Thermal stability of B-based multilayer mirrors for next generation lithography

The evolution of microstructure and reflective properties in a La/B4C and LaN/B4C multilayer was studied at elevated temperatures up to 800 °C. It was shown, that the observed opposite period thickness changes in La/B4C and LaN/B4C multilayers during annealing are based on structural modifications and chemical reactions at the interfaces. For T > 400 °C the period thickness of the La/B4C multilayer decreased, while it increased drastically in the LaN/B4C multilayer, which is explained by the formation of LaB6 crystallites and amorphous BN compounds, respectively. These thermally induced processes also lead to reflectivity drops at the wavelength of ~ 6.7 nm for both investigated systems. Even after annealing at 800 °C for 10 h the LaN/B4C multilayer showed an EUV reflectance of 12.6%, with is significantly higher than the La/B4C multilayer (2.3%), pointing up their higher thermal resistance.