalline 6H–SiC prior to its anodization on luminescence response

p-Type porous SiC layers were fabricated by anodization of resistive p-type 6H–SiC samples using HF/ethylene glycol solution. Thin films of lithium (Li) and aluminum (Al) as donor and acceptor elements were vacuum deposited and diffused onto SiC substrates prior to anodization. The aim of this work is to investigate the properties of the nanoporous SiC layer formed by this method and to deduce the effect of diffused lithium as donor and Al as acceptor atoms on their photoluminescence response (PL). The profile distribution of lithium and aluminum diffused atoms was carried out using secondary ion mass spectrometry (SIMS). The photoluminescence spectra of the anodized Al-diffused samples exhibit a broad emission band centered at about 475 nm, while the Li-diffused samples exhibit luminescence with one broad peak located at 655 nm, attributed to Li-related defect centers. In addition, the PL intensity of lithium diffused samples varies with varying the etching time. Finally, the results are expected to have important applications in modern optoelectronic devices and electrode materials of lithium batteries.     

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.     

Synchrotron PEEM and ToF‐SIMS study of oxidized heterogeneous pentlandite,pyrrhotite and chalcopyrite

Synchrotron‐based photoemission electron microscopy (PEEM; probing the surface region) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS; probing the uppermost surface layer) have been used to image naturally heterogeneous samples containing chalcopyrite (CuFeS₂), pentlandite [(Ni,Fe)₉S₈] and monoclinic pyrrhotite (Fe₇S₈) both freshly polished and exposed to pH 9 KOH for 30 min. PEEM images constructed from the metal L₃ absorption edges were acquired for the freshly prepared and solution‐exposed mineral samples. These images were also used to produce near‐edge X‐ray absorption fine‐structure spectra from regions of the images, allowing the chemistry of the surface of each mineral to be interrogated, and the effect of solution exposure on the mineral surface chemistry to be determined. The PEEM results indicate that the iron in the monoclinic pyrrhotite oxidized preferentially and extensively, while the iron in the chalcopyrite and pentlandite underwent only mild oxidation. The ToF‐SIMS data gave a clearer picture of the changes happening in the uppermost surface layer, with oxidation products being observed on all three minerals, and significant polysulfide formation and copper activation being detected for pyrrhotite.