Grazing Incidence X-ray Fluorescence Research Articles

Impact of B4C buffer layer on interface diffusion in Cr/Sc multilayers: combined study by x-ray reflectivity, scattering and fluorescence

In thin film multilayer based optical componentsof x-ray imaging system, diffusion of one material into the other degrades the reflectivity of the mirrors severely. Along with this thermodynamically driven diffusion, there are also growth generated interface roughness of different special frequencies and microstructures which can increase the diffused scattering from the multilayer and reduce the resolution of an image. Generally grazing incidence x-ray reflectivity in specular geometry (specular GIXR) and diffused x-ray scattering measurement in rocking scan geometry yield information regarding microstructure and overall diffusion at the interfaces of a multilayer. In this paper it is shown that grazing incidence x-ray fluorescence (GIXRF) measurement in standing wave condition alongwith the above measurements can give precise information regarding element-specific diffusion at the interfaces of a multilayer structure. Periodic multilayers made of 75 Cr/Sc bilayers with bilayer thickness ∼4 nm with and without B4C barrier layer of 0.2 nm thickness at the interfaces have been prepared using ion beam sputtering system and characterized by GIXR, diffused x-ray scattering and GIXRF measurements using synchrotron x-ray radiation just above the Cr K-edge. From the above measurements, drastic reduction in interface diffusion of Cr and improvement of interface morphology after addition of B4C barrier layer at the interfaces of Cr/Sc multilayers have been observed which is also corroborated by cross-sectional transmission electron microscopy of the multilayers. Finally, in the water window soft x-ray region of 2.3–4.4 nm performance of these multilayers have been tested and the Cr/B4C/Sc multilayer with improved interface quality has been found to yield ∼30.8% reflectivity at 3.11 nm wavelength which is comparable with the best reported reflectivities in the literature at this wavelength.

Grazing-emission X-ray fluorescence as a multiprobe tool for thin-film metrology

Recent research has demonstrated the high potential of grazing-emission X-ray fluorescence as a nanometrology tool for structures with complex 3D architecture. The technique now allows separate reconstruction of the spatial atomic distributions of different chemical elements both vertically and laterally, which opens up new and interesting applications in the nanofabrication industry. This study compares grazing-emission fluorescence with the well established grazing-incidence X-ray fluorescence method. The comparison uses a simple 1D thin-film structure. Reconstruction of the structure by both methods is performed and the statistical uncertainties of these reconstructions are compared by means of Monte Carlo Markov-chain simulations. For the forward model a semi-analytic approach is derived which allows simulation of the fluorescence intensity. This approach takes into account both grazing-incidence and grazing-emission cases. It also accounts for an even more complex physical phenomenon, the anomalous Kossel effect. Finally, using this semi-analytic approach, an experimental scheme combining grazing-incidence and grazing-emission X-ray fluorescence is studied theoretically.

Impact study of numerical discretization accuracy on parameter reconstructions and model parameter distributions

In optical nano metrology numerical models are used widely for parameter reconstructions. Using the Bayesian target vector optimization method we fit a finite element numerical model to a Grazing Incidence x-ray fluorescence data set in order to obtain the geometrical parameters of a nano structured line grating. Gaussian process, stochastic machine learning surrogate models, were trained during the reconstruction and afterwards sampled with a Markov chain Monte Carlo sampler to determine the distribution of the reconstructed model parameters. The numerical discretization parameters of the used finite element model impact the numerical discretization error of the forward model. We investigated the impact of the polynomial order of the finite element ansatz functions on the reconstructed parameters as well as on the model parameter distributions. We showed that such a convergence study allows to determine numerical parameters which allows for efficient and accurate reconstruction results.

Combined x‐ray reflectivity and grazing incidence x‐ray fluorescence study of Ta/Cr/Pt thin film stacks

AbstractThe Ta/Cr/Pt three‐layer system can be used as a planar x‐ray waveguide, that is to say it can guide an x‐ray beam inside its chromium layer. This property comes from the difference in density and hence in optical index between the two “heavy” or cladding tantalum and platinum layers and the “light” or guiding chromium layer. The waveguide will be efficient provided the layers are a few nanometers thick and that the interfaces are as sharp as possible. To control the quality of the stack, we combine grazing incidence x‐ray fluorescence (GIXRF) and x‐ray reflectivity (XRR) measurements on a series of Ta/Cr/Pt samples, whose only difference is the thickness of the Cr layer. The three considered samples have been deposited by magnetron sputtering and their designed structures are: Ta (8 nm)/Cr (5, 10, and 15 nm)/Pt (14 nm)/Si substrate. The combination of XRR and GIXRF tightens constraints on the parameters used to simulate the stack: thickness, roughness, composition, and density of the layers and their interlayers. For each sample we used six GIXRF curves obtained from three different characteristic x‐ray lines (Ta Lα, Cr Kα, and Pt Lβ2,15) excited at three different incident photons energies (6.25, 10, and 12 keV) as well as one XRR curve obtained at 6.25 keV. The XRR‐GIXRF combined analysis demonstrates that the Ta/Cr/Pt structure is too simplistic and that it is necessary to introduce some interlayers at the top and bottom of the stacks to obtain a reliable agreement between the experimental and simulated GIXRF and XRR curves.

A step toward calculating the uncertainties in combined GIXRF‐XRR

AbstractThe combination of X‐ray reflectivity (XRR) and grazing incidence X‐ray fluorescence (GIXRF) is a surface sensitive analytical method, which can be used for the characterization of thin films and multilayered materials. Both of these techniques are implemented on the same experimental setup and make use of similar mechanical processes and the same fundamental physical concept required for a combined data analysis. The combination of these techniques removes ambiguous results for the characterization of nanometer layers, as well as nanometer depth profiles, resulting in more accurate characterization of thickness, roughness, density, and elemental composition. Due to the vast number of fitting parameters, the estimation of the thin film sample structure is a challenging task. In this paper, we propose a recursive method for estimating the uncertainties of data from GIXRF‐XRR analysis, based on a Bootstrap statistical method. This approach relies on re‐sampling a dataset to estimate statistics on a population by applying random weights. We applied this method on an as‐deposited chalcogenide germanium, antimony, and tellurium (GST) thin film with a carbon‐capping layer. We found good agreement between the experimental and the theoretical XRR‐GIXRF values for a sample structure model, of which the parameters were determined within a confidence interval using the bootstrap method. We also propose an approach for calculating the uncertainty on the solid angle of detection based on Monte Carlo simulations.

Developing Quantitative Nondestructive Characterization of Nanomaterials: A Case Study on Sequential Infiltration Synthesis of Block Copolymers.

The sequential infiltration synthesis (SIS) of inorganic materials in nanostructured block copolymer templates has rapidly progressed in the last few years to develop functional nanomaterials with controllable properties. To assist this rapid evolution, expanding the capabilities of nondestructive methods for quantitative characterization of the materials properties is required. In this paper, we characterize the SIS process on three model polymers with different infiltration profiles through ex situ quantification by reference-free grazing incidence X-ray fluorescence. More qualitative depth distribution results were validated by means of X-ray photoelectron spectroscopy and scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy.

Study of Ni/AlN/Ni waveguide system for 8.05 keV incident energy

Ni/AlN/Ni waveguide system provides intensity enhancement by a factor of ~30 at 8.05 keV photon energy. In the present study, the effect of deviation of structural parameters (thickness of each layer) along with the surface-interface properties on electric field intensity (EFI) inside an ion beam sputter deposited Ni/AlN/Ni waveguide is investigated. EFI calculations are performed using structural parameters obtained from X-ray reflectivity (XRR) measurements performed using Cu-Kα source. Deposited waveguide structure provides intensity enhancement (~26) slightly lower than optimized value (~30). Furthermore, temporal stability of the structure is investigated using combined XRR and grazing incidence X-ray fluorescence (GIXRF) techniques (at 15 keV photon energy) after about 15 months. Some structural changes are observed which, however, do not lead to further decrease in intensity enhancement inside the waveguide, making it suitable for applications at 8.05 keV photon energy.

Assessing advanced methods in XPS and HAXPES for determining the thicknesses of high-k oxide materials: From ultra-thin layers to deeply buried interfaces

We assess advanced XPS-based methods in determining depth distribution in alumina and hafnia ALD samples of increasing thicknesses and buried layer depths. First, dose quantification of Al in ultrathin alumina is achieved through non-destructive wavelength dispersive X-ray fluorescence (WDXRF), which is confirmed by comparison to destructive liquid-phase deposition inductively coupled plasma mass spectrometry (LPD-ICPMS) (uncertainty 10 %). Second, we compare WDXRF to parallel ARXPS (pARXPS) in precise dose and thickness quantification in ultrathin alumina, and present the ability of pARXPS to accurately track linear growth during the deposition. Thickness determination of an intermediate ALD stack of silicon dioxide, hafnia, and alumina layers of 1–2 nm by pARXPS is evaluated against quantitative, reference-free grazing incidence X-ray fluorescence (GIXRF) (uncertainty 8–9 %). An alternative technique employing inelastic background analysis (IBA) is introduced as a fast and accurate method and subsequently compared with pARXPS and GIXRF thickness determinations. Finally, in thicker multilayer systems, hard X-ray photoelectron spectroscopy (HAXPES) combined with IBA is proposed for an in-depth analysis for thicknesses up to 28 nm, with generated interface locations and layer thicknesses well-aligned to nominal values. This study is a novel application of lab-based Cr-Kα HAXPES spectra for inelastic background analysis in thick metal-oxide multilayer materials.

Structural characterization of Au/Cr bilayer thin films using combined X‐ray reflectivity and grazing incidence X‐ray fluorescence measurements

Gold thin‐film mirrors are widely used as X‐ray reflective optics to manipulate X‐rays especially in synchrotron beamlines. Application of a thin underlying buffer layer of chromium enhances the adhesion of gold film to the substrate material and provides long term structural stability. In the present work, Au/Cr bilayers deposited using the DC magnetron sputtering technique with different thickness values on a Si substrate are studied to better understand the correlation between micro‐structure properties and residual stress. In order to obtain precise structural properties of Cr binding layer and gold layer, as well as in‐between nanoscaled interfacial structure, nondestructive probing is carried out by simultaneously utilizing synchrotron‐based combined X‐ray reflectivity (XRR)–grazing incidence X‐ray fluorescence (GIXRF) techniques. It has been clearly observed that when thickness of Cr layer is relatively small, a composite interlayer structure is formed at the Au/Cr boundary and at the Cr/Si interface boundary due to the interdiffusion of two layer mediums. However, when the thickness of the Cr layer is relatively larger (~90 Å), emergence of such an interlayer medium does not take place. The grazing incidence X‐ray diffraction (GIXRD) measurements have also been performed to investigate crystalline structure and residual stress of the gold layer medium. Our results show that relatively large residual stress exists in the Au layer if thickness of Cr layer is small. The surface roughness morphology of the top gold layer obtained from the atomic force microscopy measurements showed the partial agreement with the results obtained from the XRR‐GIXRF technique.

ALTECH 2021 – Analytical Techniques for Precise Characterization of Nano Materials

ALTECH 2021 – Analytical Techniques for Precise Characterization of Nano Materials

Reliable compositional analysis of airborne particulate matter beyond the quantification limits of total reflection X-ray fluorescence

Knowledge on the temporal and size distribution of particulate matter (PM) in air as well as on its elemental composition is a key information for source appointment, for the investigation of their influence on environmental processes and for providing reliable data for climate models. While cascade impactors allow for time- and size-resolved collection of airborne PM, total reflection X-ray fluorescence (TXRF) allows for element-sensitive investigation of minute sample amounts thanks to its detection sensitivity. But during quantification by means of TXRF it is crucial to be aware of the linear calibration limits of TXRF in order to identify situations where collection times or pollution levels in the different size partitions were exceedingly long or high. Indeed, TXRF can only be reliably used when the amount of matter collected on the top of the substrate is sufficiently small. By means of grazing incidence X-ray fluorescence (GIXRF), where the excitation conditions are varied in a controlled and reliable manner and include also the TXRF regime, a self consistent quantification of elemental mass depositions can be performed in order to validate or falsify TXRF quantification results. For low mass depositions an agreement within a few percent for the different excitation conditions was found, while for increasing amounts of material relative errors of up to a factor of 4 were found for TXRF as compared to GIXRF. Thus, TXRF cannot be applied to all samples regardless of their coverage and threshold values for the validity of quantification results need to be determined. As a flexible solution, GIXRF allows extending the dynamic range of reliably quantifiable mass depositions beyond the linear regime of TXRF, an important advantage when variable amounts of airborne PM need to be quantified as in the case of collection with cascade impactors. The presented more reliable quantification approach can be transferred to mobile tabletop instrumentation as well. This aspect is highly relevant for air quality monitoring in terms of supporting the introduction of appropriate legislation and measures for health and climate protection as well as for supporting their enforcement.

Depth profiling of energetic Au ions inside P-type Si 〈1 0 0〉 substrate

Gold implanted silicon substrates are widely used for the fabrication of microelectronic devices, such as detectors for infrared imaging applications, optoelectronic devices as well as for synthesis of implanted nano-particles. The continuous down scaling of physical size of these devices motivated us to develop methodologies for the reliable depth profile studies of various implanted ion species using sensitive and non-destructive methods. In this article, we present depth profile studies for gold ions implanted in silicon substrates using simultaneous X-ray reflectivity and grazing incidence X-ray fluorescence measurements. Results obtained using the XRR-GIXRF studies were found to be fairly consistent with the experimental measurements of secondary ion mass spectrometry. Furthermore, it was noticed that energetic gold ions cause substantial amorphization of crystalline Si material in the vicinity of implanted region, which was confirmed using the GIXRD measurements. Ion implantation also induces large surface modifications and roughness effects on the top of the silicon substrate, as revealed from atomic force microscope measurements.

Surface and interface characterization of Ru/C/Ru trilayer structure using grazing incidence X‐ray reflectivity and X‐ray fluorescence

In 8‐ to 20‐keV photon energy region, ruthenium and carbon thin films are used in multilayer monochromators. In the present study, this material combination is explored for X‐ray waveguide applications in hard X‐ray region. The structural parameters (thickness of each layer) of Ru/C/Ru waveguide structure are optimized to get maximum intensity enhancement of fundamental mode inside carbon guiding layer. A sample with optimized structural parameters is deposited using ion beam sputtering (IBS) technique and characterized using X‐ray reflectivity (XRR) and grazing incidence X‐ray fluorescence (GIXRF) techniques. The analysis suggests that the density of bottom Ru layer and carbon guiding layer is close to bulk density (~97% for Ru and ~95% for carbon), whereas density of top Ru layer is slightly lower (~93% of bulk density). A ~10% of thickness variation in top cladding layer along with marginal change in layer density deteriorate field enhancement in TE0 mode by more than three times. Effect of thickness and density variation on waveguide (Ru [7 nm]/C [18 nm]/Ru [20 nm]) performance is discussed.

Shape- and Element-Sensitive Reconstruction of Periodic Nanostructures with Grazing Incidence X-ray Fluorescence Analysis and Machine Learning.

The characterization of nanostructured surfaces with sensitivity in the sub-nm range is of high importance for the development of current and next-generation integrated electronic circuits. Modern transistor architectures for, e.g., FinFETs are realized by lithographic fabrication of complex, well-ordered nanostructures. Recently, a novel characterization technique based on X-ray fluorescence measurements in grazing incidence geometry was proposed for such applications. This technique uses the X-ray standing wave field, arising from an interference between incident and the reflected radiation, as a nanoscale sensor for the dimensional and compositional parameters of the nanostructure. The element sensitivity of the X-ray fluorescence technique allows for a reconstruction of the spatial element distribution using a finite element method. Due to a high computational time, intelligent optimization methods employing machine learning algorithms are essential for timely provision of results. Here, a sampling of the probability distributions by Bayesian optimization is not only fast, but it also provides an initial estimate of the parameter uncertainties and sensitivities. The high sensitivity of the method requires a precise knowledge of the material parameters in the modeling of the dimensional shape provided that some physical properties of the material are known or determined beforehand. The unknown optical constants were extracted from an unstructured but otherwise identical layer system by means of soft X-ray reflectometry. The spatial distribution profiles of the different elements contained in the grating structure were compared to scanning electron and atomic force microscopy and the influence of carbon surface contamination on the modeling results were discussed. This novel approach enables the element sensitive and destruction-free characterization of nanostructures made of silicon nitride and silicon oxide with sub-nm resolution.

Non-destructive elemental depth profiling of Ni/Ti multilayers by GIXRF technique

In thin film multilayer along with total interface widths, study of individual interface widths of the constituent elements of the multilayer is important for the knowledge of diffusion mechanism and diffusion kinetics. Element specific Grazing Incidence X-ray Fluorescence (GIXRF) technique has been used to study the individual interface diffusion of Ni and Ti in the Ni/Ti multilayer. The above measurements yield similar results on the interface qualities of the multilayer as obtained from more widely used Grazing Incidence X-ray reflectivity (GIXRR) technique and give additional information regarding Ni and Ti diffusion separately which cannot be obtained from GIXRR. Ni-fluorescence and Ti-fluorescence GIXRF spectra of 10-bilayer Ni/Ti multilayer films are fitted simultaneously to find the interface widths of Ni and Ti at the Ni-on-Ti and Ti-on-Ni interfaces. The Error function distribution has been used to describe the concentration of elements at the interfaces. Further, the variation of these individual diffusion and individual concentration profile of elements has been studied with annealing of the multilayer. It has been shown that non-destructive and relatively simpler GIXRF technique can be used for elemental depth profiling of a multilayer very precisely, which otherwise can be obtained by using complicated destructive technique like atom probe tomography.

Characterization of archaeological ceramics from eastern Siberia by total-reflection X-ray fluorescence spectrometry and principal component analysis

The study of the composition of archaeological ceramics provides information used to assess possible transportation routes and cultural connections between ancient people living in different regions. Total-reflection X-ray fluorescence (TXRF) was applied for the analysis of ceramics as a promising method due to the rapidity, cost-efficiency, and capability to analyze a very small amount of a sample (<20 mg), which is important for the preservation of archaeological material. Two techniques of sample preparation for the TXRF analysis of ceramics have been considered. The analysis of suspensions was applied to obtain information on the bulk composition of ceramics. Acid leaching was used for the elemental analysis of the clay component of the ceramics, with the separation of non-soluble mineral phase. The proposed TXRF techniques have been successfully validated by the analysis of reference material and comparison with independent analytical methods. Reproducibility (interlaboratory precision) was evaluated for the proposed TXRF leaching method involving two different laboratories. The application of grazing incidence X-ray fluorescence (GI-XRF) showed homogeneous distribution of elements in the specimen and correspondence of the measurements to the conditions of total external reflection. TXRF method was applied to collect the first data on the composition of archaeological samples of ceramics from Popovsky Lug, eastern Siberia (Russia). The obtained data on the elemental composition were used to classify the samples by statistical methods of analysis, including the method of principal components and factor analysis. Three different compositional groups of ceramic have been separated and accepted as corresponding to archaeological interpretation.

Grazing Emission X‐Ray Fluorescence: Novel Concepts and Applications for Nano‐Analytics

Angle‐resolved X‐ray fluorescence spectroscopy gives access to elemental analysis of nanoscaled materials. The two main techniques of this method are grazing incidence X‐ray fluorescence (GIXRF) and grazing emission X‐ray fluorescence (GEXRF) spectroscopy. Principles of both techniques including main applications are discussed in this Review. The main focus lies on the description of GEXRF technique and its new analytical possibilities in the nano‐world.

Towards a calibration of laboratory setups for grazing incidence and total-reflection X-ray fluorescence analysis

Grazing-Incidence X-ray fluorescence (GIXRF) analysis, which is closely related to total-reflection XRF, is a very powerful technique for the in-depth analysis of many types of technologically relevant samples, e.g. nanoparticle depositions, shallow dopant profiles, thin layered samples or even well-ordered nanostructures. However, the GIXRF based determination of the depth-dependent information about the sample is usually based on a modeling of the experimental data. This requires profound knowledge of the geometrical parameters of the setup employed, especially the incident beam profile as well as the detector aperture parameters. Together they determine the incident angle dependent so-called effective solid angle of detection which must be known in order to model any experimental data set. In this work, we demonstrate how these instrumental parameters, which are typically not known with sufficient accuracy, can be determined using dedicated experiments with a well-known calibration sample. In addition, this paves the way for a full calibration of the setup, as also information about other parameters, e.g. the incident photon flux is gained. Here, we are using a commercially available instrument for this demonstration but the principle can also be applied for other GIXRF setups.

Qualitative and Semiquantitative Determination of the Atomic and Molecular Tungsten Distributions in Hybrid Hydroxyurethanes-Poly(dimethylsiloxane) Films Containing Phosphotungstates ([PW12O40]3-).

In this study, hybrid poly(dimethylsiloxane)-derived hydroxyurethanes films (PDMSUr-PWA) containing phosphotungstic acid (H3PW12O40/PWA) were characterized using field emission gun scanning electron microscopy (FEG-SEM), in attenuated total reflectance Fourier transform mid-infrared mode (ATR FT-MIR), and analyzed using synchrotron radiation micro-X-ray fluorescence (SR-μXRF), synchrotron radiation grazing incidence X-ray fluorescence (SR-GIXRF), laser-induced breakdown spectroscopy (LIBS), and instrumental neutron activation analysis (NAA) in order to correlate the distribution patterns of tungsten and properties of PDMSUr-PWA films. PDMS constitute elastomers with good mechanical, thermal, and chemical (hydrophobicity/non-hygroscopy) resistance. Currently, products based on urethanes (e.g., polyurethanes) are widely used in many applications as plastics, fiber-reinforced polymers, high-performance adhesives, corrosion-resistant coatings, photochromic films, among others. The possibility to combine inorganic and organic components can produce a hybrid material with unique properties. PWA has an important role as agent against the corrosion of steel surfaces in different media, besides exhibiting amazing catalytic and photochromic properties in these films. PWA kept its structure inside of these hybrid films through interactions between the organic matrix of PDMSUr and silanol from the inorganic part (organically modified silica), as was shown using ATR FT-MIR spectra. The FEG-SEM/SR-μXRF/wide-angle X-ray scattering (WAXS)/X-ray diffraction (XRD)/energy dispersive X-ray results proved the presence of PWA in the composition of domains of PDMSUr-PWA films. At PWA concentrations higher than 50 wt%/wt, tungsten segregation across the thickness is predominant, while that at PWA concentrations lower than 35 wt%/wt, tungsten segregation at surface is predominant. Inhomogeneities in the tungsten distribution patterns (at micrometric and millimetric level) may play an important role in the mechanical properties of these films (elastic modulus and hardness).

Laboratory grazing-incidence X-ray fluorescence spectroscopy as an analytical tool for the investigation of sub-nanometer CrSc multilayer water window optics

Efficient multilayer optics for radiation in the water window range are difficult to manufacture due to extremely small layer thicknesses and severe intermixing of elements between the layers. Therefore, adequate analytics and short feedback loops are of utmost importance for manufacturers to improve performance and efficiency. We show the possibility for non-destructive elemental depth profiling with commercial laboratory equipment using four real-life CrSc multilayer samples. Comparative measurements at the laboratory of PTB at the synchrotron radiation facility BESSY II confirm the results and prove the potential of laboratory equipment for the reliable analysis of stratified materials with sub-nanometer layer thicknesses.