IBS deposition of dense fluoride coatings for the vacuum ultraviolet free electron laser

Abstract

Fluoride materials like Magnesium fluoride and Lanthanum fluoride exhibit unique properties for applications in mirror and anti reflecting coatings in the VUV spectral range (120- 230 nm). These large band gap materials provide low absorption and a usable refractive index contrast. Common deposition methods are thermal evaporation and electron beam evaporation. A columnar microcrystalline structure with a significant porosity is observed for such coatings. Furthermore, a high sensitivity for contamination processes resulting in an increasing absorption is often perceived. Investigations of mirror systems exposed to a harsh environment like the storage ring free electron laser at ELETTRA show a rapid degradation in respect to a reduced reflectivity, an increased hydrocarbon contamination, and a formation of colour centres. An improved performance of the fluoride coatings could possibly be expected for films, which do not have the polycrystalline columnar structure. Ion beam sputtering deposition of fluoride materials demonstrated its applicability to deposit dense amorphous fluoride coatings down to 193 nm. An IBS deposition plant with a Kaufman ion source, using a reactive fluorine environment, is used to grow fluoride layers with comparably low absorption values. Single and multilayer coatings were optically characterised after deposition, exposed to synchrotron radiation at ELETTRA, and characterized again after irradiation. However, the first set of irradiated multilayer mirrors showed a strong degradation of reflectivity and a strong hydrocarbon contamination. Colour centres were not observed.