The Scanning Transmission X-Ray Microscope at BESSY II

Abstract

Combining near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and zone plate imaging, scanning transmission X-ray microscopes (STXMs) provide both high chemical sensitivity and a spatial resolution on the order of 50nm while delivering a relatively low radiation dose to the specimen. This thesis describes the construction and characterization of the new STXM and its monochromator at the undulator U41 of the BESSY II storage ring.Calculations of the image formation in the STXM show that a very high photon rate in the focal spot on the order of 109photons/s is required to obtain images with high spatial resolution in reasonable acquisition times. To deliver such photon rates at good spectral resolution, a novel monochromator has been developed. It consists only of a plane mirror and a plane grating with varied line density and has no entrance or exit slits. The large field illuminated homogeneously by the monochromator makes it possible to acquire high resolution scan images by moving the zone plate. The sample is located in air to be able to image hydrated specimens. The transmitted radiation is measured using a pn-CCD detector. Its position resolution makes it possible to record images with advanced contrast mechanisms like darkfield or differential phase contrast.From NEXAFS spectra of carbon dioxide, a spectral resolution of 1400 has been measured at a photon energy of 296eV. First images acquired in amplitude contrast, differential phase contrast and elemental contrast have been recorded. Images of test structures show sub-100nm spatial resolution.