Advances in the Development of X-Ray Refractive Large Aperture Rolled Prism Lenses

At the Institute of Microstructure Technology (IMT) at Karlsruhe Institute of Technology (KIT) X-ray refractive line focus lenses have been developed. They consist of a large number of concave bi-parabolic lens elements made of SU8. To form a point focus two of these lens stacks, tilted by 90 degrees with respect to each other around the optical axis, need to be arranged in the optical path. To increase their transmission, the Fresnel principle can be applied to the lenses to provide higher ratios of refractive power to absorption. The lenses are fabricated by deep X-ray lithography which allows to pattern high aspect ratio structures and gives the possibility to fabricate the lens elements tilted by 90 degrees with respect to each other on a single substrate by tilted double exposure. Nevertheless, the aspect ratio is limited, due to the fact that the columns tend to collapse from capillary forces during fabrication if they exceed a certain height. To overcome this issue and to simplify the fabrication process a new type of lenses as well as a method to fabricate refractive large aperture lenses has been developed recently at IMT. These lenses are fabricated out of a structured polyimide film which is cut into a calculated shape and rolled around a glass-fiber core. The structure on the film itself consists out of triangular shaped ribs. The lenses provide the advantage of Fresnel lenses and also provide a point focus through their approximate rotational symmetry. The full width at half maximum (FWHM) of the focal spot of such lenses is mainly determined by the height of the triangular ribs. Such X-ray optical elements are well suited to be used as condenser lenses, because they provide efficient illumination of an area in the exit working distance. To increase the lens performance, the lens fabrication process has been optimized. In the paper we provide information on how the lenses where improved and present results from tests with X-ray tube sources.