Termination dependence of surface stacking at 4H-SiC (0001) -1×1: Density functional theory calculations

We study the effect of adsorbates on the relative stability of hexagonal and cubic stacking sequences at the topmost SiC bilayers of 4H-SiC (0001) -1×1 surfaces using first-principles calculations. We investigate F-terminated, OH-terminated, H-terminated, and clean surfaces, and in all cases, the cubic structure is more stable than the hexagonal structure. The energy difference between the two structures, however, significantly depends on adsorbates and is largest on the clean surface while it is smallest on the H-terminated surface. Stabilization of the cubic structure at F-terminated and OH-terminated surfaces is in contradiction to a simple argument based on the electrostatic interaction and we attribute it to orbital hybridization between occupied states of adsorbates and unoccupied states of the substrate surface. The present results suggest a possible means of controlling step bunching and the SiC stacking sequence by surface adsorbates. © 2009 The American Physical Society.Hideyuki Hara, Yoshitada Morikawa, Yasuhisa Sano, and Kazuto Yamauchi Phys. Rev. B 79, 153306 (2009)

Development of ion beam figuring system with electrostatic deflection for ultraprecise X-ray reflective optics

We developed an ion beam figuring system that utilizes electrostatic deflection. The system can produce an arbitrary shape by deterministically scanning the ion beam. The scan of the ion beam, which can be precisely controlled using only an electrical signal, enables us to avoid degradation of the mirror shape caused by imperfect acceleration or deceleration of a mechanically scanning stage. Additionally, this surface figuring method can easily be combined with X-ray metrology because the workpiece remains fixed during the figuring. We evaluated the figuring accuracy of the system by fabricating a plano-elliptical mirror for X-ray focusing. A mirror with a shape error of 1.4 nm root mean square (RMS) with a maximum removal depth of 992 nm, which corresponds to figuring accuracy of 0.14% RMS, was achieved. After the second shape corrections, an elliptical shape with a shape error of approximately 1 nm peak-to-valley, 0.48 nm RMS could be fabricated. Then, the mirror surface was smoothed by a low-energy ion beam. Consequently, a micro-roughness of 0.117 nm RMS, measured by atomic force microscopy, was achieved over an area of 1 × 1 μm2.Jumpei Yamada, Satoshi Matsuyama, Yasuhisa Sano, and Kazuto Yamauchi, "Development of ion beam figuring system with electrostatic deflection for ultraprecise X-ray reflective optics", Review of Scientific Instruments 86, 093103 (2015) https://doi.org/10.1063/1.4929323

Fabrication of ultrathin and highly uniform silicon on insulator by numerically controlled plasma chemical vaporization machining

Metal-oxide semiconductor field-effect transistors fabricated on a silicon-on-insulator (SOI) wafer operate faster and at a lower power than those fabricated on a bulk silicon wafer. Scaling down, which improves their performances, demands thinner SOI wafers. In this article, improvement on the thinning of SOI wafers by numerically controlled plasma chemical vaporization machining (PCVM) is described. PCVM is a gas-phase chemical etching method in which reactive species generated in atmospheric-pressure plasma are used. Some factors affecting uniformity are investigated and methods for improvements are presented. As a result of thinning a commercial 8 in. SOI wafer, the initial SOI layer thickness of 97.5±4.7 nm was successfully thinned and made uniform at 7.5±1.5 nm. © 2007 American Institute of Physics.Yasuhisa Sano, Kazuya Yamamura, Hidekazu Mimura, Kazuto Yamauchi, and Yuzo Mori, "Fabrication of ultrathin and highly uniform silicon on insulator by numerically controlled plasma chemical vaporization machining", Review of Scientific Instruments 78(8), 086102 (2007) https://doi.org/10.1063/1.2766836

Stitching interferometry for ellipsoidal x-ray mirrors

Ellipsoidal mirrors, which can efficiently produce a two-dimensional focusing beam with a single mirror, are superior x-ray focusing optics, especially when compared to elliptical-cylinder mirrors in the Kirkpatrick-Baez geometry. However, nano-focusing ellipsoidal mirrors are not commonly used for x-ray optics because achieving the accuracy required for the surface metrology of nano-focusing ellipsoidal mirrors is difficult due to their small radius of curvature along the short ellipsoidal axis. Here, we developed a surface metrology system for nano-focusing ellipsoidal mirrors using stitching interferometric techniques. The developed system simultaneously measures sub-aperture shapes with a microscopic interferometer and the tilt angles of the sub-aperture shapes with a large Fizeau interferometer. After correcting the systematic errors included in the sub-aperture shapes, the entire mirror shape is calculated by stitching the sub-aperture shapes based on the obtained relative angles between partially overlapped sub-apertures. In this study, we developed correction methods for systematic errors in sub-aperture shapes that originated from off-axis aberrations produced in the optics of the microscopic interferometer. The systematic errors on an ellipsoidal mirror were estimated by measuring a series of tilted plane substrates and the ellipsoidal substrate. From measurements of an ellipsoidal mirror with a 3.6-mm radius of curvature at the mirror center, we obtained a measurement repeatability of 0.51 nm (root-mean-square) in an assessment area of 0.5 mm × 99.18 mm. This value satisfies the requirements for surface metrology of nano-focusing x-ray mirrors. Thus, the developed metrology system should be applicable for fabricating nano-focusing ellipsoidal mirrors.Hirokatsu Yumoto, Takahisa Koyama, Satoshi Matsuyama, Kazuto Yamauchi, and Haruhiko Ohashi, "Stitching interferometry for ellipsoidal x-ray mirrors", Review of Scientific Instruments 87, 051905 (2016) https://doi.org/10.1063/1.4950714

Multiscale element mapping of buried structures by ptychographic x-ray diffraction microscopy using anomalous scattering

We propose an element mapping technique of nano-meso-microscale structures buried within large and/or thick objects by ptychographic x-ray diffraction microscopy using anomalous scattering. We performed quantitative imagings of both the electron density and Au element of Au/Ag nanoparticles at the pixel resolution of better than 10 nm in a field of view larger than 5 × 5 μm2 by directly phasing ptychographic coherent diffraction patterns acquired at two x-ray energies below the Au L3 edge. This method provides us with multiscale structural and elemental information for understanding the element/property relationship linking nanoscale structures to macroscopic functional properties in material and biological systems. © 2011 American Institute of Physics.Yukio Takahashi, Akihiro Suzuki, Nobuyuki Zettsu, Yoshiki Kohmura, Kazuto Yamauchi, and Tetsuya Ishikawa, "Multiscale element mapping of buried structures by ptychographic x-ray diffraction microscopy using anomalous scattering", Appl. Phys. Lett. 99(13), 131905 (2011) https://doi.org/10.1063/1.3644396

Bragg x-ray ptychography of a silicon crystal: Visualization of the dislocation strain field and the production of a vortex beam

We experimentally demonstrate the visualization of nanoscale dislocation strain fields in a thick silicon single crystal by a coherent diffraction imaging technique called Bragg x-ray ptychography. We also propose that the x-ray microbeam carrying orbital angular momentum is selectively produced by coherent Bragg diffraction from dislocation singularities in crystals. This work not only provides us with a tool for characterizing dislocation strain fields buried within extended crystals but also opens up new scientific opportunities in femtosecond spectroscopy using x-ray free-electron lasers.Yukio Takahashi, Akihiro Suzuki, Shin Furutaku, Kazuto Yamauchi, Yoshiki Kohmura, and Tetsuya Ishikawa, Phys. Rev. B 87, 121201, 2013

High-resolution and high-sensitivity phase-contrast imaging by focused hard x-ray ptychography with a spatial filter

We demonstrate high-resolution and high-sensitivity x-ray phase-contrast imaging of a weakly scattering extended object by scanning coherent diffractive imaging, i.e., ptychography, using a focused x-ray beam with a spatial filter. We develop the x-ray illumination optics installed with the spatial filter to collect coherent diffraction patterns with a high signal-to-noise ratio. We quantitatively visualize the object with a slight phase shift ([formula omitted]) at spatial resolution better than 17 nm in a field of view larger than [formula omitted]. The present coherent method has a marked potential for high-resolution and wide-field-of-view observation of weakly scattering objects such as biological soft tissues.Yukio Takahashi, Akihiro Suzuki, Shin Furutaku, Kazuto Yamauchi, Yoshiki Kohmura, and Tetsuya Ishikawa, "High-resolution and high-sensitivity phase-contrast imaging by focused hard x-ray ptychography with a spatial filter", Appl. Phys. Lett. 102, 094102 (2013) https://doi.org/10.1063/1.4794063

An abrasive-free chemical polishing method assisted by nickel catalyst generated by in situ electrochemical plating

An abrasive-free polishing method using water and a Pt catalyst, called catalyst-referred etching (CARE), has been developed for the finishing of optical and semiconductor surfaces. This method realizes well-ordered surfaces with a smoothness of several tens of picometers without crystallographic disturbance. In this study, we propose a new CARE method using a Ni catalyst with in situ electrochemical plating and dissolution, which enable enhancing the catalytic capability of Ni. This method has advantages to realize more than ten times higher removal rate and better stability compared with the conventional CARE method.Daisetsu Toh, Pho Van Bui, Ai Isohashi, Satoshi Matsuyama, Kazuto Yamauchi, and Yasuhisa Sano, "An abrasive-free chemical polishing method assisted by nickel catalyst generated by in situ electrochemical plating", Review of Scientific Instruments 91, 045108 (2020), https://doi.org/10.1063/1.5141381

Feasibility study of high-resolution coherent diffraction microscopy using synchrotron x rays focused by Kirkpatrick-Baez mirrors

High-flux coherent x rays are necessary for the improvement of the spatial resolution in coherent x-ray diffraction microscopy (CXDM). In this study, high-resolution CXDM using Kirkpatrick-Baez (KB) mirrors is proposed, and the mirrors are designed for experiments of the transmission scheme at SPring-8. Both the photon density and spatial coherence of synchrotron x rays focused by the KB mirrors are investigated by wave optical simulation. The KB mirrors can produce nearly diffraction-limited two-dimensional focusing x rays of ∼1 μm in size at 8 keV. When the sample size is less than ∼1 μm, the sample can be illuminated with full coherent x rays by adjusting the cross-slit size set between the source and the mirrors. From the estimated photon density at the sample position, the feasibility of CXDM with a sub- 1-nm spatial resolution is suggested. The present ultraprecise figuring process enables us to fabricate mirrors for carrying out high-resolution CXDM experiments.Yukio Takahashi, Yoshinori Nishino, Hidekazu Mimura, Ryosuke Tsutsumi, Hideto Kubo, Tetsuya Ishikawa, and Kazuto Yamauchi, "Feasibility study of high-resolution coherent diffraction microscopy using synchrotron x rays focused by Kirkpatrick–Baez mirrors", Journal of Applied Physics 105, 083106 (2009) https://doi.org/10.1063/1.3108997