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Improvement of tool reliability and uptime is a current focus in developmentof extreme ultraviolet lithography. The lifetime of collection mirrors forextreme ultraviolet light in tin-based plasma light sources is limitedconsiderably by contamination with thick tin deposits that cannot be removedsufficiently fast by plasma etching. For tin droplet splats sticking to largesubstrates, we have developed and compared several efficient cleaningtechniques based on cryogenic cooling. A silicon carbide substrate anddifferent silicon wafer samples with up to 6 inch diameter with the surfaceuncoated, multilayer-coated, unstructured and grating-structured were tested.After tin dripping onto heated samples, embrittlement of droplet contaminationis induced in-situ by stresses during phase transformation, following theinitiation of tin pest with seed crystals of gray tin. Conversion of initiallyadhesive deposits to loose gray tin has been reached in less than 24 hours onall tested surfaces by continuous cooling with cold nitrogen vapor totemperatures in the range of -30 to -50 {\deg}C. Alternatively,stress-initiated tin-removal by delamination of beta-Sn droplet splats has beenattained via contraction strain induced by strong cooling to temperatures ofaround -120 {\deg}C. Profilometry has been used to analyze the bottom side oftin droplet splats removed from a grating-structured wafer. The in-situ tincleaning techniques give results comparable to fast ex-situ cleaning that hasbeen achieved either by sample immersion in liquid nitrogen or by splat removalafter CO2 snowflake aerosol impact using a hand-held jet-nozzle. Theimplementation of the in-situ phase-conversion concept for the cleaning ofcollector mirrors in commercial light sources for lithography is discussed.

Cryogenic cleaning of tin-drop contamination on surfaces relevant for extreme ultraviolet light collection