Proceedings Article | 18 May 2009
KEYWORDS: Zone plates, Silicon, Free electron lasers, Optical filters, Microscopes, Polymethylmethacrylate, Zirconium, Aluminum, Absorption, Nickel
EUV- and X-ray sources with laser like properties, e.g. free electron lasers, offer possibilities for many new experiments. In
order to successfully plan and perform experiments at these high flux sources, 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, thus introducing 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 and a silicon nitride structured sample). The resulting focal spot size was estimated
from the acquired images to be in the range of 1 μm - 3μm in diameter. 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.
