Polycycloalkanes as potential third-generation immersion fluids for photolithography at 193 nm

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, perhydrophenanthrene (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 of the viscosity. Exhaustive purification of the fluids is a critical step in determining the real absorption of the different fluids at 193 nm. Even very small traces of impurities possessing a high absorption coefficient at 193 nm can lead to an unacceptably high level absorption at 193 nm, previously incorrectly attributed to the alkane instead of the absorbing impurities.