High-Index Immersion Lithography : Preventing Lens Photocontamination and Identifying Optical Behavior of LuAG

A potential extension of water-based 193-nm immersion lithography involves transition to a higher refractive index organic immersion fluid coupled with a higher index last lens element. While considerable progress has been made in improving the photo-durability of the immersion fluid itself, photo-induced contamination of the last lens element caused by laser exposure in the presence of such organic fluids remains a major concern. In this work, we study remediation strategies for such contamination, which would be compatible with conventional lithographic production environments. In general, surface photocontamination layers were found to be highly graphitic in nature, where the first monolayer is strongly bound to the substrate. We have attempted to develop a surface passivation treatment for altering the monolayer chemistry and preventing large-scale contamination, but found such treatments to be unstable under laser irradiation. On the other hand, using hydrogen peroxide as a in-situ cleaning solution has been shown to be extremely effective. We also present first laser-based durability results of LuAG, which is a leading candidate material for high index last element to be used with high index fluids.