Atomistic insights on hydrogen plasma treatment for Sn surfaces by first-principles calculations

Abstract

Extreme ultraviolet (EUV) lithography is essential for semiconductor manufacturing to create intricate patterns on silicon wafers by employing a light generated by laser irradiation of Sn droplets. Hydrogen radical etching is a promising way to clean the Sn debris on collector mirrors, but the underlying mechanism remains unclear. First-principles calculations were used to investigate the effects of various hydrogen species within Sn surfaces. The sub-surface reactive hydrogen ions (H₂⁺ and H₃⁺) significantly expanded the Sn surface areas, and they diminished the structural order because of electron transfer from Sn atoms to the reactive hydrogen ions. In contrast, the sub-surface hydrogen radicals (H·) and molecular hydrogen (H₂) induced minor structural changes, while the passivation of the Sn surfaces by hydrogen radicals strengthened the surface structure. The calculated average adsorption energies for sub-surface H₃⁺ ions closely matched the experimentally observed threshold ion energy for Sn-etching, and H₂⁺ ions required less energy to destabilize the Sn surface structures than H₃⁺ ions did. The simulation results are consistent with previous experimental observations of a low Sn-etching rate in radicals-only systems and support recent experimental studies emphasizing the importance of reactive ion etching in enhancing Sn-etching rates.