Methods for the synthesis and purification of polycycloalkane candidates for photolithography immersion fluids at 193 nm: requirements for removal of oxygen

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 be- tween refractive index and density of alkanes, which allows a prediction of the best candidates for immersion fluids based on the alkane struc- ture. 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 candi- dates 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 im- purity at 193 nm can cause the premature elimination of otherwise ex- cellent 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. © 2007 Society of