Induction of tin pest for cleaning tin-drop contaminated optics

Abstract

Highlights Tin pest induction leads to embrittlement of tin drops after β → α Sn transformation.  The purity grade influences the transformation speed of tin very strongly.  Tin drops on multilayer-coated optics disintegrate after β → α Sn transformation.  Tin drop contamination of optics is cleaned via phase transformation at -24 °C.  Reflectance of multilayer-coated mirrors is restored after tin drop transformation. Graphical Abstract AbstractTin pest, the allotropic β → α phase transformation of tin, was examined for use in cleaning of tin-contaminated optics. Induction of change in material structure led to disintegration of tin samples into pieces and powder. The transition times were studied for tin drops of different purity grades, using inoculation with α-Sn seed particles, also after prior mechanical deformation and surface oxide removal. For tin of very high purity levels fast nucleation within hours and full transformation within a day could be achieved during cooling at -24 °C, resulting in strong embrittlement of the material. Tin dripped onto samples of multilayer-coated optics as used in extreme ultraviolet lithography machines was made cleanable by phase transition after inoculation and cooling. The reflectance of multilayer-coated mirrors was found to decrease by no more than 1% with this cleaning method.

In situ transformation and cleaning of tin-drop contamination on mirrors for extreme ultraviolet light

“…After the samples were taken out of the chamber, inspection showed that the contamination was indeed fully removed without any residues. It was shown previously 20 that such fast transformation rates to α-Sn can be achieved provided the material purity grade of tin is 99.999%. The required purity for operation of the tin droplet generator in the commercial source is also at a level of at least 99,999%.…”

“…Gray tin seed powder was prepared by temperature cycling of tin pieces with repeated transformation between the α-Sn and β-Sn phase, as was described previously. 17 In most cases, but not always, inoculation of tin drops with α-Sn seeds was done within a few hours after tin dripping.…”

“…Previously, one of the authors has reported on a cleaning scheme based on the induction of tin pest by seed particles 19 where solidified drops of high-purity tin break up by embrittlement during phase transformation of β-Sn to the allotrope α-Sn. 20 This concept was also independently proposed for collector cleaning in two, meanwhile abandoned patent applications where, however, the important nucleation initiation process was not considered. 21,22 Recently, we have described first results for in-situ cleaning under high vacuum of tin drop deposits on small samples of silicon (Si) wafers with different multilayer coatings.…”

“…The basic concept for cleaning tin-drop contaminated optics in this way, and the related material science aspects of this method were presented very recently in a communication by one of us. 17 When tin in its normal metallic phase, β-Sn, is cooled to temperatures in a range of about -20 °C to -50 °C, it can undergo a phase transformation to gray tin, a rather brittle semimetal. 18 This phase, α-Sn, is the stable phase of tin with slightly lower entropy in comparison to β-Sn at temperatures below 13 °C.…”

“…The extreme brittleness of converted tin deposits and its breakup by cracking after transformation to the α-Sn phase is the attractive feature for the purpose of optics cleaning. In the previous work 17 the transformation of tin drops was studied with its dependence on material purity as well as its use for ex-situ cleaning of ML-coated EUV mirror samples from Sn drops in air. However, cleaning should ideally be done in situ inside of the source chamber to avoid long system downtimes originating from venting, pumping and module swap cycles.…”