Simulation study of the NA/σ's dependence of DOF for 193-nm immersion lithography at 65-nm node

Recently, ArF immersion lithography has been considered as a promising method after ArF dry lithography by a factor of refractive index n of the liquid filled into the space between the bottom lens and wafer, in the case of 193-nm exposure tools, water (n=1.44) has been found as the best liquid. We explore the NA/σ's dependence of depth of focus (DOF) under 3/4 annular and 3/4 quasar illumination by resist imaging simulation. Line/space pairs of line-to-space ratios 1:1, 1:2, 1:4 on binary mask are considered. Finally, we explored the high NA's dependency of DOF and gave the explanation for the peak value of DOF through three-beam imaging process, MicroCruiser 2.0, Prolith version 8.0.2 and k₂ factor based on the Rayleigh equation.