Chemical Aspects of EUV/x-ray Optics Contamination by Carbon

One of the most interesting and serious chemical problems associated with the operation of high-power extreme ultraviolet/x-ray radiation sources is the contamination of the optical elements that guide, direct or focus its beams. It is visually manifested by the formation of a brownish-black deposit on the surface of the elements, which leads to a decrease in their reflectivity or transmittance, depending on whether the radiation is reflected from them or passes through them. Chemical analyses have revealed elemental carbon as the main constituent of the deposit. It is formed by radiolysis of the carbon containing molecules (e.g., hydrocarbons and carbon oxides) among residual gases. The mechanisms of these processes represent the subject of extensive research. The decomposition can occur in the gas phase (vacuum) or in a layer condensed on an optical surface. Whether the initiation of these processes is dominated by photons or by secondary electrons is also being investigated. The results obtained are then intended to be used to develop and implement techniques to remedy the reduction (or even complete loss) of functionality of optical elements caused by the contamination.