1 January 2004 Advantage and feasibility of immersion lithography
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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.
Soichi Owa, Hiroyuki Nagasaka, "Advantage and feasibility of immersion lithography," Journal of Micro/Nanolithography, MEMS, and MOEMS 3(1), (1 January 2004). https://doi.org/10.1117/1.1637593

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