Experimental study of EUV mirror radiation damage resistance under long-term free-electron laser exposures below the single-shot damage threshold

Abstract

Edited by M. Zangrando, IOM-CNR and Elettra-Sincrotrone, ItalyKeywords: free-electron laser induced damage; EUV optics; thin films; FELs.Experimental study of EUV mirror radiation damage resistance under long-term free-electron laser exposures below the single-shot damage threshold The durability of grazing-and normal-incidence optical coatings has been experimentally assessed under free-electron laser irradiation at various numbers of pulses up to 16 million shots and various fluence levels below 10% of the single-shot damage threshold. The experiment was performed at FLASH, the Free-electron LASer in Hamburg, using 13.5 nm extreme UV (EUV) radiation with 100 fs pulse duration. Polycrystalline ruthenium and amorphous carbon 50 nm thin films on silicon substrates were tested at total external reflection angles of 20 and 10 grazing incidence, respectively. Mo/Si periodical multilayer structures were tested in the Bragg reflection condition at 16 off-normal angle of incidence. The exposed areas were analysed post-mortem using differential contrast visible light microscopy, EUV reflectivity mapping and scanning X-ray photoelectron spectroscopy. The analysis revealed that Ru and Mo/Si coatings exposed to the highest dose and fluence level show a few per cent drop in their EUV reflectivity, which is explained by EUV-induced oxidation of the surface.

Mechanism of single-shot damage of Ru thin films irradiated by femtosecond extreme UV free-electron laser

“…The energy range of 0.2 − 46 µJ per pulse was chosen so that the highest values create heavily damaged surfaces, while for the lowest values no damage was observed by means of the in-situ optical microscope. More details about the experimental set-up and procedures can be found elsewhere [19,29].…”

“…We study the nature of single-shot damage induced in ruthenium (Ru) thin films by femtosecond XUV pulses generated by FLASH. Ru is chosen as optically favorable material, since it has a high XUV reflectance in a wide range of grazing incidence angles [19] and is relatively resistant to oxidation [20]. Ru is also a standard material for grazing incidence hard X-ray optics [21].…”

Surface structure determination by x-ray standing waves at a free-electron laser

“…As a result, the number of photons per pulse at the sample was on average 5×10 7 with a beam size (≈30 cm behind the XFEL focus) of 150-200 μm FWHM, therefore the corresponding fluence was <1 μJ cm −2 . This fluence was about 5 orders of magnitude smaller than the single shot damage threshold of Mo/Si ML at normal incidence (83 mJ cm −2 ) [40]. Therefore radiation induced sample damage could be excluded.…”

Damage accumulation in thin ruthenium films induced by repetitive exposure to femtosecond XUV pulses below the single-shot ablation threshold

“…Since Ru has a high reflection coefficient for the extreme ultraviolet and soft X-ray wavelength ranges, and has a low oxidation rate, it is a very attractive material for reflective coatings. This work is a continuation of a series of studies of optics durability exposed to FEL radiation below the single-shot damage threshold published here [6] and a detailed analysis of the nature of single shot damage discussed here [7].…”

“…Exposure of the Ru films was performed at the BL2 beamline of the FLASH facility [8]. Details of the experiment can be found elsewhere [6,9]. For irradiations we used 100 fs XUV pulses with a wavelength of 13.5 nm.…”