  Improvement of tool reliability and uptime is a current focus in development
of extreme ultraviolet lithography. The lifetime of collection mirrors for
extreme ultraviolet light in tin-based plasma light sources is limited
considerably by contamination with thick tin deposits that cannot be removed
sufficiently fast by plasma etching. For tin droplet splats sticking to large
substrates, we have developed and compared several efficient cleaning
techniques based on cryogenic cooling. A silicon carbide substrate and
different silicon wafer samples with up to 6 inch diameter with the surface
uncoated, multilayer-coated, unstructured and grating-structured were tested.
After tin dripping onto heated samples, embrittlement of droplet contamination
is induced in-situ by stresses during phase transformation, following the
initiation of tin pest with seed crystals of gray tin. Conversion of initially
adhesive deposits to loose gray tin has been reached in less than 24 hours on
all tested surfaces by continuous cooling with cold nitrogen vapor to
temperatures in the range of -30 to -50 {\deg}C. Alternatively,
stress-initiated tin-removal by delamination of beta-Sn droplet splats has been
attained via contraction strain induced by strong cooling to temperatures of
around -120 {\deg}C. Profilometry has been used to analyze the bottom side of
tin droplet splats removed from a grating-structured wafer. The in-situ tin
cleaning techniques give results comparable to fast ex-situ cleaning that has
been achieved either by sample immersion in liquid nitrogen or by splat removal
after CO2 snowflake aerosol impact using a hand-held jet-nozzle. The
implementation of the in-situ phase-conversion concept for the cleaning of
collector mirrors in commercial light sources for lithography is discussed.
