TiO₂ protective capping for EUV mirrors: superior hydrogen plasma resistance and Sn contaminants removal

The long-term stability of extreme ultraviolet (EUV) lithography mirrors relies on protective capping layers capable of resisting plasma irradiation while simultaneously facilitating the removal of Sn contaminants. TiO₂, Ru, and RuO₂ coatings are comparatively assessed using density functional theory (DFT) and hydrogen-plasma cleaning experiments. TiO₂ exhibits weak Sn adhesion (0.11 eV), enabling complete removal within 8 h under non-damaging power (1 W), whereas Ru/RuO₂ retain persistent residues (>24 h) due to strong chemisorption and catalytic SnH₄ re-deposition. TiO₂ also demonstrates superior hydrogen-barrier properties, including inert adsorption (+0.34 eV), a high diffusion barrier (1.57 eV), and excellent reduction resistance. In contrast, Ru shows strong hydrogen adsorption (-0.64 eV) and low permeation barriers (1.02 eV), while RuO₂ undergoes hydrogen-induced reduction with structural degradation. Comparable Sn-removal kinetics on polycrystalline and single-crystalline TiO₂ further confirm insensitivity to grain boundaries. The unique combination of weak adsorption for contaminants, robust hydrogen impermeability, and reduction resistance in TiO₂ establishes it as a highly effective capping layer for extending EUV mirror lifetime in plasma environments.