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Article updated on 04/03/20 by Gaea Marelle Miranda
Progress in nanoscience and nanotechnology depends not only on examining the surfaces of materials but on microscopically examining biological organisms and material structures to identify what they are made of and what electronic, magnetic, optical, and chemical processes may be in play.
For measuring internal variations in shape, organization, magnetism, polarization, or chemical make-up over distances of a few nanometers (billionths of a meter), x-ray microscopy not only complements electron microscopy but also offers important advantages. For instance, the XM-1 x-ray microscope at the Advanced Light Source, located at the Department of Energy's Lawrence Berkeley National Laboratory, uses bright beams of "soft" x-rays to produce images that not only reveal structures but can identify their chemical elements and measure their electromagnetic and other properties as well.
Researchers at the Berkeley Lab's Center for X-Ray Optics (CXRO), over the past decade, were able to build and operate the XM-1 to achieve an extraordinary resolution of better than 15 nanometers, with the goal of generating an even higher resolution in the future. This goal was already achieved when developers were able to create an XM-1 solution with a 25-nm maximum resolution. According to the CXRO, its technique permits fabrication of remarkably small three-dimensional structures, which could be considered as a breakthrough in manipulating small structures using electron beam lithography.
Over the years, more innovations have been developed to accommodate x-ray equipment with at least 15-nanometer resolutions. Researchers have put more focus on Fresnel zone plates that are constructed using interlaced stacking, allowing two complementary zone plates to function near each other’s near-field.
X-Rays Using Zone Plates
Since x-rays cannot be focused by glass lenses, the XM-1 uses lenses made of zone plates, disks of concentric rings of metal from which soft x-rays are diffracted to a focus. An objective lens called a "micro" zone plate (MZP) projects a full-field image of the sample whether the interior of frozen bacteria or layers of a magnetic alloy onto a charge-coupled device. The smaller the gap between the MZP's rings, the tighter the focus, and the higher the resolution of the image.
CXRO fabricates its own zone plates with an electron-beam lithography tool called the NanoWriter. An energetic beam of electrons just 7 nanometers wide carves preprogrammed patterns in a silicon wafer coated with a resist. The carved-out circular patterns in the resist are then replaced with opaque gold to form an object that under magnification superficially resembles a long-playing gold record album but with one only 30 micrometers in diameter.
Achieving high resolution depends on the user’s ability to squeeze the zones close together, with a placement accuracy no less than one-third the width of the zones themselves. Accurate placement of 15-nanometer-wide zones allows no more than a 5-nanometer leeway. In fact, the NanoWriter is capable of a placement accuracy within 2 nanometers.
Unfortunately, no matter how accurately it is aimed, even a tight beam of electrons spreads out when it hits the resist. Electron scattering, combined with inherent limits in the resolution of the resist itself, makes it impossible at this time to maintain high contrast and optical separation between features. Historically, the best separation between zones the NanoWriter could achieve to make the XM-1's current objective lens was 25 nanometers.
Combining Two Different Zone-Plate Patterns
To overcome limits of the zone plate lenses, CXRO researchers overlaid and combined two different zone-plate patterns. Opaque zones are typically given even numbers, so in this scheme the first pattern contains zones 2, 6, 10, 14, and so on, and the second contains zones 4, 8, 12, 16, and so on. The first pattern is carved into the resist-coated wafer; the zones formed by the electron patterning are then filled with gold and the wafer is coated with resistance to make the second pattern.
When combined, the critical outer zones of the combined patterns were less than 15 nanometers apart, accurately placed to within less than 2 nanometers. Aligning the separately processed patterns was the key to success. Accuracy was achieved with software that calculated the deflection and distortion of the NanoWriter's beam as it traced out the concentric circular patterns.
The placement of the zones in the first MZP made with this technique was nearly perfect, although there was room for improvement in other areas. The opaque gold zones were broken by tiny gaps, which were wider than intended, reducing the zone plate's efficiency. Nevertheless, the experimental MZP was used to obtain images sharper than any previously achieved with an x-ray microscope.
Results showed that not only were images of test patterns (lines formed by layers of chromium and silicon in cross-section) sharper than those made with the XM-1's current 25-nanometer-resolution MZP, the new MZP was able to obtain sharp images of lines a mere 15 nanometers apart where the older zone plate had seen only a featureless field of gray.
The Need for Better Analytical Tools
Due to the need for bright beams of photons, x-ray microscopes are limited in use and application. However, CXRO researchers suggest that new sources of very bright, soft x-rays—for example, compact, laser-based x-ray sources—will make it possible to build x-ray microscopes that will fit on the bench top. Nanoscience and nanotechnology will be both the beneficiaries and the driving forces behind the widening horizon for nanoscale analysis, which has been opened by the new techniques in soft-x-ray optics.