The effect of thermal annealing on the surface roughness and optical properties of periodic Mo/Si multilayers with TiO2 layer for EUV application

In this study, the periodic Mo/Si films and TiO₂ film were deposited by RF magnetron sputtering and atomic layer deposition at Si substrate, respectively. Effects of the thickness of the TiO₂ film and annealing temperature (200~400 °C) in H₂ gas on the microstructure, surface roughness and optical properties were investigated. It was found that the EUV reflectivity of all samples were decreased after annealing in H₂ gas. The EUV reflectivity of the as-deposited, 200 °C and 300 °C annealed periodic Mo/Si films was 67.8%, 18.7% and 12.9%, respectively. The EUV reflectivity of the as-deposited, 200 °C and 300 °C annealed periodic Mo/Si films with TiO₂ film (thickness 1 nm) was 66.4%, 64.1% and 32%, respectively. The interface diffusion of annealed Mo/Si films and Mo/Si films with TiO₂ film were observed by HRTEM and it was increased with increasing the annealed temperature. It obviously caused the difference in the EUV reflectivity of the annealed Mo/Si films and Mo/Si films with TiO₂ film. Based on the reflectivity results of the as-deposited and annealed investigation, the TiO₂ thin film by ALD as the protective film for EUV application was been confirmed.