  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. Dependent on 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.
