X-ray Grating Research Articles

X-ray optics for the cavity-based X-ray free-electron laser.

A cavity-based X-ray free-electron laser (CBXFEL) is a possible future direction in the development of fully coherent X-ray sources. CBXFELs consist of a low-emittance electron source, a magnet system with several undulators and chicanes, and an X-ray cavity. The X-ray cavity stores and circulates X-ray pulses for repeated FEL interactions with electron pulses until the FEL reaches saturation. CBXFEL cavities require low-loss wavefront-preserving optical components: near-100%-reflectivity X-ray diamond Bragg-reflecting crystals, outcoupling devices such as thin diamond membranes or X-ray gratings, and aberration-free focusing elements. In the framework of the collaborative CBXFEL research and development project of Argonne National Laboratory, SLAC National Accelerator Laboratory and SPring-8, we report here the design, manufacturing and characterization of X-ray optical components for the CBXFEL cavity, which include high-reflectivity diamond crystal mirrors, a diamond drumhead crystal with thin membranes, beryllium refractive lenses and channel-cut Si monochromators. All the designed optical components have been fully characterized at the Advanced Photon Source to demonstrate their suitability for the CBXFEL cavity application.

Development of printed X-ray grating and its application to an imaging system

The short wavelength of X-rays makes them attractive for high-resolution imaging systems in fields ranging from medicine to industry. The thin-film grating required for X-ray imaging can be created using printing, which is a cost-effective method for large-area imaging. Since the grating resolution (line/space, or L/S) influences the imaging resolution, it is crucial to enhance the L/S through printing methods. In this study, thin-film gratings were fabricated from silver nanoparticles using letterpress inversion printing, achieving a maximum L/S grating resolution of 1 μm/1 μm (the highest resolution achieved through printing). Subsequently, the cross-section of a pine needle was successfully imaged.

Stabilizing high aspect ratio x-ray gratings with top layer resist grid

Stabilizing high aspect ratio x-ray gratings with top layer resist grid

(Invited) Bottom-up Gold Filling for Manufacture of X-Ray Gratings: Mechanism and Implications

Over the past four years a process for extreme superconformal deposition of gold has been demonstrated and explored for void-free filling of recessed features on patterned substrates for application in X-ray imaging. Uniform and complete void-free filling was detailed on 100 mm silicon wafers patterned with micrometer pitch vias of aspect ratio (height/diameter) 23 and trenches of aspect ratio (height/width) exceeding 60 as well as trenches more than 300 μm deep and fractal patterns of spatially varying depth. This talk discusses a proposed mechanism and model for the observed behavior. Simulations capture key features of the experimental processes, including not only the bottom-up filling itself but also an extended incubation period of conformal deposition that precedes its start as well as self-passivation of the active deposition that accompanies the completion of filling. Feature filling results with relevant electrochemistry are discussed in the context of predicted behavior for different deposition conditions. Cross-sectioned 60 μm deep trenches Au filled at fixed potentials, the associated current transients, a Au filled grating and a simulation of filling in 50 μm deep trenches are all shown in the figure. Figure 1

Spectral Modeling of the Supersoft X-Ray Source CAL87 Based on Radiative Transfer Codes

Supersoft X-ray sources (SSSs) are white dwarf (WD) binaries that radiate almost entirely below ∼1 keV. Their X-ray spectra are often complex when viewed with the X-ray grating spectrometers, where numerous emission and absorption features are intermingled and hard to separate. The absorption features are mostly from the WD atmosphere, for which radiative transfer models have been constructed. The emission features are from the corona surrounding the WD atmosphere, in which incident emission from the WD surface is reprocessed. Modeling the corona requires different solvers and assumptions for the radiative transfer, which has yet to be achieved. We chose CAL87, an SSS in the Large Magellanic Cloud, which exhibits emission-dominated spectra from the corona, as the WD atmosphere emission is assumed to be completely blocked by the accretion disk. We constructed a radiative transfer model for the corona using two radiative transfer codes: xstar for a one-dimensional two-stream solver and MONACO for a three-dimensional Monte Carlo solver. We identified their differences and limitations in comparison to the spectra taken with the Reflection Grating Spectrometer on board the XMM-Newton satellite. We finally obtained a sufficiently good spectral model of CAL87 based on the radiative transfer of the corona plus an additional collisionally ionized plasma. In the coming X-ray microcalorimeter era, it will be required to interpret spectra based on radiative transfer in a wider range of sources than what is presented here.

Nanometer flat blazed x-ray gratings using ion beam figure correction.

With the development of nanometer accuracy stitching interferometry, ion beam figuring (IBF) of x-ray mirrors can now be achieved with unprecedented performance. However, the process of producing x-ray diffraction gratings on these surfaces may degrade the figure quality due to process errors introduced during the ruling of the grating grooves. To address this challenge, we have investigated the post-production correction of gratings using IBF, where stitching interferometry is used to provide in-process feedback. A concern with ion beam correction in this case is that ions will induce enough surface mobility of atoms to cause smoothing of the grating structure and degradation of diffraction efficiency. In this study we found however that it is possible to achieve a nanometer-level planarity of the global grating surface with IBF, while preserving the grating structure. The preservation was so good, that we could not detect a change in the diffraction efficiency after ion beam correction. This is of major importance in achieving ultra-high spectral resolution, and the preservation of brightness for coherent x-ray beams.

Advanced low blaze angle x-ray gratings via nanoimprint replication and plasma etch.

We developed a new method of making ultra-low blaze angle diffraction gratings for x-ray applications. The method is based on reduction of the blaze angle of a master grating by nanoimprint replication followed by a plasma etch. A master blazed grating with a relatively large blaze angle is fabricated by anisotropic wet etching of a Si single crystal substrate. The surface of the master grating is replicated by a polymer material on top of a quartz substrate by nanoimprinting. Then a second nanoimprinting is performed using the 1st replica as a mold to replicate the saw-tooth surface into a resist layer on top of a Si grating substrate. A reactive ion etch is used to transfer the grating grooves into the Si substrate. The plasma etch provides reduction of the groove depth by a factor defined by the ratio of the etch rates for the resist and Si. We demonstrate reduction of the blaze angle of a master grating by a factor of 5 during fabrication of a 200 lines/mm blazed grating with a blaze angle of 0.2°. We investigated the quality and performance of the fabricated low blaze angle gratings and evaluate process accuracy and reproducibility. The new blaze angle reduction method preserves the planarity of the optical surface of the grating substrate and at the same time provides improvement in the grating groove quality during the reduction process.

Evaluation of Effect of Tape Edges on Tensile Properties of Chopped Carbon Fiber Tape Reinforced Thermoplastics by X-ray Grating Interferometry

Evaluation of Effect of Tape Edges on Tensile Properties of Chopped Carbon Fiber Tape Reinforced Thermoplastics by X-ray Grating Interferometry

Multimodal Image Fusion for X-ray Grating Interferometry.

X-ray grating interferometry (XGI) can provide multiple image modalities. It does so by utilizing three different contrast mechanisms-attenuation, refraction (differential phase-shift), and scattering (dark-field)-in a single dataset. Combining all three imaging modalities could create new opportunities for the characterization of material structure features that conventional attenuation-based methods are unable probe. In this study, we proposed an image fusion scheme based on the non-subsampled contourlet transform and spiking cortical model (NSCT-SCM) to combine the tri-contrast images retrieved from XGI. It incorporated three main steps: (i) image denoising based on Wiener filtering, (ii) the NSCT-SCM tri-contrast fusion algorithm, and (iii) image enhancement using contrast-limited adaptive histogram equalization, adaptive sharpening, and gamma correction. The tri-contrast images of the frog toes were used to validate the proposed approach. Moreover, the proposed method was compared with three other image fusion methods by several figures of merit. The experimental evaluation results highlighted the efficiency and robustness of the proposed scheme, with less noise, higher contrast, more information, and better details.

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.

Highly Charged Ions for High-Resolution Soft X-ray Grating Monochromator Optimisation

The energy-resolving performance of a synchrotron radiation monochromator can be characterised by measuring the fluorescence response of a gas in scans across characteristic absorption lines. Here, we describe a method using exceptionally narrow absorption features in the soft x-ray range. The features belong to helium-like ions and examples of the transition 1s → 2p in O6+ and Ne8+ are shown. We describe the instrument PolarX-EBIT and show typical data. A performance with ten times sharper effective feature width, when compared to neutral-gas absorption features, is demonstrated.

Process design for the manufacturing of soft X-ray gratings in single-crystal diamond by high-energy heavy-ion irradiation

This paper describes in detail a novel manufacturing process for optical gratings suitable for use in the UV and soft X-ray regimes in a single-crystal diamond substrate based on highly focused swift heavy-ion irradiation. This type of grating is extensively used in light source facilities such as synchrotrons or free electron lasers, with ever-increasing demands in terms of thermal loads, depending on beamline operational parameters and architecture. The process proposed in this paper may be a future alternative to current manufacturing techniques, providing the advantage of being applicable to single-crystal diamond substrates, with their unique properties in terms of heat conductivity and radiation hardness. The paper summarizes the physical principle used for the grating patterns produced by swift heavy-ion irradiation and provides full details for the manufacturing process for a specific grating configuration, inspired in one of the beamlines at the ALBA synchrotron light source, while stressing the most challenging points for a potential implementation. Preliminary proof-of-concept experimental results are presented, showing the practical implementation of the methodology proposed herein.

Shocks in the Outflow of the RS Oph 2021 Eruption Observed with X-Ray Gratings

The 2021 outburst of the symbiotic recurrent nova RS Oph was observed with the Chandra High Energy Transmission Gratings (HETG) on day 18 after optical maximum and with XMM-Newton and its Reflection Grating Spectrographs (RGS) on day 21, before the supersoft X-ray source emerged and when the emission was due to shocked ejecta. The absorbed flux in the HETG 1.3–31 Å range was 2.6 × 10−10 erg cm−2 s−1, three orders of magnitude lower than the γ-ray flux measured on the same date. The spectra are well fitted with two components of thermal plasma in collisional ionization equilibrium, one at a temperature ≃ 0.75 keV and the other at a temperature in the 2.5–3.4 keV range. With the RGS we measured an average flux of 1.53 × 10−10 erg cm−2 s−1 in the 5–35 Å range, but the flux in the continuum and especially in the lines in the 23–35 Å range decreased during the 50 ks RGS exposure by almost 10%, indicating short-term variability on a timescale of hours. The RGS spectrum can be fitted with three thermal components, respectively at plasma temperatures between 70 and 150 eV, 0.64 keV, and 2.4 keV. The post-maximum epochs of the exposures fall between those of two grating spectra observed in the 2006 eruption on days 14 and 26: they are consistent with a similar spectral evolution, but in 2021 cooling seems to have been more rapid. Iron is depleted in the ejecta with respect to solar values, while nitrogen is enhanced.

Iterative signal retrieval for X-ray grating interferometry with dual-shot.

X-ray grating interferometry normally requires multiple steps and exposures, causing a prolonged imaging time. There is motivation to use fewer steps to reduce scanning time and complexity, while keeping fidelity of the retrieved signals. We propose an iterative signal retrieval method, extracting attenuation, dark field contrast (DFC), and differential phase contrast (DPC) signals from two X-ray exposures. Two shots were captured at G2 grating positions with difference of 1/4 grating period. The algorithm consists of two stages. At the first stage, amplitude of sample phase stepping curve retrieved by virtual phase stepping (VPS) method, visibility and local phase of background phase stepping curve are used to limit the results to the proximity of the ground truth. After the second stage, three high-quality parameters, amplitude, visibility, and local phase, are retrieved through finetuning, and three signals are calculated. Simulated and real-sample experiments were conducted to validate this method. We used standard phase stepping result as benchmark and calculated structural similarity (SSIM) and peak signal-to-noise ratio (PSNR) between benchmark and parameters retrieved by our dual-shot method and virtual phase stepping (VPS) method. For both simulated and real-sample experiments, the SSIM and PSNR value of dual-shot method are higher than those of VPS method. For real-sample method, we also conducted a three-step PS, and the SSIM and PSNR value of dual-shot method are slightly lower than those of three-step PS. Using our dual-shot method demonstrates higher performance than other single-shot method in retrieving high-quality signals, and it also reduces radiation dose and time.

Beamline simulations using monochromators with high d-spacing crystals.

Monochromators for synchrotron radiation beamlines typically use perfect crystals for the hard X-ray regime and gratings for soft X-rays. There is an intermediate range, typically 1-3 keV (tender X-rays), which common perfect crystals have difficulties covering and gratings have low efficiency, although some less common crystals with high d-spacing could be suitable. To evaluate the suitability of these crystals for a particular beamline, it is useful to evaluate the crystals' performance using tools such as ray-tracing. However, simulations for double-crystal monochromators are only available for the most used crystals such as Si, Ge or diamond. Here, an upgrade of the SHADOW ray-tracing code and complementary tools in the OASYS suite are presented to simulate high d-spacing crystals with arbitrary, and sometimes complex, structures such as beryl, YB66, muscovite, etc. Isotropic and anisotropic temperature factors are also considered. The YB66 crystal with 1936 atomic sites in the unit cell is simulated, and its applicability for tender X-ray monochromators is discussed in the context of new low-emittance storage rings.

Bottom-up Gold Filling for Manufacture of X-Ray Gratings: To What Limit?

Since its discovery three years ago, a process enabling extreme superconformal deposition of gold in recessed features has been advanced to achieve void-free filling of progressively more varied and challenging features and patterned substrates for application in X-ray imaging. During the past year in particular, void-free filling in vias, trenches with aspect ratio (height/width) exceeding 60, trenches more than 300 mm deep, fractal patterns of spatially varying depth and fully patterned 100 mm wafers for all the aforementioned were demonstrated for the first time. Examples are shown in the included figure. The process and mechanism differ substantially from processes for superconformal copper filling of nanometer size damascene on-chip trenches and vias as well as micrometer size through silicon vias and even larger microvias used to connect semiconductor chips in the microelectronics industry. The bottom-up deposition can be initiated using a wide range of processing conditions so that filling is robust even to process interruption including wafer removal and rinse. Because the bottom-up evolution can also be made to self-passivate at a chosen depth below feature openings well controlled fill profiles can be obtained across wafer scale substrates, even with features of variable width and depth. I present examples of the noted feature filling results with relevant electrochemistry and discuss aspects of the deposition processes and underlying mechanism. Figure 1

Cr/C multilayer growth on a heavy metal layer for upgrading of high efficiency tender x-ray gratings.

To increase efficiency of single layer gratings used in the tender x-ray range, a high reflectance multilayer can be directly grown on single layer gratings. Multilayer growth quality was studied by depositing the Cr/C multilayer on a Pt single layer using flat substrates. Their structure quality and adhesion were characterized by atomic force microscopy (AFM), grazing incidence x-ray reflectivity (GIXRR), x-ray scattering (XRS), x-ray diffraction (XRD), and layer adhesion measurement. AFM results showed that the surface roughness was 0.218 nm for the multilayer without the Pt layer and 0.272 nm for the multilayer with the Pt layer. As GIXRR results showed, the average interface widths were 0.39 nm for the multilayer without the Pt layer and 0.42 nm for the multilayer with the Pt layer. XRS results indicated that the existence of a Pt layer enlarged slightly the roughness of the multilayer. Simulation results exhibited that these slight changes caused by the Pt layer had an insignificant effect on reflectivity. As XRD results displayed, the crystallization of the Pt layer had negligible effects on the crystallization of Cr in films. The layer adhesion measurement revealed that the critical loads to peel off the layer from the substrate were 84.64 mN for the multilayer without the Pt layer and 33.99 mN for the multilayer with the Pt layer. After 6 months, the latter layer structure is undamaged, demonstrating that the coating is not easily peeled off. This study proves the feasibility to upgrade a low efficiency single Pt layer grating to a highly efficient multilayer grating.

Investigation on the mechanical interface stability of curved high aspect ratio x-ray gratings made by deep x-ray lithography

We describe the rationale for selecting graphite as a substrate material suitable for manufacturing curved high-aspect ratio metallic x-ray gratings and experimentally validate that its properties satisfy requirements relevant for clinical phase-contrast and dark-field x-ray imaging. Selection criteria applied to two candidate materials graphite and polyimide were compliance to bending, mechanical tenacity of the attachment of the lamellar grating structure to the substrate, the substrate material’s x-ray robustness, and the compatibility with the x-ray LIGA process used to manufacture the grating structures. In contrast to other standard materials such as silicon wafers with titanium layer, graphite wafers could be bent to smaller radii and are natively electrically conductive. While polyimide wafers allowed for even smaller bending radii, we found their high risk of grating structure detachment to be a strong detractor. Minimum achievable bending radii were 55 and 70 mm for pure graphite wafers and graphite wafers with mounted grating structure, respectively. Electron microscopy of graphite surface and cross-sections revealed a fine porous structure conducting to a very stable metal–wafer interface. Grating structures with heights of more than 200 μm were bonded to graphite wafers and their integrity confirmed in flat as well as in bent state using microfocus x-ray imaging. We conclude that graphite is a very well-suited substrate material for manufacturing curved x-ray gratings.

How Do Magnetic Field Models Affect Astrophysical Limits on Light Axion-like Particles? An X-Ray Case Study with NGC 1275

Axion-like particles (ALPs) are a well-motivated extension to the standard model of particle physics, and X-ray observations of cluster-hosted AGN currently place the most stringent constraints on the ALP coupling to electromagnetism, g a γ , for very light ALPs (m a ≲ 10−11 eV). We revisit limits obtained by Reynolds et al. using Chandra X-ray grating spectroscopy of NGC 1275, the central AGN in the Perseus cluster, examining the impact of the X-ray spectral model and magnetic field model. We also present a new publicly available code, ALPro, which we use to solve the ALP propagation problem. We discuss evidence for turbulent magnetic fields in Perseus and show that it can be important to resolve the magnetic field structure on scales below the coherence length. We reanalyze the NGC 1275 X-ray spectra using an improved data reduction and baseline spectral model. We find the limits are insensitive to whether a partially covering absorber is used in the fits. At low m a (m a ≲ 10−13 eV), we find marginally weaker limits on g a γ (by 0.1–0.3 dex) with different magnetic field models, compared to Model B from Reynolds et al. (2020). A Gaussian random field (GRF) model designed to mimic ∼50 kpc scale coherent structures also results in only slightly weaker limits. We conclude that the existing Model B limits are robust assuming that β pl ≈ 100, and are insensitive to whether cell-based or GRF methods are used. However, astrophysical uncertainties regarding the strength and structure of cluster magnetic fields persist, motivating high-sensitivity RM observations and tighter constraints on the radial profile of β pl.

Single-exposure X-ray phase imaging microscopy with a grating interferometer.

The advent of hard X-ray free-electron lasers enables nanoscopic X-ray imaging with sub-picosecond temporal resolution. X-ray grating interferometry offers a phase-sensitive full-field imaging technique where the phase retrieval can be carried out from a single exposure alone. Thus, the method is attractive for imaging applications at X-ray free-electron lasers where intrinsic pulse-to-pulse fluctuations pose a major challenge. In this work, the single-exposure phase imaging capabilities of grating interferometry are characterized by an implementation at the I13-1 beamline of Diamond Light Source (Oxfordshire, UK). For comparison purposes, propagation-based phase contrast imaging was also performed at the same instrument. The characterization is carried out in terms of the quantitativeness and the contrast-to-noise ratio of the phase reconstructions as well as via the achievable spatial resolution. By using a statistical image reconstruction scheme, previous limitations of grating interferometry regarding the spatial resolution can be mitigated as well as the experimental applicability of the technique.