Amplification of the index of refraction of aqueous immersion fluids with crown ethers: a progress report

Abstract

There is a current need for high refractive index (RI) materials that can be used in aqueous systems for improving 193 nm immersion photolighography. Although heavy metal salts such as Ca 2+ and Ba 2+ have the potential to substantially increase the RI of aqueous solutions, the water solubility of these salts with common anions is often too low to achieve concentrations that significantly increase the RI to the desired values. We have therefore investigated the use of crown ethers to enhance the solubility of these cations. Most of the crown ethers are soluble in water, environmentally benign and commercial and inexpensive materials. Details of the preliminary studies on the proposed model system are presented in this paper. 15-crown-5-ether and 12crown-4-ether are liquids at room temperature and therefore can be used as neat liquids as immersion fluids without dilution in water. Saturation of crown ethers with inorganic salts do not lead to any increase of the refractive index due to the low solubility of those in such an apolar media. Thus, the use of inorganic salt as refractive index enhancement agent does not seem to be a desirable proposition in the present case. Instead, the use of crown ethers or their derivates can be alternative system since these compounds have properties, such as density, viscosity and boiling point, similar to aqueous media.