Cycloalkanes are candidates for immersion fluids because of their potential for low absorption in the 193-nm region and for a high refractive index (RI). We have developed an empirical correlation between refractive index and density of alkanes, which allows a prediction of the best candidates for immersion fluids based on the alkane structure. In particular, the correlation reveals that polycycloalkanes such as perhydrophenanthrene (PHPh) and perhydrodropyrene (PHPy), which have a higher RI than linear or cyclic alkanes, will be excellent candidates for immersion fluids at 193 nm. Therefore, PHPh and PHPy were synthesized by exhaustive hydrogenation of phenanthrene and pyrene. However, methods for the purification of the synthesized and commercial alkanes such as cyclodecane (CYD), cyclohexane (CYX), pentane (PNT), and decalin (DEC) are required in order to determine the actual absorption of candidates at 193 nm. The presence of an absorbing impurity at 193 nm can cause the premature elimination of otherwise excellent potential candidates. A rather subtle impurity is molecular oxygen, which does not itself absorb at 193 nm, but which forms complexes with alkanes that do absorb at 193 nm. In this case, the "impurity" is readily eliminated by simple purging with nitrogen or argon gas.
In a search for alkane candidates for 193 nm immersion fluids, several alkanes and cycloalkanes were synthesized,
purified and screened to ascertain their absorption at 193 nm, refractive index and temperature dispersion coefficient in
the context of the actual application. In general, cycloalkanes, and more specifically polycycloalkanes, possess a higher
refractive index than do linear alkanes. Decalin, cyclodecane, perhydrophenanthreme (PHP), perhydrofluorene (PHF)
and perhydropyrene (PHPY) are examined as potential second and third generation immersion fluids. The use of
perhydropyrene, which possesses a high refractive index of 1.7014 at 193 nm, may be limited as an immersion fluid
because of high absorption at 193 nm. Mixtures of cycloalkanes can lead to a higher enhancement of the refractive index
together with a decrease on the viscosity. Exhaustive purification of the fluids is a critical step in determining the real
absorption of the different fluids at 193 nm. Two simple purification processes of these cycloalkanes were developed
that led to low absorption fluids in the VUV region. The possibility of forming the oxygen complex in aerated fluids was
reduced by purging samples with argon or nitrogen. This easy elimination of the oxygen complex shows the weak
bonding nature of this complex.
There is a current need for high refractive index (RI) materials that can be used in aqueous systems for improving 193-nm immersion photolithography. Although heavy metal salts such as Ca2+ and Ba2+ 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 therefore investigate the use of crown ethers to enhance the solubility of these cations. Most of crown ethers are soluble in water, are inexpensive materials, and are available commercially. 15-crown-5-ether and 12-crown-4-ether are liquids at room temperature and therefore can be used as neat immersion fluids without dilution in water. Saturation of crown ethers with inorganic salts do not lead to any increase in the refractive index due to their low solubility in such an apolar medium. Thus, the use of inorganic salts as refractive index enhancement agents does not seem to be a desirable proposition in the present case. Instead, the use of crown ethers or their derivatives can be an alternative system, since these compounds have properties such as density, viscosity, and boiling point similar to aqueous media.
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 Ca2+ and Ba2+ 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 12-crown-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.
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