X-ray Standing Wave Field Research Articles

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.

Grazing incidence-x-ray fluorescence for a dimensional and compositional characterization of well-ordered 2D and 3D nanostructures

The increasing importance of well-controlled ordered nanostructures on surfaces represents a challenge for existing metrology techniques. To develop such nanostructures and monitor complex processing constraints fabrication, both a dimensional reconstruction of nanostructures and a characterization (ideally a quantitative characterization) of their composition is required. In this work, we present a soft x-ray fluorescence-based methodology that allows both of these requirements to be addressed at the same time. By applying the grazing-incidence x-ray fluorescence technique and thus utilizing the x-ray standing wave field effect, nanostructures can be investigated with a high sensitivity with respect to their dimensional and compositional characteristics. By varying the incident angles of the exciting radiation, element-sensitive fluorescence radiation is emitted from different regions inside the nanoobjects. By applying an adequate modeling scheme, these datasets can be used to determine the nanostructure characteristics. We demonstrate these capabilities by performing an element-sensitive reconstruction of a lamellar grating made of Si3N4, where GIXRF data for the O-Kα and N-Kα fluorescence emission allows a thin oxide layer to be reconstructed on the surface of the grating structure. In addition, we employ the technique also to three dimensional nanostructures and derive both dimensional and compositional parameters in a quantitative manner.

Grazing-incidence X-ray fluorescence analysis of thin chalcogenide materials deposited on Bragg mirrors

Advanced chalcogenide materials are key for state-of-the art memories, energy harvesting materials and photonics. The properties of thin chalcogenide layers are highly driven by their chemical composition, chemical depth-profiles and surface/interface effects. The combination of Grazing-Incidence X-ray Fluorescence (GIXRF) and X-ray Reflectometry (XRR) techniques allows for non-destructive access to such information, in the lab or at dedicated beamlines. However, the accuracy of the GIXRF-deduced composition slightly degrades with depth, as the enhancement of the XRF signal by the X-ray Standing Wave (XSW) field is usually limited to the first nanometers at the surface of the probed sample. In this paper, we suggest the use of (Mo/Si)*N Bragg mirrors, rather than bare silicon substrates for thin chalcogenide deposition, to improve the sensitivity of GIXRF/XRR analysis to small process-driven modifications. The aim of such multilayered substrates is to maintain high values of X-ray reflected intensity even at angles significantly higher than the critical angle, therefore generating an XSW-induced enhancement of the XRF signal not only at the surface but also in the depth of the layer of interest. The simulation of GIXRF-XRR data, collected at SOLEIL Metrology beamline and in the lab on Rigaku SmartLab diffractometer, on arsenic-free Ovonic Threshold Switch materials (Ge, Se, Sb) for advanced Phase Change memory (PCRAM) applications illustrates the interest of this approach, which is fully-compatible with numbers of PCRAM materials elaborated with low-temperature physical vapor deposition process.

Interaction of nanoparticle properties and X-ray analytical techniques

In this work, Pt–Ti core–shell nanoparticles were characterized using reference-free X-ray fluorescence analysis and used for the investigation of the modification of the X-Ray Standing Wave (XSW) field intensity with increasing NP surface coverage.

Exploring interface morphology of a deeply buried layer in periodic multilayer

Long-term durability of a thin film device is strongly correlated with the nature of interface structure associated between different constituent layers. Synthetic periodic multilayer structures are primarily employed as artificial X-ray Bragg reflectors in many applications, and their reflection efficiency is predominantly dictated by the nature of the buried interfaces between the different layers. Herein, we demonstrate the applicability of the combined analysis approach of the X-ray reflectivity and grazing incidence X-ray fluorescence measurements for the reliable and precise determination of a buried interface structure inside periodic X-ray multilayer structures. X-ray standing wave field (XSW) generated under Bragg reflection condition is used to probe the different constituent layers of the W- B4C multilayer structure at 10 keV and 12 keV incident X-ray energies. Our results show that the XSW assisted fluorescence measurements are markedly sensitive to the location and interface morphology of a buried layer structure inside a periodic multilayer structure. The cross sectional transmission electron microscopy results obtained on the W-B4C multilayer structure provide a deeper look on the overall reliability and accuracy of the XSW method. The method described here would also be applicable for nondestructive characterization of a wide range of thin film based semiconductor and optical devices.

X-ray standing wave study of Si clusters on a decagonal Al-Co-Ni quasicrystal surface

Quantitative adsorption structure determinations on quasicrystals are scarce because most techniques for measuring surface structures are not well suited to the complex and infinite unit cells of quasicrystals. The normal incidence standing x-ray wave field technique presents a solution to these problems because it can be made inherently surface sensitive and does not involve extensive computational effort. We describe a method for applying this technique to adsorbates on quasicrystals, with specific application to a submonolayer of Si atoms on a decagonal Al-Co-Ni surface. We demonstrate the sensitivity of the technique to both adsorption site and geometry, leading to the conclusion that the Si atoms, which form six-atom pentagonal clusters, have an average height of $1.77\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}\AA{}$ above pentagonal hollow sites, with a significant height variation among the Si atoms in the cluster. In particular, the central Si atom sits more deeply than the five surrounding Si atoms, which are, on average, $2.7\phantom{\rule{0.16em}{0ex}}\AA{}$ away from the central Si atom. Although this study was performed on a decagonal quasicrystal that is periodic perpendicular to the surface, we describe how the technique can be applied to cases with no periodicity.

Quantitative Adsorbate Structure Determination for Quasicrystals Using X-Ray Standing Waves

The quantitative structure determination of adsorbed species on quasicrystal surfaces has so far appeared to present insurmountable problems. The normal incidence standing x-ray wave field technique offers a simple solution, without extensive data sets or large computations. Its application to quasicrystals raises several conceptual difficulties that are related to the phase problem in x-ray diffraction. We demonstrate their solution for the case of Si atoms adsorbed on the decagonal Co-rich modification of the Al-Co-Ni quasicrystal to determine the local structure, comprising 6-atom clusters in particular hollow sites.

Characterization of High-k Nanolayers by Grazing Incidence X-ray Spectrometry

The accurate characterization of nanolayered systems is an essential topic for today’s developments in many fields of material research. Thin high-k layers and gate stacks are technologically required for the design of current and future electronic devices and can be deposited, e.g., by Atomic Layer Deposition (ALD). However, the metrological challenges to characterize such systems demand further development of analytical techniques. Reference-free Grazing Incidence X-ray Fluorescence (GIXRF) based on synchrotron radiation can significantly contribute to the characterization of such nanolayered systems. GIXRF takes advantage of the incident angle dependence of XRF, in particular below the substrate’s critical angle where changes in the X-ray Standing Wave field (XSW) intensity influence the angular intensity profile. The reliable modeling of the XSW in conjunction with the radiometrically calibrated instrumentation at the PTB allows for reference-free, fundamental parameter-based quantitative analysis. This approach is very well suited for the characterization of nanoscaled materials, especially when no reference samples with sufficient quality are available. The capabilities of this method are demonstrated by means of two systems for transistor gate stacks, i.e., Al2O3 high-k layers grown on Si or Si/SiO2 and Sc2O3 layers on InGaAs/InP substrates.

Reference-free quantification of particle-like surface contaminations by grazing incidence X-ray fluorescence analysis

The analysis of the elemental composition of aerosol particles by non-destructive grazing incidence X-ray fluorescence analysis (GIXRF) is possible if the particles are deposited on a flat substrate. If those particles exhibit surface areas parallel to the substrate surface, under certain experimental conditions, total reflection of incident X-rays might arise also at those sites thereby preventing X-rays from penetrating the particles. For a reliable quantitative analysis, this effect and the interaction with the X-ray standing wave field (XSW) has to be further investigated in detail. To study the effects occurring when nanoscaled objects are probed with GIXRF, artificial nanostructures of known size, shape and composition have been manufactured on flat silicon wafer surfaces, with the intention to simulate deposited nanoscaled aerosol particles. A reference-free quantification of the deposited mass was performed employing a simple model for the propagation of the XSW through the sample material. Depending on the quality of the manufactured structures, good agreement between nominal masses and measured values could be stated. Only moderate agreement was found for samples that were more difficult to manufacture. GIXRF measurements yield information on the physical dimensions of the structures which are well in line with results obtained by a combination of scanning electron microscopy and energy-dispersive X-ray spectrometry (SEM/EDX). The presented quantification model, which is based on existing software for XSW calculations, can be transferred to environmental nanoparticles sampled directly from the aerosol phase. All measurements were performed in the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at BESSY II using well-characterized monochromatic synchrotron radiation and calibrated instrumentation.

X-ray standing waves in a multi-trilayer system with linearly varying period

Generation of X-ray standing waves in a multi-trilayer system with a varying periodicity along its depth is described. Microstructures of a synthetic 15-period Pt/Ni/C multilayer, which has such a varying periodicity, are investigated using X-ray reflectivity (XRR) and X-ray standing wave (XSW) analysis. Microstructural parameters, e.g., thicknesses and electron densities of individual layers, interface roughnesses, varying periodicity along the depth, etc. are precisely determined by the XRR technique. The variation of period is essentially introduced in multilayer fabrication by varying the thickness of the C-layer as a function of depth. The linear increase in C-layer thickness from the bottom of the multilayer towards the surface of the multilayer structure is found by simulating the XRR data with a depth-graded multilayer model. Intensity of the XSW field is computed as a function of depth over the first order Bragg peak. Integrated intensities over the Pt- and the Ni-layers are computed and compared with the measured fluorescence yield from Pt and Ni. Sensitivity for the detection of inter-atomic diffusion across the interfaces in comparison to multi-bilayer systems, such as Pt/C multilayer, is discussed.

Detection of the standing X-ray wavefield intensity inside a thin crystal using back-diffraction topography and imaging

The standing X-ray wavefield into a single-crystal bulk is characterized by a combination of the diffracted–reflected h-beams and the diffracted–transmitted o-beam. For different angular positions on the total reflection region, the standing X-ray wavefield has its maximum from the region between the atomic planes (low photoelectric absorption) to the region on the atomic planes (high photoelectric absorption). Historically, the evidence for such a characteristic has come from experiments such as anomalous transmission (Borrmann effect, originally detected in Laue geometry) and fluorescent measurements with a single crystal under diffraction conditions. In the present work, such a characteristic is demonstrated by the direct measurement of the standing X-ray wavefield intensity into a 50 µm-thick single-crystal CCD detector (Si 800) set in back-diffraction geometry.

Probing Patterned Wafer Structures by Means of Grazing Incidence X-ray Fluorescence Analysis

The reliable characterization of both nanoscaled structures and nanoparticles deposited on semiconductor surfaces using reference-free total-reflection X-ray fluorescence needs an accurate knowledge on the influence of surface structures on X-ray standing wave field (XSW). Artificial structures (pads) of known composition with identical lateral dimensions and different heights in the 10 to 100 nm range were prepared. Their influence on the angular dependence of the XSW has been investigated employing monochromatized synchrotron radiation in the laboratory of the Physikalisch-Technische Bundesanstalt at BESSY II.

Probing the average size of self-assembled metal nanoparticles using x-ray standing waves

We report application of x-ray standing-wave field for characterization of average vertical size of metal nanoparticles dispersed on a substrate surface with accuracies better than 1 nm. This method is applied to analyze the distribution of Fe, Co, and Au nanoparticles on Si substrate and W/C multilayer substrates. Results demonstrate that the method is a valuable tool to evaluate the surface morphology of nanoparticles over a large surface area. Unlike conventional probes such as atomic force microscopy or microinterferometry, the present method provides element-specific analysis. It also has the advantage of being able to examine nanoparticles in a liquid medium, or buried inside a coating layer. We anticipate that the proposed method has great potential to infer the internal structure of nanoparticles.

Depth-resolved soft x-ray photoelectron emission microscopy in nanostructures via standing-wave excited photoemission

We extend conventional laterally-resolved soft x-ray photoelectron emission microscopy to provide depth resolution along the surface normal down to a few angstroms. This is achieved by using standing-wave excitation. The sample is a Ag/Co/Au trilayer, grown on a Si/MoSi2 multilayer mirror, with the bottom Ag layer in a wedge profile. Tuning the incident x-ray to the mirror Bragg angle sets up a standing x-ray wave field in the multilayer and the trilayer wedge structure. We demonstrate the resulting depth resolution by imaging the standing-wave fields as they move through the trilayer wedge structure.

Investigation of metal nanoparticles on a Si surface using an x-ray standing wave field

An x-ray standing wave field generated under total external reflection condition is used to characterize the average vertical dimension of metal nanoparticles as well as their nature of dispersion on a flat surface. This approach is applied to characterize the distribution of Fe nanoparticles deposited on a silicon surface using a solution dip method. The atomic force microscopy results on these nanoparticles reinforce our interpretation. The authors believe that the present method has a strong utility in characterizing, over a large area, the morphology of the surfaces coated with nanoparticles. The method also provides element specific analysis for the nanoparticulate matter.

Probing Atomic Migration in Nanostructured Multilayers: Application of X-Ray Standing Wave Fields

X-ray standing wave fields, excited in periodic nanostructured multilayers during Bragg diffraction, have been used to probe atomic migration in multilayers. Ion beam induced migration of Fe impurity atoms from the C layers to the Pt layers in a Pt(Fe)/C(Fe) multilayer, where each layer is about 2 nm thick, has been detected. With a depth resolution better than 0.2 nm of this technique, the direction of Fe migration (here outward) and the change of Fe concentration in C (also Pt) layers have been determined. The results of such measurements are important for understanding the properties of multilayers, for example, the evolution of ferromagnetism in the present example [Dev, Microelectron. Eng. 83, 1721 (2006)10.1016/j.mee.2006.01.230].

Non-dipole effects in X-ray standing wave photoelectron spectroscopy experiments

The influence of different multipole terms to the angular resolved photoelectron yield excited by an X-ray standing wave (XSW) field which is generated by two coherent waves is discussed. The contributions of dipole and quadrupole terms are analyzed for a distribution of atoms within the interference field. General expression for the photoelectron yield in the XSW field is discussed in details. It is shown how non-dipole contributions can be obtained experimentally. A comparison between theoretical and experimental results is presented.

Application of X-ray standing wave (XSW) technique for studies of Zn incorporation in InP epilayers

We present a new approach to determine the site incorporation of p-type impurities in the doped layers of microelectronic device structures based on X-ray standing wave (XSW) technique. Activation behavior of Zn in InP epitaxial layers grown by metal organic vapor phase epitaxy (MOVPE) on InP(1 0 0) substrates has been studied. The XSW experiments were performed at the A2 beamline of the Cornell high-energy synchrotron source (CHESS). Angular dependences of the Zn–K fluorescence intensity excited by the XSW field inside the InP layer for symmetrical (4 0 0) reflection have been measured along with the Indium L-edge and phosphorus K-edge fluorescence and X-ray reflectivity. Analysis of the XSW data based on the dynamical diffraction theory in layered crystal structures allowed us to determine the fractions of both Zn atoms incorporated into crystal lattice and interstitial Zn. In our example, a 1 μm thick MOCVD-grown InP layer with the nominal concentration of Zn atoms of (2.4 ± 0.2) × 10 18 cm −3 has the fraction of the substitutional Zn of (65 ± 5)%. This result is in a good agreement with electrical activation of Zn measured with a combination of SIMS and CV profilometry. The accurate knowledge of the interstitial-to-substitutional ratio as a function of growth conditions is required to optimize electrical activation of Zn and to control its diffusion in the device structures.

Characterization of amorphous-Si/1ML-Ge/Si(001) Interface Structure by X-ray Standing Waves

The interface structure of an amorphous-Si/1ML-Ge/Si(001) crystal was investigated by the X-ray standing wave (XSW) method. The sample was prepared by the molocular beam epitaxy (MBE) deposition of 1ML of Ge on a clean Si(001) surface followed by the deposition of an amorphous Si (a-Si) capping layer of 50 Å thickness. The intensity of the reflected X-rays and the yield of the fluorescent X-rays of GeKα were measured around the 111 Bragg point. The fluorescence yield curve, which indicates the interaction between the XSW field and Ge atoms, showed that most of the Ge atoms stay at a position very close to the ideal Si site. This result provides effective feedback for fabricating SixGe1-x/Si devices.

GaN polarity domains spatially resolved by x-ray standing wave microscopy

The conventional x-ray standing wave (XSW) technique [1, 2] is a phase sensitive and element specific Fourier technique, which is commonly used to analyse single crystals on the millimetre to centimetre scale. Here, we present an advanced microprobe technique based on the XSW method demonstrating that structural analysis can be achieved with chemical sensitivity on a microscopic scale. We apply the XSW microscopy technique to study the crystallographic polarity from inversion domains in GaN-based lateral polarity heterostructures. We focus the x-ray beam by a refractive lens [3] into a micrometre slice and generate the XSW field by Bragg-reflection from the (0002) diffraction planes recording the GaKα fluorescence as a function of the incidence angle. In this first demonstration of microscopic polarity determination with x-rays, we analyse the reversion of polarity across a inversion domain boundary with a spatial resolution of 1.5 μm. The new micro-XSW technique will permit microscopic examinations of the crystalline structure of modern semiconductor devices with chemical sensitivity and structural resolution on the picometre scale.