X-ray Nanobeam Research Articles

Nanometer Resolution XANES Imaging of in situ switched individual PC-RAM devices

ABSTRACTWe report on the study of single devices of phase-change (Ge2Sb2Te5) memory cells in line cell type devices. Devices were investigated employing an x-ray nanobeam of only about 150 nm diameter, which could be fully contained within the spatial extent of the active area within a single device cell. XANES spectra showing the device in the amorphous and crystalline state have been successfully collected after switching the device in situ at the synchrotron. By monitoring the fluorescence response of the sample constituent materials at a constant photon energy (corresponding to the Ge K-edge absorption edge) as a function of x-ray beam position on the sample 2D maps have been produced.

Probing Quantum Confinement within Single Core–Multishell Nanowires

Theoretically core-multishell nanowires under a cross-section of hexagonal geometry should exhibit peculiar confinement effects. Using a hard X-ray nanobeam, here we show experimental evidence for carrier localization phenomena at the hexagon corners by combining synchrotron excited optical luminescence with simultaneous X-ray fluorescence spectroscopy. Applied to single coaxial n-GaN/InGaN multiquantum-well/p-GaN nanowires, our experiment narrows the gap between optical microscopy and high-resolution X-ray imaging and calls for further studies on the underlying mechanisms of optoelectronic nanodevices.

Dynamic fluorescence imaging of the free radical products of X-ray absorption in live cells

Abstract The immediate products of x-ray absorption in aqueous biological samples are free radicals including *OH, H2O2, *H and solvated electrons. Because their lifetimes and diffusion ranges are dependent on the local bio-molecular environment, imaging these free radicals in real-time while they are produced by a scanning x-ray nanobeam may provide a biological microscopy method of high resolution. As a first step towards this goal, we investigated the feasibility of imaging the initial free radical products of x-ray absorption in live cells using fluorescent free radical sensors. We selected six commercially available fluorescent sensors for screening tests of their sensitivities towards x-ray radiation in solution form. Two of the six dyes were found to have high sensitivities. One of the two was successfully used for dynamic confocal fluorescence imaging of x-ray generated free radicals in the intracellular space of mouse embryonic fibroblasts. Time series of fluorescence images before and during x-ray radiation were acquired. The rate of increase of cellular fluorescence showed both the initial production of free radicals by the physical ionization events as well as stimulated biological production of reactive oxygen species later on. The implications of the results for future development of microscopy techniques are discussed.

Nanometer Resolution XANES Imaging of Individual PC-RAM Devices

ABSTRACTWe introduce a technique to permit x-ray absorption spectroscopy studies focusing on individual phase-change (Ge2Sb2Te5) memory cells in fully integrated PC-RAM structures. Devices were investigated employing an x-ray nanobeam of only about 300 nm diameter, which could be fully contained within the spatial extent of the active area within a single device cell and enabled us to investigate individual devices without interference from non-switching material surrounding the area of interest. By monitoring the fluorescence signals of tungsten and germanium at a photon energy corresponding to the Ge K-edge absorption edge white line position, we were successful in producing 2D area maps of the active cell region, which clearly show the imbedded tungsten heater element and the switched region of the phase change material. Additionally, position dependent changes in the phase change material could be traced by taking an array of XANES spectra at the Ge K-edge on and in the vicinity of individual devices.

Fundamental parameter based quantification algorithm for confocal nano-X-ray fluorescence analysis

A new method for the quantification of X-ray fluorescence (XRF) was derived based on the fundamental parameter method (FPM). The FPM equations were adapted to accommodate the special case of confocal nano-XRF, i.e. X-ray nano-beam excitation coupled with confocal detection, taking into account the special characteristics of the detector channel polycapillary. A thorough error estimation algorithm based on the Monte Carlo method was applied, producing a detailed analysis of the uncertainties of the quantification results. The new FPM algorithm was applied on confocal nano-XRF data obtained from cometary dust returned by NASA's Stardust mission, recorded at beamline ID13 of the European Synchrotron Radiation Facility.

The Race to X-ray Microbeam and Nanobeam Science

X-ray microbeams are an emerging characterization tool with broad implications for science, ranging from materials structure and dynamics, to geophysics and environmental science, to biophysics and protein crystallography. We describe how submicrometer hard x-ray beams with the ability to penetrate tens to hundreds of micrometers into most materials and with the ability to determine local composition, chemistry, and (crystal) structure can characterize buried sample volumes and small samples in their natural or extreme environments. Beams less than 10 nanometers have already been demonstrated, and the practical limit for hard x-ray beam size, the limit to trace-element sensitivity, and the ultimate limitations associated with near-atomic structure determinations are the subject of ongoing research.

Front Cover: Coherent X‐ray nanodiffraction on single GaAs nanowires (Phys. Status Solidi A 11/2011)

Gulden et al. (pp. 2495-2498) performed a coherent diffraction experiment on single GaAs nanowires. For this experiment, the hard X-ray nanobeam scanning X-ray microscope at ID 13 of the European Synchrotron Radiation Facility (ESRF) was used. Diffraction patterns in the vicinity of a Bragg peak were measured locally in a small region of a selected nanowire, and the origin of the different features is explained. The cover image shows a 3D isosurface of the scattered intensity in the vicinity of the Bragg peak in reciprocal space combined of 60 diffraction patterns. With further development of the method, Coherent X-Ray Diffractive Imaging (CXDI) can possibly provide high resolution images of complicated nanowire structures.

Low-temperature synthesis of Zn3P2 nanowire

Abstract

Nanoscale silicon-on-insulator deformation induced by stressed liner structures

Rotation and strain fields were mapped across silicon-on-insulator (SOI) regions induced by overlying stressed Si3N4 features using x-ray nanobeam diffraction. The distribution in SOI tilt exhibited an antisymmetric distribution with a maximum magnitude of 7.9 milliradians, representing one of the first direct measurements of the lattice tilt conducted in situ within buried layers using a spot size of less than 100 nm. The measured rotation distribution corresponds to simulated values generated by boundary element method modeling, indicating that the strain transfer into the underlying SOI primarily induces elastic deformation.

Modeling of kinematic diffraction from a thin silicon film illuminated by a coherent, focused X-ray nanobeam

A rigorous model of a diffraction experiment utilizing a coherent, monochromatic, X-ray beam, focused by a Fresnel zone plate onto a thin, perfect, single-crystal layer is presented. In this model, first the coherent wave emanating from an ideal zone plate equipped with a direct-beam stop and order-sorting aperture is computed. Then, diffraction of the focused wavefront by a thin silicon film positioned at the primary focal spot is calculated. This diffracted wavefront is propagated to the detector position, and the intensity distribution at the detector plane is extracted. The predictions of this model agree quite well with experimental data measured at the Center for Nanoscale Materials nanoprobe instrument at Sector 26 of the Advanced Photon Source.

Hard x-ray nanobeam characterization by coherent diffraction microscopy

We have carried out a ptychographic scanning coherent diffraction imaging experiment on a test object in order to characterize the hard x-ray nanobeam in a scanning x-ray microscope. In addition to a high resolution image of the test object, a detailed quantitative picture of the complex wave field in the nanofocus is obtained with high spatial resolution and dynamic range. Both are the result of high statistics due to the large number of diffraction patterns. The method yields a complete description of the focus, is robust against inaccuracies in sample positioning, and requires no particular shape or prior knowledge of the test object.

Spatially Resolved Investigation of Solution Cast Nanoparticle Films by X-ray Scattering and Multidimensional Data Set Classification

A combinatorial high-throughput approach is used to investigate a solution cast gradient consisting of colloidal gold nanoparticles on top of a silicon substrate by means of a X-ray nanobeam. Classification algorithms are used to reveal and visualize structural transitions from a frozen colloidal solution to a well-defined nanostructure. Prominent length scales on the order of 100 nm are observed. A periodic change in the nanostructure along the gradient is explained by a simplified stick-slip model.

Breaking the 10 nm barrier in hard-X-ray focusing

Hard X-rays have exceptional properties that are useful in the chemical, elemental and structure analysis of matter. Although single-nanometre resolutions in various hard-X-ray analytical methods are theoretically possible with a focused hard-X-ray beam, fabrication of the focusing optics remains the main hurdle. Aberrations owing to imperfections in the optical system degrade the quality of the focused beam1. Here, we describe an in situ wavefront-correction approach to overcome this and demonstrate an X-ray beam focused in one direction to a width of 7 nm at 20 keV. We achieved focal spot improvement of the X-ray nanobeam produced by a laterally graded multilayer mirror2. A grazing-incidence deformable mirror3 was used to restore the wavefront shape. Using this system, ideal focusing conditions are achievable even if hard-X-ray focusing elements do not achieve sufficient performance. It is believed that this will ultimately lead to single-nanometre spatial resolution in X-ray analytical methods.

Novel Scheme of Figure-Error Correction for X-ray Nanofocusing Mirror

To improve the performance of an X-ray nanobeam, we demonstrate the in situ determination and correction of the figure error of an elliptical mirror. The phase error profile on the mirror surface, which represents the figure error, is numerically reconstructed from the X-ray intensity distribution measured at the focal plane. The figure error is corrected by depositing a Pt film on the surface with subnanometer thickness controllability. Results of focusing experiments and subsequent analysis show a great improvement in focusing performance.

Hard X-ray scanning microscopy with fluorescence and diffraction contrast

Based on nanofocusing parabolic refractive x-ray lenses we have developed and built a hard x-ray scanning microscope that was tested and put to use at beamline ID13 of the ESRF. It can provide a monochromatic hard x-ray nanobeam with lateral extension below 100 nm (down to 50 nm) and a flux up to 109 ph/s in the energy range from 15 to 25 keV. The microscope exploits transmission, fluorescence, and diffraction contrast to obtain local elemental and nanostructural information from the sample. Tomographic scanning yields high resolution elemental maps from the inside of an object. Coherent x-ray diffraction imaging with nanofocused illumination yields images of objects with highest spatial resolution, e. g., 5 nm in a given example.

Wavefront Control System for Phase Compensation in Hard X-ray Optics

A highly precise adaptive optical system that can be used in the hard X-ray region was developed. To achieve highly precise control of the wavefront shape, we discussed an optical system with a bendable mirror of deformation accuracy better than 0.4 nm RMS. Using the system, we demonstrated the controllability of the wavefront of a 15 nm hard X-ray nanobeam. The intensity profile of the wavefront-modified beam was in good agreement with the wave-optically calculated profile.

Coherent X-Ray Diffraction Imaging with Nanofocused Illumination

Coherent x-ray diffraction imaging is an x-ray microscopy technique with the potential of reaching spatial resolutions well beyond the diffraction limits of x-ray microscopes based on optics. However, the available coherent dose at modern x-ray sources is limited, setting practical bounds on the spatial resolution of the technique. By focusing the available coherent flux onto the sample, the spatial resolution can be improved for radiation-hard specimens. A small gold particle (size <100 nm) was illuminated with a hard x-ray nanobeam (E=15.25 keV, beam dimensions approximately 100 x 100 nm2) and is reconstructed from its coherent diffraction pattern. A resolution of about 5 nm is achieved in 600 s exposure time.

Direct determination of the wave field of an x-ray nanobeam

We present a remarkably accurate method for determining the wave field of an x-ray nanobeam. The intensity profile of a beam was directly measured over a range of three orders of magnitude while its phase distribution was successfully recovered using an iterative algorithm. The evolution of the wave field along the beam propagation direction was precisely simulated, and there was good agreement with the experimental results.

Characterization of Oxide Scale on Alloy 446 by X-Ray Nanobeam Analysis

To understand the potential degradation of metallic interconnects, tubes of alloy 446 were exposed to air/hydrogen dual atmosphere at elevated temperatures for 1030 h. The scale developed on the air side consisted of two layers. Because the two sublayers in the oxide scale were too thin for evaluation by regular X-ray diffraction, nanobeam X-ray diffraction was used to study the phase compositions in the scales that formed on both the air and hydrogen sides of alloy 446. The results indicate that Mn{sub 1-x}Cr{sub 2-x}O{sub 4} spinel is the major phase in the oxide scale on alloy 446. The valence of manganese in the oxide scale was also evaluated by X-ray near-edge absorption.

Short focal length Kirkpatrick-Baez mirrors for a hard x-ray nanoprobe

We describe progress in the fabrication of short-focal-length total-external-reflection Kirkpatrick-Baez x-ray mirrors with ultralow figure errors. The short focal length optics produce nanoscale beams (&amp;lt;100nm) on conventional (∼64m long) beamlines at third generation synchrotron sources. The total-external reflection optics are inherently achromatic and efficiently focus a white (polychromatic) or a tunable monochromatic spectrum of x rays. The ability to focus independent of wavelength allows novel new experimental capabilities. Mirrors have been fabricated both by computer assisted profiling (differential polishing) and by profile coating (coating through a mask onto ultra-smooth surfaces). A doubly focused 85×95nm2 hard x-ray nanobeam has been obtained on the UNICAT beamline 34-ID at the Advanced Photon Source. The performance of the mirrors, techniques for characterizing the spot size, and factors limiting focusing performance are discussed.