Removal of surface contamination from EUV mirrors using low-power downstream plasma cleaning

Abstract

The problem of carbon contamination on extreme ultraviolet (EUV) optics, causing unacceptably low reflectivity in mirrors, must be solved before industry will adopt the technology on a production scale. Breaking vacuum, removing and then cleaning mirrors is a time-consuming and expensive method for dealing with the problem. A safe yet effective in situ method for cleaning EUV optics and maintaining vacuum chamber cleanliness is important for progress in EUV lithography. Carbon contamination has also been a problem for the scanning electron microscopes (SEMs) leading to reduced image quality. The use of low power downstream plasma cleaner has been shown to be effective in removing carbon contamination from SEMs. The plasma dissociates oxygen molecules into neutral oxygen radicals. These radicals flow throughout the SEM vacuum chamber and chemically remove the carbon contamination. Since the process works by chemical etch and not by sputter etch, the capping layer on EUV mirrors will not be damaged by the cleaning process. The production of chemically etching oxygen radicals by plasma cleaning was measured using a quartz crystal microbalance. The effectiveness of the downstream plasma cleaning process was also tested on EUV mirrors.