Sn etching with hydrogen radicals to clean EUV optics

Abstract

Extreme ultraviolet (EUV) collector optics are an essential part of EUV lithography machines, but are subject to substantial Sn contamination. We demonstrate a method to clean EUV optics from Sn contamination. We use hydrogen radicals to remove Sn at >50 nm/min etch rate. For a Ru surface, the Sn etch rate drops with two orders of magnitude when the Sn layer becomes thinner. We found that a very thin (∼2 nm) layer of Si 3N 4 between the Sn and the Ru layer increases the Sn etch rate to its maximum value. Finally, we have demonstrated that this technique may be used to fully recover the reflectivity of Sn-contaminated EUV mirrors.