Michael J. Wojcik

Nanoscale 3D imaging at the Advanced Photon Source

Over the past decade, technology breakthroughs in the field of x-ray optics have enabled the development of advanced imaging nanoprobes at third-generation synchrotrons.1–11 X-rays have unique capabilities in terms of resolution, sensitivity, and speed, and by combining these properties with their ability to penetrate matter, these new instruments have played an important role in the recent advent of nano-material-related research.12 The gap— in terms of spatial resolution—between such x-ray instruments and electron microscopes, however, still needs to be reduced. In addition, it remains a challenge to offer in situ measurement capabilities while simultaneously pushing the spatial resolution limits. Conceptually, transmission x-ray microscopes (TXMs) are similar to optical visible light microscopes. In these instruments, tunable monochromatic x-rays illuminate the condenser—either an ellipsoidal glass mono-capillary or special type of diffraction grating known as a beam-shaping condenser (BSC)—and a Fresnel zone plate (FZP) is used as the objective lens to magnify the images or radiographs (see Figure 1). TXMs are also full-field imaging instruments, meaning that the snapshot images of absorption contrasts inside samples are acquired with 2D detectors (commonly four megapixel sensors). It is this type of full-field imaging—much faster than raster scan modes of pencil beam nanoprobes—which makes dynamic studies possible. To take on the challenge of nano-materials science in the fields of energy storage, microelectronics, nano-porous material functions, as well as life, Earth, and environmental sciences, we have developed a new in-house TXM at the Advanced Photon Source (sector 32-ID) of the Argonne National Laboratory. This instrument has replaced an older, first-generation commercial system,13 by providing a superior analytical imaging performance and in situ capabilities. In addition, our TXM supports a Figure 1. (a) Schematic representation of a transmission x-ray microscope (TXM) used for nano-tomography studies. (b) Photograph of the TXM that has been developed at sector 32-ID of the Argonne National Laboratory’s Advanced Photon Source. -CT: Micro-computed tomography.

Angular spectrum simulation of X-ray focusing by Fresnel zone plates

An efficient computing simulation routine has been implemented to model explicitly several types of Fresnel zone plate taking advantage of the circular symmetry. This code was used to evaluate an optimized approach for stacking of two high-resolution Fresnel zone plates.

Fresnel zone plate stacking in the intermediate field for high efficiency focusing in the hard X-ray regime

Focusing efficiency of Fresnel zone plates (FZPs) for X-rays depends on zone height, while the achievable spatial resolution depends on the width of the finest zones. FZPs with optimal efficiency and sub-100-nm spatial resolution require high aspect ratio structures which are difficult to fabricate with current technology especially for the hard X-ray regime. A possible solution is to stack several zone plates. To increase the number of FZPs within one stack, we first demonstrate intermediate-field stacking and apply this method by stacks of up to five FZPs with adjusted diameters. Approaching the respective optimum zone height, we maximized efficiencies for high resolution focusing at three different energies, 10, 11.8, and 25 keV.

Probing transverse coherence of x-ray beam with 2-D phase grating interferometer.

Transverse coherence of the x-ray beam from a bending magnet source was studied along multiple directions using a 2-D π/2 phase grating by measuring interferogram visibilities at different distances behind the grating. These measurements suggest that the preferred measuring orientation of a 2-D checkerboard grating is along the diagonal directions of the square blocks, where the interferograms have higher visibility and are not sensitive to the deviation of the duty cycle of the grating period. These observations are verified by thorough wavefront propagation simulations. The accuracy of the measured coherence values was also validated by the simulation and analytical results obtained from the source parameters. In addition, capability of the technique in probing spatially resolved local transverse coherence is demonstrated.

Circular grating interferometer for mapping transverse coherence area of X-ray beams

A circular grating interferometer was used to map the transverse coherence area of an X-ray beam. Due to the radial symmetry of the circular grating, coherence lengths along all transverse directions were obtained simultaneously by measuring the visibility decay of interferograms recorded at different distances behind a single circular π/2 phase grating. The technique is model-free and provides direct measurement of the complex coherence factor of the beam. The use of a circular grating also enables the unique capability of measuring the source shape profile. Sensitivity of this technique was demonstrated by detecting the small source tilt of a few degrees.

Efficiency of a multilayer-Laue-lens with a 102 μm aperture

A multilayer-Laue-lens (MLL) comprised of WSi2/Al layers stacked to a full thickness of 102 μm was characterized for its diffraction efficiency and dynamical diffraction properties by x-ray measurements made in the far field. The achieved aperture roughly doubles the previous maximum reported aperture for an MLL, thereby doubling the working distance. Negative and positive first orders were found to have 14.2% and 13.0% efficiencies, respectively. A section thickness of 9.6 μm was determined from Laue-case thickness fringes in the diffraction data. A background gas consisting of 90% Ar and 10% N2 was used for sputtering. This material system was chosen to reduce grown-in stress as the multilayer is deposited. Although some regions of the full MLL exhibited defects, the presently reported results were obtained for a region devoid of defects. The data compare well to dynamical diffraction calculations with Coupled Wave Theory (CWT) which provided confirmation of the optical constants and densities assumed for the CWT calculations.

Development and implementation of a portable grating interferometer system as a standard tool for testing optics at the Advanced Photon Source beamline 1-BM

We developed a portable X-ray grating interferometer setup as a standard tool for testing optics at the Advanced Photon Source (APS) beamline 1-BM. The interferometer can be operated in phase-stepping, Moiré, or single-grating harmonic imaging mode with 1-D or 2-D gratings. All of the interferometer motions are motorized; hence, it is much easier and quicker to switch between the different modes of operation. A novel aspect of this new instrument is its designed portability. While the setup is designed to be primarily used as a standard tool for testing optics at 1-BM, it could be potentially deployed at other APS beamlines for beam coherence and wavefront characterization or imaging. The design of the interferometer system is described in detail and coherence measurements obtained at the APS 34-ID-E beamline are presented. The coherence was probed in two directions using a 2-D checkerboard, a linear, and a circular grating at X-ray energies of 8 keV, 11 keV, and 18 keV.

Fabrication of hard x-ray zone plates with high aspect ratio using metal-assisted chemical etching

Fresnel zone plates are widely used as nanofocusing optics for x-ray microscopy, where the spatial resolution is set by the width of the finest rings while the efficiency is set by their thickness. This leads to the requirement for high aspect ratio nanofabrication. Metal-assisted chemical etching and atomic layer deposition has already been used to produce high aspect ratio zone plate structures on unthinned silicon wafers. The authors demonstrate here a substantial improvement on the achieved aspect ratio up to a value of 500:1, by producing 16 nm wide platinum zones with thicknesses up to 8 μm. At the same time, the silicon substrate was thinned to 15 μm as required for a practically useful optic. First tests have shown 4.8% diffraction efficiency using 20 keV x rays. This x-ray focusing efficiency is higher than most Fresnel zone plates for this photon energy and near what has been achieved with multilayer-coated Kirkpatrick–Baez mirrors, and multilayer Laue lenses. Hard x-ray zone plates offer the ad...

Development of single grating x-ray Talbot interferometer as a feedback loop sensor element of an adaptive x-ray mirror system

The initial result of using a single 2-D checkerboard phase-grating Talbot interferometer as a feed-back loop sensor element of an adaptive x-ray mirror system is reported. The test was performed by measuring the surface profile of a deformable Pt coated Silicon mirror at five different actuation states. The reflected beam was detected at the fractional Talbot distance of a π/2 phase grating. The measured interferograms were de-convolved using the spatial harmonic imaging technique to extract the phase and amplitude of the reflected wavefront. The wavefront was then propagated to the mirror center to retrieve the surface profile of the mirror. The activation of a single actuator was easily detected from the reconstructed surface profile of the mirror. The presented results indicate that the single phase-grating x-ray Talbot interferometer is capable of sensing nano-meter scale profile changes of an adaptive mirror.

X-ray optics testing beamline 1-BM at the advanced photon source

Beamline 1-BM at the APS has been reconfigured in part for testing of synchrotron optics with both monochromatic and white beams. Operational since 2013, it was reconfigured to accommodate users of the APS as well as users from other DOE facilities. Energies between 6 and 28 keV are available. The beamline was reconfigured to remove two large mirrors and to provide a 100 mm wide monochromatic beam at 54 m from the source. In addition a custom white beam shutter was implemented for topography exposures as short as 65 millisec over the full available horizontal width. Primary agendas include both white beam and monochromatic beam topography, Talbot grating interferometry, and tests of focusing optics. K-B mirrors, MLLs, and FZPs have been characterized. Measurements of the spatial coherence lengths on the beamline were obtained with Talbot interferometry. Topography data has been reported.