Hard x-ray nanoprobe facility at the National Synchroton Light Source II

In scanning x-ray microscopy beamlines (such as that illustrated in Figure 1), monochromatic x-rays are focused to produce a nanobeam. X-ray imaging can be performed by scanning a sample across the focused beam, and the resulting x-ray signals are collected to visualize (with the use of a variety of contrast mechanisms) elemental, structural, and chemical details of the sample. Fluorescent x-rays, for example, are emitted by excited electrons in the sample and provide a unique fingerprint of its elemental composition. Bragg-diffracted x-rays yield detailed information on the crystalline phase, crystallite (i.e., grain) orientation, and strain distribution. To obtain comprehensive structural and chemical images of a sample, scanning x-ray instruments must be capable of making simultaneous measurements of these different signal types.

Figure 1. Schematic representation of the scanning x-ray microscopy capabilities available at the Hard X-ray Nanoprobe (HXN) beamline.

Over the last decade, the scientific requirements for scanning x-ray microscopy beamlines at synchrotron facilities have increased dramatically, i.e., there is a need for increasingly high spatial resolutions.^1–4 The Hard X-ray Nanoprobe (HXN) beamline at the National Synchrotron Light Source II (NSLS-II)⁵ is the newest addition to the growing number of nanoprobes. This beamline was developed with unprecedented goals for hard x-ray resolutions. The initial performance target for the HXN is to enable x-ray imaging experiments at a resolution of 10nm, with an ultimate goal of about 1nm. In addition, an alternative imaging method—known as ptychography—is offered at the HXN beamline. In ptychography, coherently diffracted signals are used to mathematically reconstruct the nanostructure of a sample. Spatial resolutions that are better than the size of the focused beam can be achieved with this methodology.

We have undertaken several substantial development efforts to assure the high-performance scanning x-ray microscopy capabilities of the new HXN beamline. For instance, we have produced nanofocusing optics that allow 10nm resolutions to be achieved,^6–8 and we have constructed a versatile x-ray microscope.^{9, 10} We use a new class of x-ray nanofocusing optics—known as multilayer Laue lenses (MLLs)—to achieve our particularly high spatial resolutions. We will offer ptychographic analysis¹¹ at the HXN beamline for transmitted x-rays (to visualize the electronic density of non-crystalline samples) and for Bragg-diffracted x-rays (to visualize the strain distribution within crystalline grains). Once we have completed the HXN commissioning, the beamline will operate over an energy range of 6–25keV. The focused beam will have a flux of 5×10⁸ photons per second over the 10×10nm² beam. Spectro-microscopy and 3D imaging capabilities will also be offered.

We conceived MLLs to overcome the well-known aspect-ratio limit of x-ray zone plates (i.e., that prevents ultra-high resolutions being achieved), but while retaining good efficiency. With MLLs we are able to avoid this aspect-ratio limit because these lenses are fabricated using thin-film deposition of depth-graded multilayers. We section and thin the MLL thin film with the use of a focused ion beam. In this way we are able to obtain the optimal thickness for the lens. A scanning electron microscopy image of a sectioned MLL is shown in Figure 2(A). In our setup, monochromatic x-rays are focused to a point—see Figure 2(B)—by a pair of orthogonally arranged MLLs.

Figure 2. (A) Scanning electron microscope image of a sectioned multilayer Laue lens (MLL). The active region of the MLL is the thinner portion, which is prepared with the use of a focused ion beam. (B) Two MLLs are arranged to produce a point focus of x-rays. (C) Photograph of the MLL microscope module for the HXN x-ray microscope. The vertical MLL (vMLL) sample, interferometer heads, retro reflectors, and x-ray fluorescence (XRF) detector components are labeled. The horizontal MLL and order-sorting aperture are not visible in this photograph. Nine independent interferometers are used to monitor the position of the two MLLs and the sample.

A close-up view of the MLL microscope module for our HXN x-ray microscope is shown in Figure 2(C). This module has compact dimensions—about the size of a small coffeemaker—and a closed-loop feedback mechanism that uses laser interferometers. These characteristics of the MLL microscope module allow us to achieve sub-nanometer positioning stability with the microscope. The HXN x-ray microscope also houses a zone plate microscope module. We operate this module with a spatial resolution of 30nm, but it does offer more scientifically flexible capabilities.

Of our HXN beamline imaging techniques that are illustrated in Figure 1, the scanning fluorescence imaging capability is now fully commissioned and available to general users. One of the first sets of nanoscale images that we produced at the beamline, in April 2015, is shown in Figure 3. For these measurements, we used a platinum (Pt) resolution test pattern—with a circular structure array that was 20nm wide and 200nm tall—as a sample. In addition, we made the measurements with the use of monochromatic x-rays at 12keV and a synchrotron electron beam current of 50mA. Our measured image of the Pt L-edge (spectroscopic measure of electronic structure) clearly shows the individual circular structures. Our preliminary analysis of the results indicates that a range of spatial resolutions (about 12–15nm) was achieved. It is also important to note that we produced this image using a fly-scanning method.¹¹ In this approach, we obtain each horizontal line of the image by continuously scanning the sample, while simultaneously triggering the detectors. We required a total of 40 minutes to collect this image, which consists of 101×101 pixels (5nm per pixel and 200ms exposure time per pixel). By October 2015 we will have a higher electron beam current (150mA) and will have completed further instrument optimizations. At that point we will be able to obtain a comparable image in about five minutes.

Figure 3. One of the first sets of nanoscale x-ray images taken at the HXN beamline in April 2015. These images were obtained by collecting fluorescence (platinum L-edge) x-rays that were emitted from a platinum resolution test pattern. The circular shapes correspond to nano-fabricated structures that are 20nm wide and 200nm tall. The color scale indicates the fluorescence intensity over the acquisition period (200ms per pixel).

We have designed, built, and recently commissioned the HXN beamline, which offers high-performance scanning x-ray microscopy at unprecedented resolutions. Our development efforts include a new class of x-ray nanofocusing optics, known as multilayer Laue lenses. Our first nanoscale x-ray fluorescence images were successfully obtained with the beamline in April 2015. Additional scientific capabilities will be offered on the HXN beamline as we progress through our rapid commissioning schedule. In October 2015, a differential phase contrast imaging technique¹² will become available. This will allow the analysis of transmitted x-rays so that nanoscale morphology and elemental distribution of a sample can be imaged simultaneously. In 2016, we plan for transmission ptychography, nanodiffraction, and Bragg ptychography to be commissioned and subsequently made available for general user experiments.

We thank Deming Shu and Ray Conley of the Advanced Photon Source for their substantial contributions to the development of our instrumentation. We also thank Ming Lu of the Center for Functional Nanomaterials for the fabrication of the resolution test pattern used in this work. The work conducted at NSLS-II and the Center for Functional Nanomaterials at Brookhaven was supported by the Department of Energy, Office of Basic Energy Sciences, under contract DE-SC00112704.