William A. Wojtczak

The effect of added salts on the optical properties of water for high index immersion lithography fluids

In immersion lithography, a high index fluid is used to enable increases in the numerical aperture (NA) of the imaging system and decrease the minimum printable feature size. Water has been used in first generation immersion lithography at 193nm to reach the 45nm node. To reach the 38 and 32nm nodes, fluids with a higher index than water are needed. Ionic additives have a higher index than water and can be added to water in high concentration to obtain a high index fluid. Generally, addition of a high index additive leads to a higher absorbance. The next generation immersion fluid should have both a high index and low absorbance. Therefore, both the index and absorbance of a series of ionic additives in water were characterized to identify an additive with a high index and low absorbance at 193nm. This study revealed that the anion dictates the optical properties of the additive. Of the anions studied, the methylsulfonate anion has the desired characteristics and was dissolved in water at high concentrati...

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.