Advances in X-ray microscopy at FLASH using transmissive optics

Abstract

In order to successfully plan and perform experiments at high flux EUV- and X-ray sources, e.g. free electron lasers, it is necessary to know which kind of optics, exposed to the full beam, can be used. Due to the high intensities, it is not clear, whether transmissive diffractive optics are applicable, because these optics are usually fabricated on thin membranes which are therefore subject to additional absorption in the desired energy range. Since diffractive optics, especially zone plates, offer the possibility to achieve small spots when used as a focussing element and can also achieve good image quality in microscopic setups, their usage would facilitate many experiments, especially for their easy handling. As a proof of concept, we set up a zone plate based scanning transmission microscope at the unfocussed beamline BL3 at FLASH (DESY/Hamburg). The operating wavelength was 32 nm and 13.8 nm, respectively. While the first attempt, utilizing a zone plate composed of PMMA on silicon substrate failed due to ablation of the PMMA, a second zone plate (chromium on silicon nitride) was successfully used to focus the beam onto different samples (e.g. nickel-mesh, a silicon nitride structured sample and nickel structures on silicon nitride). The resulting focal spot size was estimated from the acquired images to be in the range of 1 μ–3 μ in diameter. Beside the scanning approach, a full-field transmission microscopy setup was tested. A preliminary image in full field mode was taken with a second zone plate, showing features of a Siemens-star pattern down to approximately 120 nm. After several hours of exposure, no damage was visible to the optics. Beside the optics, different filters (Silicon/Zirconium, Zirconium and Aluminum) have been placed in the beam to evaluate possibilities to further reduce intensity which may be necessary if sensitive detectors are involved. All of the filters withstood the irradiation during the whole experiment.