Tin-drop contamination was cleaned from multilayer-coated mirrors byinduction of phase transformation. The {\beta} - {\alpha} phase transition oftin was induced to initiate material embrittlement and enable facile removal ofthick tin deposits. The necessary steps were performed under high-vacuumconditions for an in-situ demonstration of the removal of severe tincontamination from optics used for reflection of extreme ultraviolet light.Molten tin of high purity was dripped onto mirror samples, inoculated withsmall seed particles of gray tin and then cooled to temperatures in the rangeof -25 {\deg}C to -40 {\deg}C. As recorded by photographic imaging, the dropswere converted in an evacuated chamber to gray tin by induction of tin pestleading to their disintegration within a few hours. They could then be easilycleaned or fell off from the surface without causing any damage of themultilayer coating. Cleaning of tin contamination from the mirrors with almostcomplete structural transformation of the tin drops and subsequent removal bypuffs of dry gas could be achieved within a day. The fraction of area coverageof untransformed tin remaining on the samples after cleaning was evaluated fromthe images and generally found to be well below 1%. After tin dripping, phasetransition and cleaning, analysis of the reflectance of a Mo/Si-coated mirrorwith measurements at wavelengths of 13.6 nm and 13.5 nm showed a reduction byonly 0.5%, with an upper limit of 1%.
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