Advantage and feasibility of immersion lithography

Immersion lithography has an advantage in the numerical aperture of optics by a factor of refractive index n of the liquid filled into the space between the bottom lens and wafer. In case of 193-nm exposure tools, water $(n=1.44)$ has been found as the best liquid. It is shown, by using imaging simulations, that ArF (193-nm) immersion lithography $(NA=1.05$ to 1.23) has almost equivalent performance to F2 (157-nm) dry $(NA=0.85$ to 0.93) lithography. Issues in the ArF immersion exposure tools are discussed with fluid-dynamic and thermal simulations results. In the fundamental issues, there seems to be no showstoppers so far, however, there exist several challenges to realize viable exposure tools. © 2004 Society of Photo-Optical Instrumentation Engineers.