Bragg Fresnel Research Articles

Development of Field Alignment Methods for Electron-Вeam Lithography in the Case of X-Ray Bragg–Fresnel Lenses

A method providing more than a tenfold improvement in the precision of exposure field alignment in electron-beam lithography is proposed. The method uses a scanning electron microscope with a standard mechanical positioning system and alignment markers produced by contact photolithography. Bragg–Fresnel lens (1–3–5 orders) with a tenfold decrease in the error of exposure field alignment through the use of mechanical stages are created via this technique.

Investigation by coherent X-ray section topography of ferroelectric domain behaviour as a function of temperature in periodically poled Rb:KTP

The behaviour of ferroelectric domains at high temperatures near the Curie temperature in a periodically poled rubidium-doped potassium titanyl phosphate crystal (Rb:KTP) has been studied by Bragg–Fresnel X-ray diffraction imagingin situusing a compact coherence-preserving furnace. The development and partial disappearance of the inverted domain structure as the temperature increases has been successfully modelled, and is explained by invoking a built-in electric field produced under heating in a low vacuum by out-diffusion of atoms from the sample.

Sharp-focusing Bragg–Fresnel zone plate with Laue diffraction geometry

The impact of decreased zone height on the focal properties of hard X-ray Bragg-Fresnel zone plates has been studied by numerical simulation. Decreased zone height allows for smaller zone widths and, although the efficiency of the lens is decreased, the signal-to-background ratio in the focal plane of the lens remains at a comparatively high level. This is distinct from an analogous case of ordinary phase zone plates.

Nanometer focusing of X-rays with modified reflection zone plates

Transmission Fresnel zone plates and x-ray waveguides are presently the best high-resolution optical elements used in x-ray microscopy and micro-probing. The spatial resolution of zone plates is defined not only by the technological limit of about 10 nm but also by the effects of volume diffraction, even for high-order zone plates. The physical nature of a waveguide limits the theoretical spatial resolution to 10 nm, corresponding to the minimum thickness of film able to fulfil the conditions necessary for optical mode formation. Here, we show that reflection zone plates and Bragg–Fresnel lenses can overcame the resolution limit of transmission zone plates and waveguides and make it possible to achieve spatial resolution into the sub-nanometer range, subject to source size limitations. Such optics can be used with existing third-generation synchrotron radiation sources, as well as with x-ray laser sources, e.g., free-electron lasers.

Double focusing of hard x rays using combined multilayer and Bragg–Fresnel optics

A multilayer Bragg–Fresnel x-ray lens (MBFL) structure that combines diffraction and geometric focusing on a single device is presented. With the MBFL, the linear BFL focuses the x-ray beam vertically while the bent multilayer focuses x rays horizontally. This combination eliminates the need for a second mirror in the conventional Kirkpatrick–Baez-based microprobe, which will be advantageous in x-ray fluorescence microscopy as well as microdiffraction applications. The characterization of the MBFL was carried out at the European Synchrotron Radiation Facility and a 1.6 μm (vertical)×12 μm (horizontal) focus was obtained from a 250 μm (V)×200 μm (H) incident beam. With a binary Fresnel lens efficiency of about 35% this yields a gain of almost three orders of magnitude compared to obtaining the same spot size by a raw aperture.

Metal layer Bragg–Fresnel lenses for diffraction focusing of hard x-rays

A thin-film Bragg–Fresnel lens (BFL) was developed for diffractive focusing of hard x-rays into submicron to nanometer spots for scanning x-ray spectromicroscopy. The lens is made of metal-layer Fresnel zones deposited on an x-ray reflective substrate. The use of a high-density lens structure reduces the thickness of the lens and simplifies the fabrication process. Linear and elliptical lenses made of a 200-nm-thick Au film were fabricated using e-beam lithography and a metal deposition process. The focusing capabilities of the Au layer BFLs were demonstrated at the Advanced Photon Source.

Structural matching of ferroelectric domains and associated distortion inpotassium titanyl phosphate crystals

The surface deformation and atomic-level distortions associated withcrystal structural matching at ferroelectric inversion domain wallsare investigated in periodically poled potassium titanyl phosphate(KTP) crystals. A deformation, of the order of 10−8 min scale and having the periodicity of the domains, is observed at the surfaces byoptical interferometry. It is discussed in terms of the piezoelectric effect. Thematching of the crystal structures at the domain walls is studied by combining thehard x-ray Fresnel phase-imaging technique with Bragg diffraction imagingmethods (‘Bragg–Fresnel imaging’) and using synchrotron radiation. Quantitativeanalysis of the contrast of the Bragg–Fresnel images recorded as a function of thepropagation distance is demonstrated to allow the determination of how thedomains are matched at the atomic (unit cell) level, even though the spatialresolution of the images is on the scale of micrometres. The atom P(1) isdetermined as the linking atom for connecting the inversion domains across thewall in KTP crystals for domain walls forced by electric-field processing to beparallel to (100). In addition to this, it is shown that a shift of 1/2(a + b)between atoms in the original and inverted structures is introduced as a result ofthe domain inversion operation.

Bent Bragg–Fresnel lenses for x-ray imaging diagnostics

Bent Bragg–Fresnel lenses were tested under conditions relevant to the NRL Nike Laser monochromatic backlighting experiments. This hybrid two-dimensional (2D) x-ray imaging diagnostics incorporates high resolution and 1D focusing of the linear Fresnel structure as well as monochromatic and 1D focusing nature of cylindrically bent Bragg crystals. Lenses made of silicon (111) and quartz (101_1 and 101_0) with focal length of 125 mm were tested on our x-ray test bench. First images of backlit targets were obtained. Experimental issues, possible applications and future developments including extension of these diagnostics to the harder x rays (>10 keV), are discussed.

Synchrotron Microbeam Diffraction Study of the Microstructure and the Chemical Composition in a Monocrystalline Ni-Base Turbine Blade After a Thermomechanical Mission Test

High-performance turbine blades composed of monocrystalline Ni-base γ/γ′ superalloys are often protected by coatings to resist the high-temperature corrosive atmosphere environment applications. During applications severe thermal and mechanical loads cause drastic and complicated changes in the microstructure and the chemical composition, also combined with strong lateral gradients of both temperature and stress. Microbeam X-ray diffraction experiments with a spatial resolution of about 5 μm enabled the determination of both the microstructural and the chemical composition gradients within material volumes of the sizes of the order of about 1 mm3 or smaller. The high angular and spatial resolution was achieved by a special focusing technique using a Bragg–Fresnel focusing monochromator at the BM5 beamline of the ESRF synchrotron at Grenoble. The X-ray diffraction experiments were complemented and supported by electron microscopy investigations of the microstructure and the chemical composition.

Diffractive x-ray optics using production fabrication methods

Zone plates are the key focusing element for many x-ray (7–20 keV) and soft x-ray (200–500 eV) applications, yet, production with electron-beam lithography poses obstacles to their widespread availability. In addition, fabrication processes to date have limited the studies of amplitude Bragg–Fresnel-type elements in the hard x-ray regime. We report new processes that couple 100 keV electron-beam lithography with established production methods to achieve two goals: (1) improving the overall yield and volume of ultrahigh-resolution soft x-ray zone plates and (2) applying deep silicon etching techniques to extend the state of the art in high aspect ratio Bragg–Fresnel optics required to create high efficiency focusing of high-energy x rays.

Source size and emittance monitoring on BESSY II

Three optical monitors image the transverse cross section of the electron beam at BESSY II. The emittance of the electron beam can be determined from source images generated by hard X-ray pinhole cameras without additional measurement of the energy width of the beam. Vertical emittance changes down to 3 pmrad can be detected. Typical emittance values of 5<ε x <6 nmrad horizontal and of ε y< 1 0 0 pmrad vertical are provided in user runs with up to 300 mA ring current. These results agree with higher resolution measurements of the Bragg–Fresnel telescope. Measurements of the source size versus ring current as well as the influence of feedbacks and the relation between emittance coupling and the properties of undulator radiation are given.

Characterizing the hard x-ray diffraction properties of a GaAs linear Bragg–Fresnel lens

We investigated the diffractive focusing properties of (111) GaAs linear Bragg–Fresnel lenses (BFLs) developed for hard x-ray microscopy and microdiffraction of complex materials in confined geometries. We demonstrated that the use of GaAs yields significant processing advantages due to the reduced zone depth. Focal plane diffraction patterns of linear BFLs measured at the advanced photon source using 8–40 keV x rays were compared to a simple model based on Kirchhoff–Fresnel diffraction theory. Good agreement was obtained between experimental data and model calculations using only zones within an effective aperture defined by the transverse coherence of the source.

High-efficiency Bragg–Fresnel lenses with 100 nm outermost zone width

In this article, we describe the manufacture and characterization of linear silicon Bragg–Fresnel lenses (BFLs) with outermost zone widths of 100 nm. This is, to our knowledge, a factor of 3 smaller than any other device of this kind reported so far. Furthermore, we give the first detailed description of an accurate and robust method to measure the diffraction efficiency of BFLs. The first-order diffraction efficiency of our lenses at 13.25 keV photon energy was 26%±0.5%. The combination of wide aperture angle, high efficiency, and large size of our devices results in a significant gain in achievable x-ray flux.

High-efficiency multilevel zone plates for keV X-rays

The development of high brilliance X-ray sources coupled with advances in manufacturing technologies has led to significant improvements in submicrometre probes for spectroscopy, diffraction and imaging applications. The generation of a small beam spot size is commonly based on three principles1: total reflection (as used in optical elements involving mirrors or capillaries), refraction (such as in refractive lenses2) and diffraction. The latter effect is employed in Bragg–Fresnel or Soret lenses, commonly known as Fresnel zone plate lenses. These lenses currently give the best spatial resolution, but are traditionally limited to rather soft X-rays—at high energies, their use is still limited by their efficiency. Here we report the fabrication of high-efficiency, high-contrast gold and nickel multistep (quaternary) Fresnel zone plates using electron beam lithography. We achieve a maximum efficiency of 55% for the nickel plate at 7 keV. In addition to their high efficiency, the lenses offer the advantages of low background signal and effective reduction of unwanted diffraction orders. We anticipate that these lenses should have a significant impact on techniques such as microscopy3, micro-fluorescence4 and micro-diffraction5, which require medium resolution (500–100 nm) and high flux at fixed energies.

X-ray diagnostic applied to the study of shock wave propagation in foams

A new radiographic technique has been studied in order to analyze the continuous evolution of shock wave propagation in low density foams. The technique is based on a Bragg–Fresnel multilayer lens. Preliminary static results, obtained by imaging a grid of suitable size, are presented.

2-D X-ray laser-plasma imaging using Bragg Fresnel multilayer zone plates

Two types of Bragg Fresnel multilayer zone plates operating in the keV range have been investigated for X-ray laser-plasma imaging. Elliptical on-axis and off-axis Bragg Fresnel multilayer zone plates were characterised. With the on-axis zone plate all the reflection orders are superposed in the same plane. The image contrast is then degraded as the zone plate focus distance is not the same for all orders. The problem is avoided with an off-axis zone plate with which the different orders in the image plane are spatially separated. The Al He α strong emission line (7.757 Å) of a laser produced aluminium plasma has been selected for the imaging tests. The first order reflected intensity has been optimized by varying the Bragg and azimuthal angles. As a result, an X-ray image of a fine gold grid backlighted by the aluminium plasma emission has been obtained with a spatial resolution very close to the theoretical limit of the imaging mounting.

Imaging of laser produced plasma at 1.43 keV using Fresnel zone plate and Bragg–Fresnel lens

X-ray imaging of plasmas with a resolution on the order of 1 μm could not be achieved with pinholes because the light flux on the detector would be too low. We tested two different types of diffractive lenses derived from the circular grating based on the Fresnel zones. Compared to pinholes, they can have an equivalent diameter of about 100 μm with a resolution of about 1 μm. The two kinds of devices tested were: (1) a transmission phase Fresnel zone lens (PFZL) associated with a multilayer mirror; (2) a reflective Bragg–Fresnel lens (BFL) which combines a multilayer mirror and the grating. The PFZL works at normal incidence by transmission; an additional mirror is used to reflect only a small bandwidth within the spectrum; the angle of reflection of the multilayer of the imaging beam on the mirror is set as to adjust the center of the useful bandwidth. The BFL works at fixed grazing incidence and we only use an off-axis part of the BFL in order to avoid the illumination of the detector by zeroth order diffracted light. We computed the spatial response of both devices and found that the aberrations were very low within an object field of 100 μm for the BFL device and 1 mm for the PFZL device. Their theoretical resolution is given by the width of their last zone: below 0.1 μm for the BFL device and 0.4 μm for the PFZL device. We also made images of plasma with both devices and using back-lighted grids, we obtained a resolution on the order of 4–5 μm. In the discussion, we try to explain the difference between the theoretical and experimental results.

A linear Bragg–Fresnel optics fabricated for 1D focusing of 18 nm X-ray radiation

The theoretical and technique results of a 1D focusing linear Bragg–Fresnel optics working at 18 nm X-ray radiation are given in the present paper. A theoretical method describing its optical performances is introduced briefly, the parameters of the diffraction pattern are presented. The preparation of multilayer mirrors, the decision of X-ray absorber layer thickness and the fabrication process of the diffraction pattern are described. The studied results of thermal and chemical stabilities for Mo/Si multilayer structure are shown. Some measured results for the 1D focusing linear Bragg–Fresnel optics are given.

Bragg–Fresnel optics for hard x-ray microscopy: Development of fabrication process and x-ray characterization at the Advanced Photon Source

Results are presented on development of processes for fabricating linear and circular Bragg–Fresnel lenses (BFLs) on Si and III–V compound material substrates, and on x-ray characterization of linear BFLs at the Advanced Photon Source (APS). Processes were developed for fabricating long (zone length &amp;gt;5 mm) linear BFLs on Si with enhanced capability for focusing high-energy x rays. By stitching together 20 sequentially exposed 400-μm-long linear BFLs, we were able to fabricate 8-mm-long linear BFLs with 0.5-μm finest zone width. BFLs were also fabricated on III–V compound semiconductor substrates GaAs and InP, with improved process control due to the substantially reduced zone thickness required (∼50% less than Si). Reduction of the zone aspect ratio (thickness/width) lessens the demand on the process technology, and may lead to higher lens resolution and pattern transfer accuracy. A process was explored to fabricate BFLs on a GaAs/AlGaAs heterostructure incorporating a built-in “etch stop” layer to ensure uniform zone thickness. Experimental characterization of the focusing properties of a field-stitched 8-mm-long linear BFL on Si (zone aperture=150 μm) was conducted at APS undulator beamline 1-ID-C using 10-keV x rays. Based on measured focal plane intensity distribution, the focus was estimated to be 1.2 μm, comparable to the geometrically demagnified source size of ∼1 μm. Lens efficiency was estimated to be ∼30%. Work is underway to incorporate the BFL-microprobe in x-ray microdiffraction and fluorescence microscopy experiments to study spatially confined complex materials.

Micro x-ray small-angle scattering with synchrotron radiation

Three small-angle scattering cameras based on mirror/monochromator optics, a circular Bragg-Fresnel (BF) lens, and a microcollimation system combined with a double-focusing mirror have been tested for microfocusing applications. In order to reach a focal spot less than or equal to 10 μm. a microcollimation system provides a flexible solution for medium-resolution applications. When using a glass capillary as a collimation system, a minimum s-value of 5·10−2 nm−1 was attained. Scanning small-angle x-ray scattering (SAXS) patterns from a poly(tetramethyl-p-silphenylene)-siloxane spherulite and a 40-μm polyethyl-eneterephthalate fiber were obtained in a few seconds per pattern with a 4 μm diameter beam at a wavelength of λ = 0.095 nm.