New high index fluids for immersion lithography

Immersion lithography at 193nm has rapidly evolved from a novel technology to the top contender for the 45nm device node. The likelihood of immersion implementation in semiconductor manufacturing has raised interest in expanding its capabilities. Extending resolution requires immersion fluids with higher refractive indices than those currently available. We have therefore sought substances which, when added to water, increase the refractive index at 193nm without increasing the absorbance and viscosity beyond acceptable limits. This work explores the relationship between index of refraction and absorbance, with specific focus on the identification of fluids that have a high index and low absorbance. The majority of the fluids studied either have prohibitively high absorbance values or material properties that would be incompatible with current fluid handling systems. However, a class of methylsulfonate salts was identified with optical and material properties approaching the target values. Fluid testing and imaging is included to confirm the resolution enhancing capability of these new high index fluids.