Induction of tin pest for cleaning tin-drop contaminated optics

TL;DR

This paper explores using the tin pest phase transformation to clean tin-contaminated optics, demonstrating effective removal of tin with minimal impact on mirror reflectance, especially in high-purity tin samples.

Contribution

It introduces a novel method for cleaning tin-contaminated optics by inducing tin pest transformation, with detailed study on transformation times and effectiveness across different tin purities.

Findings

High-purity tin transforms within a day at -24°C

Optics remain nearly unaffected with less than 1% reflectance loss

Effective cleaning achieved through phase transition and cooling

Abstract

Tin pest, the allotropic {\beta} - {\alpha} 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 {\alpha}-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 {\deg}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.