TiO 2 Protective Capping for EUV Mirrors: Superior Hydrogen Plasma Resistance and Sn Contaminants Removal

Abstract

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. TiO 2 , Ru, and RuO 2 coatings are comparatively assessed using density functional theory (DFT) and hydrogen-plasma cleaning experiments. TiO 2 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 SnH 4 re-deposition. TiO 2 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 RuO 2 undergoes hydrogeninduced reduction with structural degradation. Comparable Sn-removal kinetics on polycrystalline and single-crystalline TiO 2 further confirm insensitivity to grain boundaries. The unique combination of weak adsorption for contaminants, robust hydrogen impermeability, and reduction resistance in TiO 2 establishes it as a highly effective capping layer for extending EUV mirror lifetime in plasma 2 environments.