Properties of ultrathin films appropriate for optics capping layers exposed to high energy photon irradiation

The contamination of optical surfaces by photon irradiation in the presence of background vacuum gases shortens optics lifetime and is one of the main concerns for reflection optics used in conjunction with intense light sources, such as high power lasers, 3rd and 4th generation synchrotron sources or plasma sources used in extreme ultraviolet lithography (EUVL) tools. This paper focuses on properties and surface chemistry of different materials, which as thin films could be used as capping layers to protect and extend the lifetime of multilayer mirror optics, especially those exposed to 13.5 nm (92 eV) radiation for EUVL applications. The most promising candidates include single element materials such as ruthenium and rhodium, and oxides such as TiO2 and ZrO2.