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Posted: Jun 13, 2014
X-ray lenses for nanoanalytics
(Nanowerk News) Progress in the field of nanotechnology can only be achieved, if analytical methods for the characterization of nanostructures continuously improve. Due to the structure’s small size visible light cannot be utilized for nanoanalytical techniques. In point of fact, microscopic methods with electron waves or X-rays must be applied here, as the wavelengths, they work with, are small enough. Multilayer Laue lenses (MLL) offer a most promising approach to developing highest resolution X-ray optics.
Principle of two crossed MLL (a), SEM image of two crossed MLL after FIB processing (in cooperation with Fraunhofer IKTS-MD) (b). (Image: Fraunhofer IWS Dresden)
The electron microscopy already offers a unique insight into the nanoworld and allows for the observation of single atoms. However, a technical limitation is that the electron microscopy can only analyze objects at the surface. In order to get volume information the objects under investigation must be sophisticatedly prepared and are often destroyed. These limitations can be avoided if X-rays are used as “light” instead of electron waves. X-rays have an excellent penetration capacity and have the short wavelength, required for a high resolution. Although it is more than 100 years ago that Wilhelm Conrad Roentgen detected X-ray technology, it is still very difficult to fabricate X-ray optics today. A particularly challenging experimental task is focusing hard X-rays to sizes smaller than 50 nm. That’s the reason why X-ray based microscopes find only still niche applications.
During the recent decades enormous efforts have been made to develop efficient Xray optics, utilizing different physical principles. The multilayer Laue lenses concept, used at the Fraunhofer IWS and TU Dresden, represents a very promising approach to efficiently focusing hard X-rays. The physical principle of beam focusing is the same that is applied in Fresnel’s zone plates, meaning scientists utilize the diffraction of electromagnetic waves on a concentric ring structure. In contrast to zone plates, where lithographic processes are used, MLL are fabricated by means of thin film coating technology and subsequent processing with a focused ion beam (FIB).
Principle of X-ray focusing using MLL. (Image: Fraunhofer IWS Dresden)
The high resolution is achieved by fabricating thin and smooth films with thicknesses as low as 1 nm. The film thickness corresponds to the zone width and for highly resolving zone plates is typically in the rage of > 20 nm. Furthermore, when fabricating MLL, the ratio between zone width and lens thickness a most important factor for the lenses’ high efficiency, can be almost freely selected. A single MLL, however, enables only a one-dimensional beam focusing. To achieve a point focus or a twodimensional optical imaging, 2 MLL must be perpendicularly positioned. (Fig. 1)
The Dresden scientists have achieved very promising research results. An initial test at the electron storage ring “PETRA III” revealed focus sizes of 40 nm x 50 nm for focal lengths > 10 mm. (Fig. 2) Further reductions of the focal diameter are expected through improvements to the thickness accuracy during the MLL’s coating process. Even now the thickness deviations from the mathematical ideal are only about ± 0.3 %.
Recent MLL measuring results and further research studies will be presented at the10th Nanofair, International Congress Center from July 1 - 3, 2014. The congress and its included exhibition will bring together renowned international nanotechnology experts from science and industry.