Sticking behavior and transformation of tin droplets on silicon wafers and multilayer-coated mirrors
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Silicon wafer and multilayer-coated mirror samples were exposed to impact of drops of molten tin to examine the adhesion behavior and cleaning possibilities. The sticking of tin droplets to horizontal substrates was examined for different surface conditions in a high vacuum chamber. Silicon wafers without a coating, with thick oxide top layer, and also with differently capped Mo/Si multilayer coatings optimized for reflection at a wavelength of 13.5 nm were exposed to tin dripping. Depending on the substrate temperature and coating, adhesion as well as detachment with self-peeling and self-contraction of spreaded drops was observed. The adhesion strength of solidified tin splats decreased strongly with decreasing substrate temperature. Non-sticking surface conditions could be generated by substrate super-cooling. The morphology of non-sticking tin droplets was analyzed by profilometry. Adhering deposits were converted in situ via induction of tin pest by infection with gray tin powder and cooling of the samples. The phase transition was recorded by photographic imaging. It caused material embrittlement and detachment after structural transformation within several hours and enabled facile removal of tin contamination without coating damage. The temperature-dependent contamination behavior of tin drops has implications for the preferred operating conditions of extreme ultraviolet light sources with collection optics exposed to tin debris.
This study was motivated by the ongoing industrial development of EUV light sources. We are grateful to the molecular and surface physics group at Bielefeld University for general support and for supplying Mo/Si-coated EUV mirror samples. Furthermore, we would like to thank Torsten Feigl and his team at optiX fab for generously providing several ML-coated mirror samples at our request. This research did not receive any specific grant from funding sources in the public, commercial, or not-for-profit sectors.
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