Cleaning Sn off from EUV optics by plasma etching method
Sn is more preferable among fuel materials for EUV light source since Sn has better conversion efficiency than other materials (Xe and Li). However, Sn is a condensable material so that it builds up on the EUV optics (collectors or mirrors). Debris build-up will roughen the surface and accordingly result in reflectivity loss. Therefore, cleaning is very essential step for HVM (high volume manufacture) level Sn EUV source.
The house modified chamber, GALAXY was first used for this study. With chlorine and argon plasma, etch rates of different material were measured and the optimal condition has been found so that Sn can be selectively removed from Ru mirror surface. Using a small mock-up collector in the GALAXY system, cleaning rate dependence on the distance from a plasma source was investigated as well.
Based on the preliminary studies, more study about cleaning and plasma transport are in progress. For the more realistic experiment, a real commercial DPP EUV system (XTS 13-35) is used. By measuring the cleaning rates along the distance from the (cleaning) plasma source using a comparable size of mock-up collector to real ones, we investigate the means of cleaning Sn debris with plasma based method. Understanding plasma transportation through the mirror shells which have different gap width and modeling surface physics between Sn and chlorine plasma is a key contribution to science during this project.