Stuttgart – Scientists at the Max-Planck-Institute for Intelligent Systems (MPI-IS) in Stuttgart have developed lenses for X-ray microscopes that helps them see structures of only fifteen nanometers in size, about one thousand times smaller than an average human cell. Focusing X-rays is a very challenging task due to the very low refraction in addition to strong absorption. The everyday magnifying glass cannot be used to focus X-rays. An elegant yet challenging approach is to use the phenomenon of diffraction for focusing X-rays. The scientists at the MPI-IS took up the challenge and succeeded: with a unique 3D nanofabrication method, they are able to construct highly efficient X-ray focusing lenses layer-by-layer, atom-by-atom using a method known as “Atomic Layer Deposition” (see Figure 1).
Figure 1. Schematic overview of the concept
This invention is a huge step forward in further advancing X-ray microscopy, explains Kahraman Keskinbora, leader of the “Micro/Nano Optics” Research Group in the Department of Gisela Schütz, Director of the Modern Magnetic Systems Department at the MPI-IS. “These optics are especially interesting for high resolution hard X-ray microscopy, where nanofabrication of focusing optics are extremely challenging.”
“Its importance will grow in the future owing to the emergence of next generation high brilliance laboratory X-ray sources or further improvements of X-ray free electron lasers”, Umut T. Sanli adds. He is a Ph.D. student in the Modern Magnetic Systems Department and the lead author of the publication “3D Nanofabrication of High-Resolution Multilayer Fresnel Zone Plates”, which was published in Advanced Science beginning of June.
The first building block of their 3D nanofabrication method is a core or base onto which Sanli and Keskinbora apply the layers. They use optical glass fibers about 1 cm long and with a diameter between 30 and 50 micrometers – about half the diameter of a human hair (Figure 2a). They use optical glass fibers because of their roundness and smooth surface.
Figure 2. Fabrication stages of a lens (which the scientists call “Multi Layer Fresnel Zone Plates” or ML-FZP) a) A bunch of high quality optical fibers are deposited with alternating layers of absorbing and transparent materials via ALD as depicted in b) for Al2O3. Finally, in step c) a focused beam of Ga+ ions are used to slice a diffractive lens slab out of the multilayer structure.
In this new approach, a first layer of Aluminium oxide (Al2O3) wraps evenly around the core and no rotation during the layering process of the core is required. This eliminates any potential uneven thickness distribution. “That´s important, because even the smallest imperfection in one layer can worsen the resolution,” Sanli explains. “But, through the atom by atom assembly, we can achieve atomic scale precision in the deposited layers”. Then, they alter the material: the second material used is Hafnium oxide (HfO2). Finally, they add several hundred alternating layers of Al2O3 and HfO2 in total. The scientific name Sanli and Keskinbora use is “Multilayer Fresnel Zone Plates” in short ML-FZP. From the wrapped core, the scientists can now cut off “pieces” of lenses with the help of a focused ion beam (Figure 2 c). The higher the x-ray energy the thicker the slice has to be. Whenever a new lens is needed, it can be cut from the centimeter long deposited fiber. About one thousand lenses can be prepared from a single glass fiber; enough to cover the entire world’s need for X-ray lenses, had everyone used the same type of lens that is.
But there was further room for improvements: It is known, that in principle by a small tilt of the zones of the multilayer zone plate (see Figure 3a), the incoming X-rays can be much better focused with much larger efficiency. Making tilted Fresnel zone plates has been a nanofabrication challenge since a very long time. Fortunately, Sanli and Keskinbora found a way to make Fresnel zone plates with precisely tilted zones. The idea starts with milling out tapered micro-pillars (Figure 3b). “Through optimizing the focused ion beam parameters, we can precisely alter the tilt angle when building the core”, Sanli explains. “We use 0.8 degrees of tilt angle, nearly not noticeable to our eyes, however, it makes a huge difference in the focusing of soft X-rays.
Figure 3. Fabrication stages of Multilayer Fresnel Zone Plates that have tilted zones.
On an array of these tilted micropillars the alternating zones are deposited via ALD similar to the previous Fresnel zone plates. Afterwards, a micromanipulator is used to lift a deposited micropillar as shown in Figure 3c. The micropillar is mounted on a sample holder for easier handling (see Figure 3d). In the final step (Figure 3e) the mounted slice is thinned down to the desired thickness and the back and front surfaces are polished using a focused ion beam. The X-ray lens is now ready to be plugged in the X-ray microscope for exciting experiments. The fabrication technique allows making X-ray lenses optimized for high energy X-rays, which have shorter wavelengths and can theoretically image much smaller features. “It is just a matter of time until we succeed single nanometer resolutions”, Sanli believes.
The successful development of the multilayer Fresnel Zone Plates resulted in two international awards. In a previous publication on the same project, Umut T. Sanli won a Best Paper Award by SPIE, the international society for optics and photonics. Additionally, he created a poster which he presented at the Microscopy and Microanalysis conference organized in Portland/USA and won the Best Poster Award by the Microscopy Society of America. The Micro/Nano Optics Group holds several international patents and patent applications on the fabrication of novel X-ray optics.
The paper "3D Nanofabrication of High-Resolution Multilayer Fresnel Zone Plates" was published in the open access journal of Wiley, Advanced Science. Find out more here:
Umut T. Sanli
Umut T. Sanli received his B.Sc. in Materials Science and Engineering from Anadolu University in Turkey and M.Sc. in Materials Science from CAU Kiel in Germany. He is currently a Ph.D. candidate at the Max-Planck-Institute for Intelligent Systems working together with Dr. Kahraman Keskinbora, Leader of the Micro/Nano Optics Group, and Prof. Gisela Schütz, Director of the Modern Magnetic Systems Department at the MPI-IS. His Ph.D. research is focused on developing X-ray optics using innovative approaches for advancing X-ray microscopy. He is skilled in various nano-engineering and characterization techniques including atomic layer deposition, focused ion beam lithography, multi-photon lithography, microscopy and spectroscopy using electrons and X-rays.
Dr. Kahraman Keskinbora has a B.Sc. degree in Materials Science and Engineering from Anadolu University in Turkey. After finishing his M.Sc. thesis in the same university, he was admitted to the International Max Planck Research School on Advanced Materials in 2011, and moved to the Max-Planck-Institute for Intelligent Systems (former MPI for Metals Research) for his Ph.D. studies. Here, he worked on the development of new nano-fabrication approaches for novel X-ray focusing optics together with Prof. Gisela Schütz, Director of the Modern Magnetic Systems Department at the MPI-IS. Keskinbora finished his Ph.D. with distinction in July 2015, and since then, he is leading the Micro/Nano Optics Group in the Department of Modern Magnetic Systems. His research covers the development of materials, lithography processes as well as innovative kinds of X-ray optics, mainly by utilizing ion beam lithography and micro/nano-machining.
Professor Dr. Gisela Schütz is a Director at the Max Planck Institute for Intelligent Systems in Stuttgart, where she heads the "Modern Magnetic Systems" department. Her research interests cover the application of synchrotron radiation in X-ray spectroscopy and microscopy and the development of advanced spintronic/magnonic systems and new supermagnets.
Schütz was born in Ottobeuren in Southern Germany in 1955. She studied physics at the Technical University of Munich (TUM) until 1979, and in 1984 she received her doctorate from the Chair of Nuclear Physics at TUM.
There she started with research activities in the field of condensed matter with synchrotron radiation. She worked at several synchrotron laboratories and developed new methods of studying magnetic structures and phenomena with polarized X-rays. After qualifying as university lecturer in 1992 in experimental physics, she became a professor at the University of Augsburg in 1993 and received a chair at the Institute for Experimental Physics at the University of Würzburg in 1997. In 2001, the mother of three children became Director of the Max Planck Institute for Metal Research, now the Max Planck Institute for Intelligent Systems.