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The performance of EUV scanners in the field testifies that formidable obstacles to high-volume EUVL manufacturing have been overcome. A key element in this progress has been the virtual elimination of optics contamination as a major hurdle, a development involving several efforts worldwide and stretching over more than a decade. We will describe the chemical processes that lead to contamination, some of the research efforts directed at understanding them, and the resulting mitigation techniques that were developed. Ionizing radiation creates chemical reactions that contaminate optical surfaces in an atmosphere with trace amounts of water vapor or organic molecules. Organic molecules adsorbed on the optical surface can be cracked by both the energetic radiation and the secondary electrons it produces, creating a contaminating carbon film. Adsorbed water molecules can be activated to generate strong oxidizers that attack the optical surface itself. However, these two processes can counteract each other, and in the presence of a significant partial pressure of organic molecules or a carbon overlayer, the oxidation by radicals from the water vapor can be reduced, or the carbon overlayer can be etched.
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