Here we report on the effort to develop a hard x-ray grating interferometry technique for application to hard x-ray optics
and wavefront characterization at the Advanced Photon Source (APS), Argonne National Laboratory, USA. We will
mention the motivation for developing an x-ray interferometer at the APS and discuss the design of the interferometer.
We will also describe the efforts in fabricating 2-D gratings and a new type of grating having nanometer periods for
high-energy x-ray applications. The preliminary results obtained using x-ray Talbot interferometers built at APS, using a
broadband (pink) beam and a monochromatic beam demonstrate the importance of this tool as a metrology instrument
for optics and beam wavefront diagnostics.
Kirkpatrick-Baez (KB) mirrors consist of two individual mirrors: one vertical focusing mirror and one horizontal
mirror at separate positions. Nested (Montel) KB mirrors consist of two mirrors arranged perpendicularly to each
other and side-by-side. We report our results from the fabrication and tests of the first set of nested KB mirrors for a
synchrotron hard x-ray micro/nano-focusing system. The elliptically shaped nested Platinum KB mirrors include
two 40 mm long mirrors fabricated by depositing Platinum on Silicon substrates using the magnetron sputtering
technique. Hard x-ray synchrotron tests have been performed at 15 keV and 2D focal spots of approximately 150 nm
x 150 nm (FWHM) were achieved from both monochromatic and polychromatic beams at the 34 ID beamline of the
Advanced Photon Source (APS) at Argonne National Laboratory. The side-by-side arrangement of nested KB
mirrors requires them to have good surfaces and low figure errors at the intersection of the two mirrors' surfaces. It
is very challenging to fabricate substrates that fit the nested KB mirror's arrangement and to deposit thin films to
ideal elliptical shapes at the edge of the mirrors. Further research and development will be performed in the areas of
fabrication and testing with respect to nested KB mirrors used in micro/nano-focusing systems. In particular,
substrate processing and deposition techniques should be examined to improve the performance of the mirrors.
X-ray imaging techniques based on grating interferometers rely on transmission gratings to detect x-ray refraction and
scattering in a sample. Gratings periods below 2 microns are challenging to realize due to the high aspect ratio of the
structures. We propose a method to fabricate transmission gratings with sub-micron periods over centimeter areas by
multilayer coating of a staircase (echelle) substrate. The advantage of this approach is the high aspect ratio of multilayer
coating and the large area of the echelle substrate. The staircase pattern is etched on the surface of a silicon wafer
through anisotropic etching. Multiple layers are deposited on the horizontal surfaces of the stairs by magnetron
sputtering in a single run. The layers alternate between two materials of different absorption coefficients or refractive
indices. The layer thickness d is designed to be (stair height)/2N, where 2N is the total number of layers. The incident xray
beam is parallel to the layers and oblique to the wafer surface. Each stair of the echelle substrate forms a micro
grating of period 2d, and the array of micro gratings together act as a single grating over a large area given the right
continuity conditions. The grating period potentially can be below 100 nm. We present theoretical description of wave
diffraction by the grating array, and results of the first fabrication test with magnetron sputtering deposition.
Fizeau interferometer is the most commonly used interferometer for testing optical components. The aim of this work is
to apply this technique to the measurement of elliptical Kirkpatrick-Baez (KB) mirrors during their fabrication process.
KB mirrors are widely used at synchrotron radiation facilities around the world for x-ray focusing. Fizeau interferometer
can provide accurate measurements for KB mirrors. Recently a KB mirror that can focus X-ray down to 150 nm has
been fabricated in the Argonne National Laboratory.
We report a successful fabrication and testing of the first set of Platinum (Pt)-coated Kirkpatrik-Baez (KB) mirrors for a
submicrofocusing x-ray polychromatic beam from a conventional beamline (64 m long) at the 34-ID of Advanced
Photon Source (APS). The set includes one 80 mm long mirror and one 40 mm short mirror fabricated by depositing Pt
on finely polished spherical Silicon (Si) substrates using the APS-developed profile coating technique with the
magnetron sputtering system. Profile coating masks were calculated through the coating profile data from metrology
measurements acquired using interferometric stitching technique. Instead of flat substrates, spherical substrates (with
shapes approximately mimicking the tangential profiles of the desired ellipses) were used, reducing the coating thickness
and, thus, stress. The mirror pair was commissioned on the beamline and generated a 2-D spot with full width at half
maximum (FWHM) 280 nm (V) x 150 nm (H). The detailed fabrication methods, metrology measurements, and
calculations are discussed.
Fabrication and evaluation of elliptical X-ray mirrors, such as Kirkpatrick-Baez (K-B) mirrors
produced by the profile-coating technique, requires accurate surface figure measurements over a wide range of
spatial frequencies. Microstitching interferometry has proven to fulfill this requirement for length scales from a
few μm up to the full mirror length. At the Advanced Photon Source, a state-of-the-art microroughness
microscope interferometer that incorporates advanced microstitching capability has been used to obtain
measurements of profile-coated elliptical K-B mirrors. The stitched surface height data provide previously
unattainable resolution and reproducibility, which has facilitated the fabrication of ultrasmooth (< 1 nm rms
residual height) profile-coated mirrors, whose hard X-ray focusing performance is expected to approach the
diffraction limit. This paper describes the system capabilities and limitations. Results of measurements obtained
with it will be discussed and compared with those obtained with the Long Trace Profiler.
We report our progress in the growth of periodic and depth-graded multilayers in the APS rotary deposition system, a
machine designed for fabrication of films tens of microns thick with thousands of layers. A computational method was
employed to design depth-graded multilayers for use as wide-angular bandpass reflective optics. We present
experimental results for a 154-layer WSi2/Si multilayer system with bilayer thickness ranging from 2.2 nm to 5.5 nm that
closely match theoretical flat-top reflectivity predictions of 9.8% from 15.6 mrad to 23.3 mrad at 8 keV.
A Multilayer Laue Lens (MLL) is a new type of linear zone plate, made by sectioning a planar depth-graded multilayer and used in Laue transmission diffraction geometry, for nanometer-scale focusing of hard x-rays. To produce an MLL, a depth-graded multilayer consisting of thousands of layers with a total thickness of tens of microns is needed. Additionally, the multilayer wafer has to be sectioned and polished to a thickness of ~10 to 25 microns to yield a diffracting grating to focus x-rays. The multilayers must have both low stress and good adhesion to survive the subsequent cutting and polishing processes, as well as sharp interfaces and accurate layer placement.
Several partial MLLs using WSi2/Si multilayers with precise zone-plate structures have been successfully fabricated. A W/Si multilayer with the same structure, however, cracked and peeled off from the Si substrate after it was grown. Here we report results of our film stress studies of dc magnetron-sputtered WSi2, W, and Mo thin films and WSi2/Si, W/Si, and Mo/Si multilayers grown on Si(100) substrates. The stress measurements were carried out using a stylus profiler to measure the curvatures of 2-inch-diameter, 0.5-mm-thick Si(100) wafers before and after each coating. The physical origins of the stress and material properties of these systems will be discussed.
Using Fresnel zone plates, a spatial resolution between 20 nm for soft x-rays and 70 nm for hard x-rays has been achieved. Improvement of the spatial resolution without loss of efficiency is difficult and incremental due to the fabrication challenges posed by the combination of small outermost zone width and high aspect ratios. We describe a novel approach for high-resolution x-ray focusing, a multilayer Laue lens (MLL). The MLL concept is a system of two crossed linear zone plates, manufactured by deposition techniques. The approach involves deposition of a multilayer with a graded period, sectioning it to the appropriate thickness, assembling the sections at the optimum angle, and using it in Laue geometry for focusing. The approach is particularly well suited for high-resolution focusing optics for use at high photon energy. We present a theory of the MLL using dynamic diffraction theory and Fourier optics.
Zone plates with depth to zone-width ratios as large as 100 are needed for focusing of hard x-rays. Such high aspect ratios are challenging to produce by lithography. We are investigating the fabrication of high-aspect-ratio linear zone plates by multilayer deposition followed by sectioning. As an initial step in this work, we present a synchrotron x-ray study of constant-period multilayers diffracting in Laue (transmission) geometry. Data are presented from two samples: a 200 period W/Si multilayer with d-spacing of 29 nm, and a 2020 period Mo/Si multilayer with d-spacing of 7 nm. By cutting and polishing we have successfully produced thin cross sections with section depths ranging from 2 to 12 μm. Transverse scattering profiles (rocking curves) across the Bragg reflection exhibit well-defined interference fringes originating from the depth of the sample, in agreement with dynamical diffraction theory for a multilayer in Laue geometry.
To develop narrow-bandpass multilayer monochromators, we have studied small d-spacing WSi2/Si multilayers. We found that WSi2/Si is an excellent multilayer system for achieving both the desired spectral resolution and peak reflectivity. Compared to other traditional multilayer systems such as W/Si, WSi2/Si not only has a lower density and lower absorption, but also is a chemically more stable system, since WSi2 is already a silicide. One thus expects better thermal stability and sharper interfaces for WSi2/Si multilayers. There are two approaches to achieve high-resolution multilayers: either decrease the d spacing or use low absorption materials. By using WSi2/Si, we can utilize both approaches in the same system to achieve good energy resolution and peak reflectivity. Another advantage of this system is that the sputtering rate for Si is much higher than other traditional low-Z materials. Several WSi2/Si multilayers have been fabricated at the Advanced Photon Source (APS) deposition lab using dc magnetron sputtering with constant currents of 0.5 A in Ar at a pressure of 2.3 mTorr. A test sample of [9.65Å-WSi2/10.05Å-Si] × 300 was studied at four institutions: using laboratory x-ray diffractometers with Cu Kα (8.048 keV) wavelength at the APS x-ray lab and at European Synchrotron Radiation Facility (ESRF), and using synchrotron undulator x-rays at 10 keV at MHATT-CAT and at 25 keV at ChemMatCARS-CAT of the APS. The measured first-order reflectivity was 54% with a bandpass of 0.46% at 10 keV and 66% reflectivity with a bandpass of 0.67% at 25 keV of undulator x-rays. Similar results were obtained from Cu Kα x-rays. This result is very attractive for the design of a multilayer monochromator for the ChemMatCARS-CAT to be used in the 20 to 25 keV range. Other small d-spacing multilayers are being studied. Comparison between WSi2/Si and W/Si multilayers will be discussed.
We describe x-ray Kirkpatrick-Baez mirror designs with the potential to produce hard x-ray beams of 40 nm or smaller. The x-ray quality mirrors required to achieve the desired performance can be fabricated by differential deposition on ultra-smooth surfaces, or by differential polishing. Various mirror systems designed for nanofocusing to ~40 nm and below are compared. The performance limits of total-external-reflection mirrors are compared with the limits of multilayer mirrors that can potentially focus to an even smaller spot size. The advantages of side-by-side Kirkpatrick-Baez mirrors are evaluated and more advanced, four-mirror systems with significantly greater geometrical demagnification are discussed. These systems can potentially reach 5 - 20 nm focal spot sizes for multilayer and total-external-reflection optics respectively.
Synchrotron bending magnet radiation at the Advanced Photon Source was used to measure x-ray diffuse scattering of tungsten/carbon multilayers having period of 28 Å. Scattering not only near the first Bragg sheet in reciprocal space, but also near sheets corresponding to Kiessig fringes was simulated in the Born approximation. Full roughness propagation starting with the substrate was used in the simulation. We conclude that the differential equation that describes the kinetic roughening during sputtering is second order, i.e., the Langevin equation. For thermal-diffusion-related kinetic roughening either a third-or fourth-order equation should apply, and we conclude that thermal mechanisms for kinetic roughening need not be invoked
For microfocusing x-ray mirrors, an ellipse shape is desirable for aberration-free optics. However, it is difficult to polish elliptical mirrors to x-ray quality smoothness. A differential coating method to convert a cylindrical mirror to an elliptical one has been previously reported The differential coating was obtained by varying the sputter source power while the mirror was passed through. Here we report a new method of profile coating to achieve the same goal more effectively. In the profile coating, the sputter source power is kept constant, while the substrate is passed over a contoured mask at a constant speed. The mask is placed very close to the substrate level (within 1.0 mm) on a shield-can over the sputter gun. Four-inch-diameter Si wafers were coated through a 100-mm-long by 152-mm-wide aperture on the top of the shield-can. The thickness distribution was then obtained using a spectroscopic ellipsometer with computer-controlled X-Y translation stages. A model has been developed to fit the measured thickness distribution of stationary growth. The relative thickness weightings are then digitized at every point 1 mm apart for the entire open area of the aperture. When the substrate is moving across the shield-can during a deposition, the film thickness is directly proportional to the length of the opening on the can along the moving direction. By equating the summation of relative weighting to the required relative thickness at the same position, the length of the opening at that position can be determined. By repeating the same process for the whole length of the required profile, a contour can be obtained for a desired thickness profile. The contoured mask is then placed on the opening of the shield-can. The number of passes and the moving speed of the substrate are determined according to the required thickness and the growth-rate calibration. The mirror coating profile is determined from the ideal surface figure of a focus ellipse and that obtained from a long trace profiler on the substrate. Preliminary test results using Au as a coating material are presented.
We are developing freestanding high-aspect-ratio, focused, two-dimensional antiscatter grids for mammography using deep x-ray lithography and copper electroforming. The exposure is performed using x-rays from bending magnet beamline 2-BM at the Advanced Photon Source (APS) of Argonne National Laboratory. A 2.8-mm-thick prototype freestanding copper antiscatter grid with 25μm-wide parallel cell walls and 550 μm periodicity has been fabricated. The progress in developing a dynamic double-exposure technique to create the grid with the cell walls aligned to a point x-ray source of the mammography system is discussed.
A double crystal, multilayer monochromator was designed and fabricated for a wiggler beamline at the Cornell High Energy Synchrotron Source (CHESS). The monochromator consists of an internally water-cooled first substrate and a fixed-radius sagittally focusing second substrate, each coated with a multilayer consisting of 100 bilayers of Tungsten/Carbon with a 27 angstrom d-spacing. The wide energy bandpass of this multilayer along with sagittal focusing provides the best available flux for time resolved experiments. A flux 100 times that of conventional silicon monochromators is possible and allows for a finer time resolution for the crystal growth studies on this beamline. For other experimental uses, the higher intensity allows for more rapid data collection.
Previously we reported the ability to lift off, via electroforming, multilayers deposited on gold coated flat mandrels. We had shown the multilayers remained intact after separation as the X-ray reflectivity produced peaks in the reflectivity versus energy measurements. Since gold is high a Z material and is not easy to remove via chemical etching, we have now developed the ability to use a low-Z material that is also etchable. Moreover, we improved the quality of the smoothing with the process of amorphous carbon nitride (CNx) deposition on electroless nickel in both our test chamber and our much larger fabrication chamber. Our method of mandrel preparation is to first deposit CNx on electroless nickel, then a low-Z release layer, and then the multilayers. We have demonstrated that flat mandrels produced in this manner can be cooled to liquid nitrogen temperatures without harm. This is important as the usual practice is to shrink mandrels that have Wolter I shape via liquid nitrogen. We have coated a truncated-cone-shaped 'engineering' (not high quality in terms of smoothness) mandrel and have removed the layers, intact, on the inside of an electroform with a cylindrical, truncated- cone geometry. We report the details of the fabrication, engineering, and X-ray tests.
We describe a tunable multilayer monochromator with an adjustable bandpass to be used for reflectivity and grazing incidence diffraction studies on surfaces at energies near 10 keV. Multilayers have a bandpass typically 100 times larger than the Si(111) reflection, and by using multilayers an experimenter can significantly increase data collection rates over those available with a Si monochromator. The transmission through 1 and 2 laterally graded multilayer (LGML) reflections was recorded versus photon energy. The identical LGMLs were comprised of 60 bilayers of W and C on 100 X 25 X 3 mm float glass with a bilayer spacing varying from 35 to 60 angstrom. The average gradient was 0.27 angstrom/mm along the long dimension. The rms deviation of the data for the bilayer spacing from a linear fit was 0.36 angstrom. Data were obtained for a nondispersive (plus or minus) double-multilayer arrangement. The relative bandpass width (FWHM) when the two multilayers exposed the same bilayer spacing was measured to be 2.2% with a transmission of 78.7 plus or minus 1.6%. This value is consistent with the transmission of 88.9% that we also measured for a single LGML at HASYLAB beamline D4. The bandpass was tunable in the range 1.1% to 2.2%.
A set of internally water-cooled multilayers were fabricated for the double crystal monochromator of the A2 wiggler beamline at CHESS. With over 3 kW total deposited on the first multilayer, conventional contact cooled multilayers proved unable to cope with the incident heat load. Cooled silicon substrates were fabricated with internal water cooling channels to reduce or eliminate thermal distortion. The highly polished top surfaces were coated with a sputtered W/C or W/B4C multilayer film deposited at the Advanced Photon Source and Osmic, Inc. respectively. Measured reflectivities over 60% and bandwidths of 1.3 - 1.8% were obtained. Results to beam currents of 250 mA show the effectiveness of the internal cooling design and have provided x-ray fluxes of 1 X 1013 photons/sec/mm2.
We discuss the possibilities for using Kirkpatrick-Baez (K-B) multilayer elements to directly image the fluorescence distribution from a specimen under x-ray illumination. X-ray fluorescence would be collected by K-B elements close to the specimen, with a magnified image formed at an area detector about 0.5 m away from the source. This is in contrast to the use of K-B optics to form a microprobe beam that is rastered across a surface while the x-ray fluorescence is recorded to form a fluorescent image. Tungsten-carbon multilayers on curved substrates have been fabricated at the Advanced Photon Source, and their focusing properties have been characterized by a laboratory x-ray source. Synchrotron applications would illuminate specimens with lithographic patterns, for example, with the image being formed at an x-ray CCD camera. The ability to form an x-ray fluorescent image, and therefore a map of the specimen's elemental distribution near the surface, could provide a useful analytical tool without the usual need for a microfocusing beam. Furthermore, there are interesting possibilities offered by combining fluorescence imaging with x-ray near edge absorption spectroscopy (XANES), extended x- ray absorption spectroscopy (EXAFS), and x-ray standing waves.
X-ray scattering measurements at 10 keV from multilayers having a period of 24.8 Angstrom and consisting of 100 W/C bilayers are reported. Specular scans revealed first order reflectivities in the range 73.5% to 78.0% with bandpasses in the range of 1.5% to 1.7%. Total roughness (or interface grading) values deduced from fitting were in the range 2.5 to 3.0 angstrom for the last-to-grow surface of the W layers. Diffuse scattering measurements were made in a novel geometry that permitted investigation of in-plane momentum transfers up to 0.2 Angstrom-1. This is roughly an order of magnitude larger than is possible in conventional rocking scans. A power law dependence of the diffuse scattering after integration over a 'Brillioun zone' is found. The exponent of this power law, 1.75, when interpreted using a logarithmic correlation function leads to a value of 1.0 angstrom for the correlated roughness.
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