The great promise of EUV lithography (EUVL) comes from its tremendous reduction in wavelength, small numerical apertures (NAs), and large operational k1 factors. The latter two parameters lead to the very important conclusion of long extendibility of the technology.
Being simply an extension of conventional optical projection lithography, the standard lithographic resolution equation holds. Namely, the resolution R can be expressed as R=k 1 Î» NA , where Î» is the imaging wavelength, NA is the numerical aperture, and k1 is the Rayleigh constant. The Rayleigh constant is affected by several variables, including illumination conditions and mask architecture. Assuming a conservative k1 factor of 0.5, which can be achieved readily with conventional illumination and a binary amplitude mask, the half-pitch resolution of a 0.3-NA EUV (13.5-nm wavelength) system would be 22.5 nm. Using resolution enhancement techniques such as off-axis illumination andâor phase-shift masks, the ultimate resolution k1 factor can be decreased to 0.25, corresponding to a 0.3-NA resolution limit of 11.25 nm.
Even further extendibility can be obtained by pushing the NA to 0.5. Practical 0.5-NA optical designs have recently been demonstrated. At this NA, the resolution limits become 13.5 and 6.75 nm for the k1 = 0.5 and k1 = 0.25 cases, respectively. It is illustrative to view the simulated aerial images to understand the significance of the larger k1 factor compared to that we have grown accustomed to with 248- and 193-nm lithography.
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