Multilayer X-ray Research Articles

Data quality in laboratory convergent-beam X-ray total scattering

Measurement of laboratory atomic pair distribution function data has improved with contemporary X-ray sources, optics and detectors, with acquisition times of the order of minutes for ideal samples. This paper examines resolution effects in pair distribution function data obtained using a convergent-beam configuration and an Ag X-ray tube from standard silicon powder and from 10 nm BaTiO3 nanocubes. The elliptical multilayer X-ray mirror reflects a non-trivial X-ray spectrum and introduces resolution effects not commonly treated in ordinary parafocusing divergent-beam laboratory diffraction. These resolution effects are modeled using the fundamental parameters approach, and the influence this has on interpretation and modeling of the resulting reduced atomic pair distribution function data is demonstrated.

Fast and efficient hard X-ray projection imaging below 10 nm resolution.

High-resolution X-ray imaging of noncrystalline objects is often achieved through the approach of scanning coherent diffractive imaging known as ptychography. The imaging resolution is usually limited by the scattering properties of the sample, where weak diffraction signals at the highest scattering angles compete with parasitic scattering. Here, we demonstrate that X-ray multilayer Laue lenses with a high numerical aperture (NA) can be used to create a strong reference beam that holographically boosts weak scattering from the sample over a large range of scattering angles, enabling high-resolution imaging that is tolerant of such background. An imaging resolution of sub-10 nm was achieved at a photon energy of 17.4 keV with lenses of 0.014 NA from a Siemens star test object and a sample of hierarchical nanoporous gold, recording projection holograms at an effective magnification of more than 30,000 directly on a pixel-array detector. A numerical study compared this approach to low-NA far-field ptychography, indicating significant advantages for using high-NA lenses in the presence of background noise. This imaging modality is particularly fast and efficient at recording high-resolution transmission phase-contrast images over large fields of view in a facile manner.

Hard X-ray HfO2/Al2O3 multilayers fabricated by atomic layer deposition

Multilayer is a widely used X-ray optics that can be designed and optimized for specific X-ray application. Atomic layer deposition (ALD) has unique advantage in coating homogeneous ultra-thin film on both flat and curved substrates, which shows great potential in the fabrication of X-ray multilayer with small period. In this paper, hard X-ray HfO2/Al2O3 multilayers on Si substrate prepared by ALD were investigated. The X-ray (0.154 nm) reflectivity of HfO2/Al2O3 multilayer with 3.44 nm period, 60 bilayers is 24 %. Atomic force microscope results show that surface roughness of HfO2/Al2O3 multilayer with total thickness more than 200 nm is slightly larger than that of the substrate. High resolution transmission electron microscope images reveal that amorphous HfO2/Al2O3 material pair forms multilayer with sharp interfaces even for 1.69 nm period. Further study indicates that lowering substrate roughness can reduce the deviation between actual and theoretical X-ray reflectivity. These results demonstrate that for multilayer-based hard X-ray optics, ALD-grown HfO2/Al2O3 is a promising candidate.

Emission of quasi-monochromatic ultrasoft X-ray of multilayer target in the Bragg direction under 5.7 MeV electron irradiation

Previously, it was experimentally shown that VUV radiation generated in a periodic structure of a multilayer X-ray mirror by relativistic electrons exhibits a quasi-monochromatic spectrum depending on the angle of inclination of the target and the direction of emission of radiation inside the radiation cone. Here, we present new experimental results regarding the properties of angular density of radiation. A comparison between the measured radiation intensity over a wide range of target inclination angles and theoretical calculations is provided.

Multistrip multilayer mirrors for sources based on inverse Compton scattering

A technique has been developed for calculating the periods of multi-strip multilayer mirrors for effective use with sources based on the inverse Compton effect. A model for constructing the multi-strip mirror is presented. The model has been developed in accordance with modern capabilities for creating multilayer X-ray mirrors. The reflection coefficients and fractions of the energy emitted by the source that are reflected are compared for total external reflection, for multilayer Goebel mirrors and for multi-strip mirrors. It is shown that the multi-strip design of multilayer mirrors proposed in the work is almost an order of magnitude superior to Goebel mirrors and to mirrors based on the total external reflection effect in terms of the efficiency of use of the source energy.

EUV Radiation in the Range of 10–20 nm from Liquid Spray Targets Containing O, Cl, Br and I Atoms under Pulsed Laser Excitation

The article describes the results of an investigation to determine the values of radiation intensities emitted by O-, Cl-, Br-, and I-containing liquid spray targets in absolute units in the wavelength range 10–20 nm when excited by pulsed laser radiation. The conversion coefficients of laser radiation into the EUV radiation are given for some wavelengths. The authors’ specially designed pulse extrusion liquid supply system was used to form the liquid spray targets. An Nd:YAG laser with λ = 1064 nm, τ = 8.4 ns, and Epulse = 0.8 J was used to excite the targets. Spectral measurements were made using a grazing incidence grating spectrometer–monochromator. The absolute intensities of a number of emission lines were also measured using a Bragg spectrometer based on a Mo/Be multilayer X-ray mirror, calibrated by both sensitivity and wavelength. The high values of absolute intensities of the liquid targets in the extreme ultraviolet wavelength range were demonstrated.

Design of a monocapillary with an inner Al2O3/HfO2 multilayer to obtain focused monochromatic hard X-rays

An X-ray monocapillary with an inner multilayer can be a promising optical device to obtain focused monochromatic X-rays. A focused beam is acquired via controlling the shape of the monocapillary meanwhile monochromatic X-rays are screened out by the inside multilayer. For hard X-rays such as Cu-kα line 8.04 keV and Mo-kα line 17.44 keV, Al2O3/HfO2 is an effective material pair for the X-ray multilayer that can reflect the X-rays at an acceptable efficiency. In this work, four tapered-monocapillaries with inner Al2O3/HfO2 multilayers are designed to focus and monochromatize X-rays (8.04 keV and17.44 keV, respectively) from the point source and collimated beam. The theoretical transmission performance, including the beam size, reflectivity, and monochromaticity of the device, is also calculated. The results show that the ideal optics can focus desired X-rays with efficiency of about 60%. It provides a reference for fabricating this optics in the future, especially via the atomic layer deposition (ALD) technique, which represents great potential to coat uniform film on a curved surface.

Spectrum of coherent VUV radiation generated by 5.7 MeV electrons in a multilayer X-ray mirror

The spectral and angular properties of diffracted transition radiation (DTR) and parametric radiation (PXR) in the ultrasoft X-ray region generated by the periodic structure upon interaction with a relativistic electron beam with energy of 5.7 MeV are numerically studied using a sample of the periodic structure [Mo/Si]50, known as a multilayer X-ray mirror. Based on calculations, an experimental approach is proposed to separate and identify the contributions of PXR and DTR. The ultrasoft X-ray radiation can be used to eliminate coherent effects occurring in the optical range when diagnosing the submicron electron beam size.

Reflective x-ray masks for x-ray lithography

Application of x-ray multilayers as reflective x-ray masks for x-ray lithography is proposed. The mask is a specially prepared multilayer mirror capable to selectively reflect x-rays. The use of grazing geometry allows a pattern design on the mask to be compressed in one direction. Application examples are given for the masks (WC/Si multilayers) with two types of a radiation source: an x-ray tube (λ = 0.154 nm) and a synchrotron (λ ∼ 0.35 nm). The compression of the mask segments by 14–33 times with the imprint size in the resist plane 3.5–4 μm is obtained. The advantages of the proposed masks are given. The possibilities of obtaining submicron imprints are discussed.

Investigation of structural and reflective characteristics of short-period Mo/B4C multilayer X-ray mirrors.

The results of a study of the structural and reflective characteristics of short-period multilayer X-ray mirrors based on Mo/B4C at wavelengths 1.54 Å, 9.89 Å and 17.59 Å are presented. The period of the samples varied in the range 8-35 Å. The average widths of the interfaces were ∼3.5 and 2.2 Å at one and the other boundaries, with a tendency for weak growth with any decrease in the period. The interlayer roughness was ∼1 Å. The research results indicate promising prospects for the use of multilayer Mo/B4C mirrors for synchrotron applications.

Study of the Effect of Si and Be Barrier Layers on Crystallization of Cr/Sc Multilayer X-ray Mirror

Study of the Effect of Si and Be Barrier Layers on Crystallization of Cr/Sc Multilayer X-ray Mirror

EFFECT OF ARGON WORKING GAS PRESSURE ON MECHANICAL STRESSES IN Mo/Si MULTILAYER X-RAY MIRRORS

By the method of X-ray tensometry (=0.154 nm), the dependences of residual stresses in Mo/Si multilayer Xray mirrors (MXMs) obtained by direct-current magnetron sputtering were studied as a function of the sputtering Ar pressure in the range of 1…4 mTorr. It is shown that an increase in pressure is accompanied by an increase in stresses from ~ 0.4 to ~ 0.8 GPa for a group of mirrors with periods near 7 nm and from ~ 0.9 to ~ 1.6 GPa for a group of mirrors with periods near 14 nm. The angles of texture misorientation for Mo crystallites were measured as a function of argon pressure. Mechanisms have been proposed to explain the reasons for the increase in stresses.

Laboratory X-ray Microscopy of 3D Nanostructures in the Hard X-ray Regime Enabled by a Combination of Multilayer X-ray Optics.

High-resolution imaging of buried metal interconnect structures in advanced microelectronic products with full-field X-ray microscopy is demonstrated in the hard X-ray regime, i.e., at photon energies > 10 keV. The combination of two multilayer optics-a side-by-side Montel (or nested Kirkpatrick-Baez) condenser optic and a high aspect-ratio multilayer Laue lens-results in an asymmetric optical path in the transmission X-ray microscope. This optics arrangement allows the imaging of 3D nanostructures in opaque objects at a photon energy of 24.2 keV (In-Kα X-ray line). Using a Siemens star test pattern with a minimal feature size of 150 nm, it was proven that features < 150 nm can be resolved. In-Kα radiation is generated from a Ga-In alloy target using a laboratory X-ray source that employs the liquid-metal-jet technology. Since the penetration depth of X-rays into the samples is significantly larger compared to 8 keV photons used in state-of-the-art laboratory X-ray microscopes (Cu-Kα radiation), 3D-nanopattered materials and structures can be imaged nondestructively in mm to cm thick samples. This means that destructive de-processing, thinning or cross-sectioning of the samples are not needed for the visualization of interconnect structures in microelectronic products manufactured using advanced packaging technologies. The application of laboratory transmission X-ray microscopy in the hard X-ray regime is demonstrated for Cu/Cu6Sn5/Cu microbump interconnects fabricated using solid-liquid interdiffusion (SLID) bonding.

A modular table-top setup for ultrafast x-ray diffraction.

We present a table-top setup for femtosecond time-resolved x-ray diffraction based on a Cu Kα (8.05keV) laser driven plasma x-ray source. Due to its modular design, it provides high accessibility to its individual components (e.g., x-ray optics and sample environment). The Kα-yield of the source is optimized using a pre-pulse scheme. A magnifying multilayer x-ray mirror with Montel-Helios geometry is used to collect the emitted radiation, resulting in a quasi-collimated flux of more than 105 Cu Kα photons/pulse impinging on the sample under investigation at a repetition rate of 10Hz. A gas ionization chamber detector is placed right after the x-ray mirror and used for the normalization of the diffraction signals, enabling the measurement of relative signal changes of less than 1% even at the given low repetition rate. Time-resolved diffraction experiments on laser-excited epitaxial Bi films serve as an example to demonstrate the capabilities of the setup. The setup can also be used for Debye-Scherrer type measurements on poly-crystalline samples.

Numerical analysis of X-ray multilayer Fresnel zone plates with high aspect ratios

Abstract A partition calculation method (PCM) for calculating the diffraction efficiency of multilayer Fresnel zone plate with high aspect ratio is proposed. In contrast to the traditional theory, PCM designs and evaluates Fresnel zone plate (FZP) considering material pairs, all zone widths, thicknesses and X-ray energy more completely. The results obtained through PCM are validated by comparing them with the complex amplitude superposition theory and coupled wave theory numerical results. The PCM satisfies the requirements of the theoretical investigation of FZP with small outermost zone width (drN) and large thickness (t). Combining proper numerical analysis with the experimental conditions will present a great potential to break through the imaging performance of X-ray microscopy.

Preparation of X-Ray Multilayers Based on Atomic Layer Deposition

Preparation of X-Ray Multilayers Based on Atomic Layer Deposition

Self-aligned multi-layer X-ray absorption grating using large-area fabrication methods for X-ray phase-contrast imaging

X-ray phase-contrast (XPCi) imaging methods are an emerging medical imaging approach that provide significantly better soft tissue contrast and could function as a viable extension to conventional X-ray, CT, and even some MRI. Absorption gratings play a central role in grating-based XPCi systems, especially because they enable the acquisition of three images in a single exposure: transmission, refraction, and dark-field. An impediment to commercial development and adoption of XPCi imaging systems is the lack of large area, high aspect ratio absorption gratings. Grating technology development, primarily due to technological limitations, has lagged system development and today prevents the scaling up of XPCi system into a footprint and price point acceptable to the medical market. In this work, we report on a self-aligned multi-layer grating fabrication process that can enable large-area X-ray absorption gratings with micron-scale feature sizes. We leverage large-area fabrication techniques commonly employed by the thin-film transistor (TFT) display industry. Conventional ITO-on-glass substrates are used with a patterned film of Cr/Au/Cr that serves as a self-aligned lithography mask for backside exposure. Commonly available SU-8 photoresist is patterned using the backside exposure mask followed by an electroplating step to fill the gaps in the SU-8 with X-ray attenuating material. Consequently, the electroplated patterned material acts as a self-aligned photomask for subsequent SU-8 layer patterning and so forth. The repeatability of the reported process makes it suitable for achieving higher aspect ratio structures and is advantageous over previously reported X-ray LIGA approaches. A prototype three-layer grating, with a thickness of around 40,upmu text{m}, having a visibility of 0.28 at 60,text{kV}_p with a 70,text{mm}times 70,text{mm} active area was fabricated on a 4-inch glass substrate and demonstrated by modifying a commercially available 3D propagation-based XPCi Microscope. The scalable and cost-effective approach to build larger area X-ray gratings reported in this work can help expedite the commercial development and adoption of previously reported Talbot-Lau, speckle-tracking, as well as coded-aperture XPCi systems for large-area clinical and industrial applications.

Исследование свойств многослойных зеркал на основе пары материалов Mo/B-=SUB=-4-=/SUB=-C

The properties of Mo/B4C multilayer X-ray mirrors with periods of 3.74 – 3.84 nm are studied in this work. The dependences of the transmission band, reflection coefficient and the magnitude of internal stresses in mirrors on the ratio of material thicknesses in the period were investigated. The results of a study of the effect of thermal annealing on the structural parameters and reflective characteristics of mirrors are also presented in the paper.

Эмиссионные характеристики лазерно-плазменного источника экстремального ультрафиолетового излучения с тонкопленочными мишенями

The radiation spectra in the soft X-ray and extreme ultraviolet wavelength ranges of thin-film (0.15 microns thick) targets made of light materials (Si, C, Be) were studied when excited by a Nd:YAG laser pulse with a duration of 5.2 ns focused to an intensity of ~10^12 W/cm^2. Line spectra of BeIII, BeIV, CV, and SiV ions were recorded using a spectrometer based on a multilayer X-ray mirror. A comparison with the spectra of bulk solid-state targets of the same materials is carried out. In all cases, there was a decrease in the intensity of the lines of the soft X-ray spectrum of film targets compared to monolithic ones; the decrease was, depending on the material, from several tens of percent to several times, with more than an order of magnitude less mass of the vaporized substance.

Микроструктура и остаточные напряжения многослойных покрытий ZrN/CrN, полученных плазменно-ассистированным вакуумно-дуговым методом

Introduction. The current state of the art in the field of hard coatings application requires the formation of nanostructured compositions using different chemical elements. Modern hard coatings are able to combine different properties such as high hardness, wear resistance, corrosion resistance. At present, coatings formed by layer-by-layer deposition of zirconium and chromium nitrides are promising. When depositing combinations of chemical elements on various substrates, studies are required aimed at investigating its microstructure and, mainly, residual stresses formed during the deposition of multilayer coatings. The purpose of this work is to investigate the structural-phase state and residual stresses of ZrN/CrN system coatings formed by plasma-assisted vacuum-arc method from the gas phase. Research methods. Samples with coatings of zirconium and chromium nitrides deposited on substrates of hard alloy VК8 are investigated. Transmission electron microscopy is used to study the microstructural characteristics of multilayered coatings and X-ray diffraction analysis is used to quantify macroscopic stresses. Results and discussion. Based on the experimental results obtained it is found that changing the modes of deposition of multilayer ZrN/CrN coatings with regard to rotation speeds of table and substrate holder leads to variations in microstructure, morphology and internal stresses of surface layers of multilayer coatings. It is shown that by changing conditions for the multilayer coating deposition the possibilities of forming ZrN/CrN coatings on the substrate made of VK8 alloy with nanoscale thickness of coating layers open up. X-ray diffraction analysis indicates mainly insignificant stresses, and at high table and substrate rotation speeds – high compressive stresses in the multilayer coating. Transmission electron microscopy revealed that CrN and ZrN coatings have a common multilayer coating growth texture at low rotation speeds, and at high speeds a textural misorientation of the phases of the coating layers is observed. Based on the results obtained it is possible to recommend coatings of ZrN/CrN system as hard coatings.