Advanced Photon Source's Hard X-Ray Scanning Microprobe
|With suboptical spatial resolution, the microprobe offers unprecedented benefits via quantitative elemental, chemical, structural, and tomographic analysis for application to an unlimited range of samples. The hydrated bacteria (pseudomonas fluorescens) in these fluorescence images are about 1 x 4 microns in size; elemental maps of potassium and chromium are shown, following treatment with a chromium (+6) solution.
Conventional X-ray techniques, including X-ray imaging, diffraction, fluorescence, and spectroscopy — though their spatial resolution is generally limited to 0.1-1.0 mm — have profoundly affected our lives. The benefits of X-ray technology include advanced materials, new drug designs, and more accurate medical diagnostic methods. Imagine the potential for X-rays if the spatial resolution were to surpass that of the optical microscope. Superior resolution is just one of the key features of the hard X-ray scanning microprobe (HXRM), developed at Sector 2 of the SRI-CAT at the Advanced Photon Source (APS).
The APS HXRM offers unprecedented capabilities, among them
- The ability to image objects at a suboptical 150-nm spatial resolution, with improvement to 50-nm resolution expected in the near future;
- An elemental detection sensitivity of 10-100 parts per billion (ppb) for mapping trace-element distributions in cells and bacteria in their natural hydrated state;
- The ability to measure structural phases and strains in materials and microelectronic devices in a diffraction gauge volume of less than 0.1 cubic micrometer, with a strain sensitivity of better than one part in ten thousand.
The Door Is Open to New Applications
These capabilities open many exciting new applications in microelectronics and in the materials, biomedical, and environmental sciences.
Conceptually, the APS HXRM functions much like a scanning electron microscope (SEM), except that a tightly focused X-ray beam takes the place of the electron beam. A phase zone plate (PZP), a novel optical component developed by our group in collaboration with two other institutions, focuses the incident X-rays with a flux density gain of about five orders of magnitude at the focus to deliver a flux of 5 x 1010 photons per second per square micrometer at the focal spot. The microprobe operates in the classical hard X-ray regime of 4-15 keV, providing high penetration power; this allows studies to be performed in vacuum or in ambient pressure, or even in an aqueous environment, with applications to a broad range of samples and configurations.
The X-ray microprobe’s diverse modes of operation include transmission, diffraction, fluorescence, spectroscopy, and tomography:
- In transmission mode, we measure the attenuation of the X-ray beam by the sample. This maps the sample’s electron and mass density, but we can also locate particular elemental constituents by tuning to the sample's absorption edges and generating an element-specific image. This allows non-destructive X-ray imaging of any sample with submicron resolution.
- By measuring X-rays diffracted from the sample, we obtain local structural information, such as strain, crystallographic phases, and textures, with an accuracy 100 times better than with electron diffraction. Applications include development of new materials and devices, and local study of tiny protein crystals.
- X-ray fluorescence from the sample reveals the spatial distribution of individual elements. Because it offers 1000 times higher sensitivity than electron probes, the fluorescence technique is a powerful tool for trace-element analysis, important in hazard detection, understanding materials properties, and study of anticancer drugs.
- In spectroscopy mode, the primary X-ray beam’s energy is scanned across the absorption edge of an element, providing information on its chemical speciation or its local atomic arrangement. This allows speciation of samples with environmental concern or biological importance.
|Competitive Advantages of the APS Hard X-ray Scanning Microprobe
- Combination of high spatial resolution with high elemental and structural sensitivity
- Relaxed sample preparation and operating environment, with application to an unlimited class of samples
- Imaging of thick and optically opaque samples
- Nonintrusive, nondestructive techniques supports studies of in situ and in vivo processes, dynamics, and kinetics
In X-ray tomography, one of these modes is combined with sample rotation to produce a series of 2-D projection images, used for reconstructing the sample’s internal 3-D structure. Better 3-D imaging supports defect analysis of integrated circuits (ICs) and better understanding of cellular functions.
Thanks to reduced scattering and higher fluorescence cross section, the APS HXRM provides a ten thousand times higher signal-to-noise ratio than that of SEM, detecting elemental concentrations in the low parts-per-billion range. The SEM’s primary electron beam can be focused to 5-10 nm, but for thick samples its spatial resolution in the fluorescence mode is actually worse than the APS HXRM’s, due to scattering and the effect of secondary electrons. For this reason, elemental analysis on SEM and TEM require very thin (< 1 µm) sections of biological specimens, whereas the X-ray microprobe can examine specimens as thick as tens to hundreds of microns without sectioning. Argonne’s APS HXRM has also demonstrated the ability to detect masses as small as 10 attograms (10-17 g) in biological cells, while reducing radiation damage by a factor of 10-3 to 10-5.
The APS HXRM also lends itself to microspectroscopy. Combining the suboptical spatial resolution of the X-ray microprobe with techniques for tuning across a particular absorption edge and examining the near-edge (XANES) or extended fine structures (EXAFS), one can obtain detailed information on chemical speciation and the atomic environment. X-ray microspectroscopy can be used to investigate local chemistry, phase separation, confined geometry, etc.
Using APS HXRM technology, scientists and industrial developers can view specimens in an unperturbed environment. The X-ray microprobe allows nondestructive measurements, even for embedded samples or regions of interest. First-time users are always amazed when their samples are simply mounted in the apparatus with adhesive tape or nail polish. Because samples can be studed in air, with no vacuum requirements, in-situ characterization under realistic conditions is possible for such important industrial processes as vapor deposition, etching, and annealing. Charging is not a concern, so insulators, ceramics, semiconductors, and organic materials can be examined without special sample preparation. In fact, live biological cells and bacteria can be studied in-vivo, in their native hydrated environment, thus avoiding the artifacts associated with staining, chemical fixation, drying, and tagging with fluorescence dyes. The penetrating power of APS HXRM X-rays also allows microtomography and 3-D reconstruction of specimens to be carried out without physical sectioning.
Biomedical Applications: No staining or tagging is required in using Argonne’s X-ray microprobe for biomedical studies. The APS HXRM has been used for mapping trace-metal distribution in various cells, where the trace metals could be either cell constituents (e.g., zinc proteins or enzymes, copper in chromatin, iron in macrophages) or from an external source (e.g., in anticancer drugs such as cisplatin, or anti-inflammatory agents such as copper-indomethacin complexes). The APS HXRM's high sensitivity allows study of unstained cells subjected to a clinical doses of drugs. Anticipated benefits include:
- Imaging tumor growth, cell division, etc. could provide new mechanism for early detection of cancer; monitoring response of biological units to experimental drugs/treatment.
- Defining functions, pathways, and dynamic behavior of proteins in vivo may lead to better drug design and testing.
- More effective anticancer drugs, antiviral treatments, etc. could result from APS HXRM studies.
- The APS HXRM will markedly improve diagnostic technologies, resulting in more effective diagnosis scheme and treatment of disease.
Environmental Monitoring and Remediation. The high sensitivity and noninvasive character of Argonne’s APS HXRM have proven valuable in studying bacterial response to metal contamination. This has important implications for determining the fate of metal contaminants and for developing effective bioremediation strategies. The APS HXRM allows the study of fully hydrated bacteria in their natural state and, using microspectroscopy, it reveals the chemical speciation of contaminants in vivo. Fine airborne particles having an average size of only 0.3 µm have been studied; these extremely light particles can travel long distances, but by revealing their elemental content with the APS HXRM, we identified their origin. Impacts on everyday life include
- Improved bioremediation technologies.
- Faster, more sensitive detection of hazards in local and global environments.
- Capacity to predict the spread of contaminants and monitor/verify compliance with environmental regulations.
- Improved understanding of climate and warming via local-chemistry/speciation capabilities of our technology.
Microelectronics. The spatial resolution of the APS HXRM is well matched to the size of microelectronic devices. Microdiffraction can be used to study stress in these devices — for instance, around gates, interconnects, or optoelectronics. This becomes ever more important as device size shrinks; the control of stress affects device performance and lifetime. Examples of real-world impacts include the following:
- Development of smaller and more reliable computers, telecommunications equipment, etc.; enhancing the performance.
- Helping to resolve the problem of electromigration in interconnects which is a major reliability issue. With production semiconductor devices shrinking toward sub-100-nm widths, electromigration will accelerate open-circuit failure of many devices.
Materials Science. Both the fluorescence and diffraction capabilities of the APS HXRM have been used to study local residual stress and phase composition, content of intermetallic alloys, structure of secondary phase particles, crack propagation, agglomeration of impurities, and magnetic domains and interactions. The Argonne technology offers the following benefits:
- Detection of flaws/strains via noninvasive APS HXRM, leading to purer, stronger, more reliable materials for extreme environments, e.g. storage, machining, aviation, etc.
- Lighter, sturdier, safer transportation through advanced materials with tailored properties.
- Improved sensors and coatings, as well as smaller devices, through investigation of thick and thin films.
Life sciences. The major genetics revolution so often predicted for the 21st century cannot take place without some means to observe ongoing life processes in vivo – Argonne's APS HXRM technology provides this capability.
Microtomography. Greater reliability of microelectronic components can be attained through the microprobe’s capacity to detect microflaws in realistic multi-level circuits (for instance, via 3-D X-ray microtomography).
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