ArF water immersion exposure systems with a numerical aperture (NA) of over 1.3 are currently being developed and
are expected to be used for the node up to 45-nm half-pitch. Although there are multiple candidates for the next
generation node, we here focus on ArF immersion lithography using high-index materials. The refractive index of highindex
fluids is typically about 1.64 and is larger than that of fused silica (~1.56). In this situation, the NA is limited by
the refractive index of silica and is at most 1.45. An exposure system with 1.45 NA is not suitable for 32-nm hp node,
but may be used for 37-nm hp node. In spite of this limitation, the system has the advantage of slight alterations from
the current system using water as immersion fluid. On the other hand, high-index lens material is effective to increase
the NA of projection optics further. At present, LuAG, whose refractive index is 2.14, is most promising as high-index
lens material. The combination of high-index fluid and high-index lens material can enhance the NA up to about 1.55
and the exposure system would be available for the 32-nm half-pitch node.
Although high-index immersion lithography is attractive since it is effective in raising resolution, such new materials
should be examined if these materials can be used for high precision projection optics. Here, we have investigated
optical characteristics of high-index materials in order to realize high-index immersion systems.