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

TL;DR

This study demonstrates an in-situ method to clean severe tin-drop contamination from EUV mirrors by inducing a phase change in tin, enabling easy removal without damaging the mirror's multilayer coating.

Contribution

It introduces a novel in-vacuum technique to transform and remove tin contamination from optics used in EUV lithography, preserving mirror reflectance.

Findings

Effective removal of tin drops via phase transformation within a day

Less than 1% residual tin coverage after cleaning

Mirror reflectance reduced by only 0.5% after cleaning

Abstract

Tin-drop contamination was cleaned from multilayer-coated mirrors by induction of phase transformation. The {\beta} - {\alpha} phase transition of tin was induced to initiate material embrittlement and enable facile removal of thick tin deposits. The necessary steps were performed under high-vacuum conditions for an in-situ demonstration of the removal of severe tin contamination from optics used for reflection of extreme ultraviolet light. Molten tin of high purity was dripped onto mirror samples, inoculated with small seed particles of gray tin and then cooled to temperatures in the range of -25 {\deg}C to -40 {\deg}C. As recorded by photographic imaging, the drops were converted in an evacuated chamber to gray tin by induction of tin pest leading to their disintegration within a few hours. They could then be easily cleaned or fell off from the surface without causing any damage of the multilayer coating. Cleaning of tin contamination from the mirrors with almost complete structural transformation of the tin drops and subsequent removal by puffs of dry gas could be achieved within a day. The fraction of area coverage of untransformed tin remaining on the samples after cleaning was evaluated from the images and generally found to be well below 1%. After tin dripping, phase transition and cleaning, analysis of the reflectance of a Mo/Si-coated mirror with measurements at wavelengths of 13.6 nm and 13.5 nm showed a reduction by only 0.5%, with an upper limit of 1%.