Thin-film optimization strategy in high numerical aperture optical lithography, part 2: applications to ArF

Abstract

The methodology of the optimization of a thin-film stack for high-NA optical lithography is different from that of the conventional low-NA case, since there are many new factors that need to be considered simultaneously. These include polarization dependence of the reflection coefficient, impact from variation of the angle of incidence, low energy coupling efficiency at the air/resist interface for TE waves, and critical dimension (CD) variation resulting from the bulk effect. In this work, we show that in the extreme high-NA regime, it is hardly possible with existing materials to realize a perfect top antireflection coating that satisfies simultaneous requirements of minimizing the swing effect as well as maximizing throughput and image contrast. Fortunately, for immersion lithography, it is very probable to realize such a top antireflection coating, even when NA is close to the physical limit. We also show that the impact from variation of the angle of incidence can be overcome by implementing top/bottom antireflection coating optimized at low/high angles of incidence, or vice versa. We finally show that CD variation resulting from the bulk effect can be compensated in half a swing period by fine tuning the swing effect.