High-accuracy experimental determination of photon mass attenuation coefficients of transition metals and lithium fluoride in the ultra-soft energy range.

In the field of quantitative X-ray analysis techniques, such as electron probe microanalysis, precise knowledge of fundamental parameters is crucial. Especially, the accurate determination of photon mass attenuation coefficients is essential to perform correct elemental quantification. While the widely used databases offer agreement for the hard X-ray range, significant differences arise for lower photon energies. Furthermore, addressing the uncertainties of the tabulated data, which can be of several hundreds of percent, is of urgent need. Driven by recent advances in analytical techniques in the low energy range including investigation of materials containing lithium, the interest in a reliable set of photon mass attenuation coefficients is steadily increasing. In this study, we experimentally determine photon mass attenuation coefficients for lithium fluoride, aluminium, and different transition metals in the extreme low energy range from 40 eV to a several hundreds of eV. This high-precision experimental determination allows a comparison with the existing data tables. We observe differences that turn out to be significant, especially around the absorption edges.

Study of interface reaction in a B4C/Cr mirror at elevated temperature using soft X-ray reflectivity.

Boron carbide is a prominent material for high-brilliance synchrotron optics as it remains stable up to very high temperatures. The present study shows a significant change taking place at 550°C in the buried interface region formed between the Cr adhesive layer and the native oxide layer present on the silicon substrate. An in situ annealing study is carried out at the Indus-1 Reflectivity beamline from room temperature to 550°C (100°C steps). The studied sample is a mirror-like boron carbide thin film of 400 Šthickness deposited with an adhesive layer of 20 ŠCr on a silicon substrate. The corresponding changes in the film structure are recorded using angle-dependent soft X-ray reflectivity measurements carried out in the region of the boron K-edge after each annealing temperature. Analyses performed using the Parratt recursive formalism reveal that the top boron carbide layer remains intact but interface reactions take place in the buried Cr-SiO2 region. After 300°C the Cr layer diffuses towards the substrate. At higher temperatures of 500°C and 550°C the Cr reacts with the native oxide layer and tends to form a low-density compound of chromium oxysilicide (CrSiOx). Depth profiling of Si and Cr distributions obtained from secondary ion mass spectroscopy measurements corroborate the layer model obtained from the soft X-ray reflectivity analyses. Details of the interface reaction taking place near the substrate region of boron carbide/Cr sample are discussed.

A Simple Approach for Thickness Measurements Using Electron Probe Microanalysis.

A simple and fast method for thickness measurements using electron probe microanalysis (EPMA) is described. The method is applicable on samples with a thickness smaller than the electron depth range and does not require any knowledge of instrumental parameters. The thickness is determined by means of the distance that electrons travel inside the sample before crossing through it. Samples are first deposited on a substrate that, when reached by the transmitted electrons, produces an X-ray signal. The measured characteristic X-ray line intensity of the substrate is later used to determine the energy of transmitted electrons, which is proportional to the distance that electrons travel inside the sample. The study was performed on spherical K411 glass particles and cylindrical particles of U­Ce oxide with a size ranging from 0.2 to 4 µm. The measured thicknesses of all the studied particles showed good agreements with the real values. Although the method is only validated on particles with usual shapes, it can be applied to determine a local thickness of thin samples with irregular morphologies. This can help solving multiple issues in analysis with EPMA of non-bulk samples exhibiting complex geometries. Three dimensional microscopic imaging could also find a good utility in the described method.

Characterization of the Chemical Composition of Uranium Microparticles with Irregular Shapes Using Standardless Electron Probe Microanalysis and Micro-Raman Spectrometry.

We describe an approach enabling the identification of the elemental composition of uranium microparticles with undefined geometry using standardless quantitative electron probe microanalysis (EPMA) and micro-Raman spectrometry (MRS). The standardless procedure is based on a ZAF peak-to-background quantitative method in combination with Monte Carlo simulations. The experimental X-ray spectra were measured with an energy-dispersive spectrometer attached to a scanning electron microscope. To account for the X-ray intensity loss due to the transmission of electrons in microparticles with irregular shapes, a method was developed enabling the determination of an apparent thickness of the particle by means of the mean distance that electrons travel inside the particle before being transmitted. Size effects were further taken into account by using peak-to-background ratios and performing simulations on a particle with a thickness equal to the apparent thickness. To assess the validity of the standardless procedure in EPMA, weight fractions were determined for NIST homogeneous spherical microparticles of K411 glass and compared to certified ones. The correction of size effects was achieved and lead to accurate quantitative results with absolute relative deviations less than 9%. The model used for the determination of the apparent thickness was validated on the set of spherical K411 particles and enabled us to conduct quantifications on irregularly shaped uranium microparticles. The chemical composition of uranium particles was further investigated using MRS which enabled us to verify the reliability of the results obtained by the standardless approach.

Improving the soft X-ray reflectivity of Cr/Ti multilayers by co-deposition of B4C.

The `water window', covering 2.4-4.4 nm, is an important wavelength range particularly essential to biology research. Cr/Ti multilayers are one of the promising reflecting elements in this region because the near-normal-incidence reflectivity is theoretically as high as 64% at 2.73 nm. However, due to multilayer imperfections, the reported reflectivity is lower than 3% for near-normal incidence. Here, B and C were intentionally incorporated into ultra-thin Cr/Ti soft X-ray multilayers by co-deposition of B4C at the interfaces. The effect on the multilayer structure and composition has been investigated using X-ray reflectometry, X-ray photoelectron spectroscopy, and cross-section electron microscopy. It is shown that B and C are mainly bonded to Ti sites, forming a nonstoichiometric TiBxCy composition, which hinders the interface diffusion, supresses the crystallization of the Cr/Ti multilayer and dramatically improves the interface quality of Cr/TiBxCy multilayers. As a result, the near-normal-incidence reflectivity of soft X-rays increases from 4.48% to 15.75% at a wavelength of 2.73 nm.

Kossel Effect in Periodic Multilayers.

The Kossel effect is the diffraction by a periodically structured medium, of the characteristic X-ray radiation emitted by the atoms of the medium. We show that multilayers designed for X-ray optics applications are convenient periodic systems to use in order to produce the Kossel effect, modulating the intensity emitted by the sample in a narrow angular range defined by the Bragg angle. We also show that excitation can be done by using photons (X-rays), electrons or protons (or charged particles), under near normal or grazing incident geometries, which makes the method relatively easy to implement. The main constraint comes from the angular resolution necessary for the detection of the emitted radiation. This leads to small solid angles of detection and long acquisition times to collect data with sufficient statistical significance. Provided this difficulty is overcome, the comparison or fit of the experimental Kossel curves, i.e., the angular distributions of the intensity of an emitted radiation of one of the element of the periodic stack, with the simulated curves enables getting information on the depth distribution of the elements throughout the multilayer. Thus the same kind of information obtained from the more widespread method of X-ray standing wave induced fluorescence used to characterize stacks of nanometer period, can be obtained using the Kossel effect.

Ultra-short and ultra-intense X-ray free-electron laser single pulse in one-dimensional photonic crystals.

The propagation within a one-dimensional photonic crystal of a single ultra-short and ultra-intense pulse delivered by an X-ray free-electron laser is analysed with the framework of the time-dependent coupled-wave theory in non-linear media. It is shown that the reflection and the transmission of an ultra-short pulse present a transient period conditioned by the extinction length and also the thickness of the structure for transmission. For ultra-intense pulses, non-linear effects are expected: they could give rise to numerous phenomena, bi-stability, self-induced transparency, gap solitons, switching, etc., which have been previously shown in the optical domain.

Nitridated Pd/B4C multilayer mirrors for soft X-ray region: internal structure and aging effects.

Reactive sputtering with a mixture of argon and nitrogen (N2 partial pressure of 4%, 8%, and 15%) as the working gas is used to develop the high reflectance Pd/B4C multilayers for soft X-ray region application. Compared to the pure Ar fabricated sample, the interface roughness of the nitridated multilayer is slightly increased while the compressive stress is essentially relaxed from -623 MPa (pure Ar) to -85 MPa (15% N2). A maximum reflectance of 32% is measured at the wavelength of 9.5 nm for the multilayer fabricated with 15% N2. After storing the multilayers in an air environment for 6-17 months, a distinct aging effect is observed on the nitridated samples. The transmission electron microscopy results indicate that a large part of the top layers of the nitridated samples is deteriorated with severe interdiffusion, essential decrease in d-spacing, and compacted multilayer structure. The deterioration is less pronounced for the multilayers fabricated with a higher ratio of N2. Energy dispersive X-ray spectroscopy reveals that the concentration of nitrogen and boron in the degraded area is much reduced compared to the intact layers. A primitive model of upward diffusion of nitrogen and boron is proposed to explain the aging effects of the nitridated structure.

High reflectance Cr/V multilayer with B(4)C barrier layer for water window wavelength region.

To develop the high reflectance mirror for the short wavelength range of the water window region (λ=2.42-2.73 nm), Cr/V multilayers with B4C barrier layers are studied. The grazing incidence x-ray reflectometry results show that the multilayer interface widths are significantly reduced down to 0.21-0.31 nm, after the introduction of 0.1 nm B4C barrier layers at both interfaces. The [B4C/Cr/B4C/V] multilayer with a large number of bilayers of N=300 maintains the same small interface widths while the surface roughness is only 0.2 nm. According to the transmission electron microscope measurements, the layer structure improvement with barrier layers can be attributed to the suppression of the crystallization of vanadium inside the structure. Using the interface engineered multilayer, a maximum soft x-ray reflectance of 24.3% is achieved at λ=2.441 nm, under the grazing incidence of 42°.

X-ray fluorescence induced by standing waves in the grazing-incidence and grazing-exit modes: study of the Mg-Co-Zr system.

The characterization of Mg-Co-Zr tri-layer stacks using X-ray fluorescence induced by X-ray standing waves, in both the grazing-incidence (GI) and the grazing-exit (GE) modes, is presented. The introduction of a slit in the direction of the detector improves the angular resolution by a factor of two and significantly improves the sensitivity of the technique for the chemical characterization of the buried interfaces. By observing the intensity variations of the Mg Kα and Co Lα characteristic emissions as a function of the incident (GI mode) or detection (GE mode) angle, it is shown that the interfaces of the Si/[Mg/Co/Zr] × 30 multilayer are abrupt, whereas in the Si/[Mg/Zr/Co] × 30 multilayer a strong intermixing occurs at the Co-on-Zr interfaces. An explanation of this opposite behavior of the Co-on-Zr and Zr-on-Co interfaces is given by the calculation of the mixing enthalpies of the Co-Mg, Co-Zr and Mg-Zr systems, which shows that the Co-Zr system presents a negative value and the other two systems present positive values. Together with the difference of the surface free energies of Zr and Co, this leads to the Mg/Zr/Co system being considered as a Mg/CoxZry bi-layer stack, with x/y estimated around 3.5.

Enhancement of soft X-ray reflectivity and interface stability in nitridated Pd/Y multilayer mirrors.

Pd/Y multilayer mirrors operating in the soft X-ray region are characterized by a high theoretical reflectance, reaching 65% at normal incidence in the 8-12 nm wavelength range. However, a severe intermixing of neighboring Pd and Y layers results in an almost total disappearance of the interfaces inside the multilayer structures fabricated by direct current magnetron sputtering and thus a dramatic reflectivity decrease. Based on grazing incidence X-ray reflectometry and X-ray photoelectron spectroscopy, we demonstrate that the stability of the interfaces in Pd/Y multilayer structures can be essentially improved by adding a small amount of nitrogen (4-8%) to the working gas (Ar). High resolution transmission electron microscopy shows that the interlayer width is only 0.9 nm and 0.6 nm for Y(N)-on-Pd(N) and Pd(N)-on-Y(N) interfaces, respectively. A well-defined crystalline texture of YN (200) is observed on the electron diffraction pattern. As a result, the measured reflectance of the Pd(N)/Y(N) multilayer achieves 30% at λ = 9.3 nm. The peak reflectivity value is limited by the remaining interlayers and the formation of the YN compound inside the yttrium layers, resulting in an increased absorption.

Enhancement of the reflectivity of Al/Zr multilayers by a novel structure.

The reflectivity of Al/Zr multilayers is enhanced by the use of a novel structure. The Al layers are divided by insertion of Si layers. In addition, Si barrier layers are inserted at the Al/Zr interfaces (Zr-on-Al and Al-on-Zr). As a result, crystallization of the Al layer is inhibited and that of Zr is enhanced. In grazing incidence x-ray reflectometry, x-ray diffraction, and extreme ultraviolet measurements, the novel multilayers exhibit lower interfacial roughness compared with traditional multilayer structures, and their reflectivity is increased from 48.2% to 50.0% at a 5° angle of incidence. These novel multilayers also have potential applications in other multilayer systems and the semiconductor industry.

Optical and structural performance of the Al(1%wtSi)/Zr reflection multilayers in the 17-19nm region.

Two kinds of Al/Zr (Al(1%wtSi)/Zr and Al(Pure)/Zr) multilayers for extreme ultraviolet (EUV) optics were deposited on fluorine doped tin oxide coated glass by using direct-current magnetron sputtering technology. The comparison of the two systems shows that the Al(1%wtSi)/Zr multilayers have the lowest interfacial roughness and highest reflectivity. Based on the X-ray diffraction, the performance of the two systems is determined by the crystallization of Al layer. To fully understand the Al(1%wtSi)/Zr multilayer, we built up a two-layer model to fit situation of the AFM images, and simulate the grazing incident x-ray reflection-measurements of multilayers with various periods (N = 10, 40, 60, 80). Below 40 periods, the roughness components are lowered. After 40 periods, both surface and interfacial roughness increase with the period number, and decrease the reflectance. According to transmission electron microscope images, the model can represent the variable structure of the system.

Thermal stability of Mg/Co multilayer with B4C, Mo or Zr diffusion barrier layers.

The efficiency of B(4)C, Mo and Zr barrier layers to improve thermal stability of Mg/Co multilayer up to 400 °C is investigated. Multilayers were deposited by direct current magnetron sputtering and characterized using X-ray and extreme ultraviolet reflection. The results suggest that B(4)C barrier layer is not effective due to drastic diffusion at Mg-B(4)C interface. Although introducing Mo barriers improves the thermal stability from 200 to 300 °C, it increases the interface roughness and thus degrades the optical performances. On the contrary, Zr barriers can significantly increase the thermal stability of Mg/Co up to 400 °C without optical performance degradation. Thus, Mg/Zr/Co/Zr is suitable for EUV applications requiring both optimal optical performances and heat resistance.

Structural properties of Al/Mo/SiC multilayers with high reflectivity for extreme ultraviolet light.

We present the results of an optical and chemical, depth and surface study of Al/Mo/SiC periodic multilayers, designed as high reflectivity coatings for the extreme ultra-violet (EUV) range. In comparison to the previously studied Al/SiC system, the introduction of Mo as a third material in the multilayer structure allows us to decrease In comparison to the previously studied Al/SiC system with a reflectance of 37% at near normal incidence around 17 nm, the introduction of Mo as a third material in the multilayer structure allows us to decrease the interfacial roughness and achieve an EUV reflectivity of 53.4%, measured with synchrotron radiation. This is the first report of a reflectivity higher than 50% around 17 nm. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and x-ray photoelectron spectroscopy (XPS) measurements are performed on the Al/Mo/SiC system in order to analyze the individual layers within the stack. ToF-SIMS and XPS results give evidence that the first SiC layer is partially oxidized, but the O atoms do not reach the first Mo and Al layers. We use these results to properly describe the multilayer stack and discuss the possible reasons for the difference between the measured and simulated EUV reflectivity values.

Comparison of Mg-based multilayers for solar He II radiation at 30.4 nm wavelength.

Mg-based multilayers, including SiC/Mg, Co/Mg, B(4)C/Mg, and Si/Mg, are investigated for solar imaging and a He II calibration lamp at a 30.4 nm wavelength. These multilayers were fabricated by a magnetron sputtering method and characterized by x-ray reflection. The reflectivities of these multilayers were measured by synchrotron radiation. Near-normal-incidence reflectivities of Co/Mg and SiC/Mg multilayer mirrors are as high as 40.3% and 44.6%, respectively, while those of B(4)C/Mg and Si/Mg mirrors are too low for application. The measured results suggest that SiC/Mg, Co/Mg multilayers are promising for a 30.4 nm wavelength.

Realization of wafer-scale nanogratings with sub-50 nm period through vacancy epitaxy.

Gratings, one of the most important energy dispersive devices, are the fundamental building blocks for the majority of optical and optoelectronic systems. The grating period is the key parameter that limits the dispersion and resolution of the system. With the rapid development of large X-ray science facilities, gratings with periodicities below 50 nm are in urgent need for the development of ultrahigh-resolution X-ray spectroscopy. However, the wafer-scale fabrication of nanogratings through conventional patterning methods is difficult. Herein, we report a maskless and high-throughput method to generate wafer-scale, multilayer gratings with period in the sub-50 nm range. They are fabricated by a vacancy epitaxy process and coated with X-ray multilayers, which demonstrate extremely large angular dispersion at approximately 90 eV and 270 eV. The developed new method has great potential to produce ultrahigh line density multilayer gratings that can pave the way to cutting edge high-resolution spectroscopy and other X-ray applications.

Note: Observation of the angular distribution of an x-ray characteristic emission through a periodic multilayer.

We present the observation of the angular distribution of a characteristic x-ray emission through a periodic multilayer. The emission coming from the substrate on which the multilayer is deposited is used for this purpose. It is generated upon proton irradiation through the multilayer and detected with an energy sensitive CCD camera. The observed distribution in the low detection angle range presents a clear dip at a position characteristic of the emitting element. Thus, such a device can be envisaged as a spectrometer without mechanical displacement and using various ionizing sources (electrons, x-rays, and ions), their incident direction being irrelevant.

EUV stimulated emission from MgO pumped by FEL pulses.

Stimulated emission is a fundamental process in nature that deserves to be investigated and understood in the extreme ultra-violet (EUV) and x-ray regimes. Today, this is definitely possible through high energy density free electron laser (FEL) beams. In this context, we give evidence for soft-x-ray stimulated emission from a magnesium oxide solid target pumped by EUV FEL pulses formed in the regime of travelling-wave amplified spontaneous emission in backward geometry. Our results combine two effects separately reported in previous works: emission in a privileged direction and existence of a material-dependent threshold for the stimulated emission. We develop a novel theoretical framework, based on coupled rate and transport equations taking into account the solid-density plasma state of the target. Our model accounts for both observed mechanisms that are the privileged direction for the stimulated emission of the Mg L2,3 characteristic emission and the pumping threshold.

High Reflectance Nanoscale V/Sc Multilayer for Soft X-ray Water Window Region.

V/Sc multilayer is experimentally demonstrated for the first time as a high reflectance mirror for the soft X-ray water window region. It primarily works at above the Sc-L edge (λ = 3.11 nm) under near normal incidence while a second peak appears at above the V-L edge (λ = 2.42 nm) under grazing incidence. The V/Sc multilayer fabricated with a d-spacing of 1.59 nm and 30 bilayers has a smaller interface width (σ = 0.27 and 0.32 nm) than the conventional used Cr/Sc (σ = 0.28 and 0.47 nm). For V/Sc multilayer with 30 bilayers, the introduction of B4C barrier layers has little improvement on the interface structure. As the number of bilayers increasing to 400, the growth morphology and microstructure of the V/Sc layers evolves with slightly increased crystallization. Nevertheless, the surface roughness remains to be 0.25 nm. A maximum soft X-ray reflectance of 18.4% is measured at λ = 3.129 nm at 9° off-normal incidence using the 400-bilayers V/Sc multilayer. According to the fitted model, an s-polarization reflectance of 5.2% can also be expected at λ = 2.425 nm under 40° incidence. Based on the promising experimental results, further improvement of the reflectance can be achieved by using a more stable deposition system, exploring different interface engineering methods and so on.