In-Situ and Non-Destructive hydrogen plasma cleaning of Sn from the Protection layers of EUV Mirrors: The mechanisms of surface interaction

EUV lithography, capable of achieving 2 nm nodes, faces challenges of reduced reflectivity caused by Sn deposition on EUV mirrors. RuO 2 has demonstrated superior performance as a protective layer for EUV mirrors. This study employs density functional theory calculations and plasma cleaning experiments to assess the efficacy of hydrogen plasma in removing strongly adsorbed Sn contaminants from RuO 2 surface. Theoretical results indicate that Sn desorption from RuO 2 occurs via interactions with hydrogen atoms, transitioning Sn from a metallic state to an H-hybridized molecular state. Experiments confirm that RuO 2 exhibits lower catalytic activity than Ru, mitigating Sn re-deposition induced by SnH x dissociation. Notably, residual Sn on RuO 2 surface forms SnO 2 crystals due to ion thermal effects. Complete Sn removal and suppression of re-deposition were achieved using a combination of superficial blowing and enhanced active particle methods. However, compared to Ru, reducing H atoms in RuO 2 induces oxygen vacancies, severely degrading its performance. Based on the properties of Ru/RuO 2 , this study identifies TiO 2 as a promising protective layer, exhibiting efficient cleanability (for Sn), resistance to re-deposition, and strong reduction stability, as preliminarily validated by experiments. These findings provide valuable insights into the maintenance of EUV mirrors and the selection of protective coatings.