Refractive Optics for Hard X-ray Transmission Microscopy

Abstract

For hard x‐ray transmission microscopy at photon energies higher than 15 keV we design refractive condenser and imaging elements to be used with synchrotron light sources as well as with x‐ray tube sources. The condenser lenses are optimized for low x‐ray attenuation—resulting in apertures greater than 1 mm—and homogeneous intensity distribution on the detector plane, whereas the imaging enables high‐resolution (<100 nm) full‐field imaging. To obtain high image quality at reasonable exposure times, custom‐tailored matched pairs of condenser and imaging lenses are being developed. The imaging lenses (compound refractive lenses, CRLs) are made of SU‐8 negative resist by deep x‐ray lithography. SU‐8 shows high radiation stability. The fabrication technique enables high‐quality lens structures regarding surface roughness and arrangement precision with arbitrary 2D geometry. To provide point foci, crossed pairs of lenses are used. Condenser lenses have been made utilizing deep x‐ray lithographic patterning of thick SU‐8 layers, too, whereas in this case, the aperture is limited due to process restrictions. Thus, in terms of large apertures, condenser lenses made of structured and rolled polyimide film are more attractive. Both condenser types, x‐ray mosaic lenses and rolled x‐ray prism lenses (RXPLs), are considered to be implemented into a microscope setup. The x‐ray optical elements mentioned above are characterized with synchrotron radiation and x‐ray laboratory sources, respectively.