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...