Keita Sakai

Feasibility of 37-nm half-pitch with ArF high-index immersion lithography

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.

Immersion exposure system using high-index materials

ArF water immersion systems with a numerical aperture (NA) of over 1.3 have already introduced for the node up to 45- nm half-pitch production. For the next generation of lithography, we focus on ArF immersion lithography using high-index materials. At present, LuAG (n=2.14) is the most promising candidate as a high-index lens material. Second-generation fluids (n=1.64) have the sufficient performance as a high-index immersion fluid. The combination of LuAG and a second-generation fluid can enhance the NA up to 1.55 and the exposure system would be available for the 34-nm half-pitch node when k1 is 0.27. Although high-index immersion lithography is attractive since it is effective in raising resolution, there are some issues not encountered in water immersion system. The issues of LuAG are its availability and the intrinsic birefringence. Fluid degradation induced by dissolved oxygen or laser irradiation, lens contamination, and residual fluid on a wafer are the specific issues of the immersion system. In this article, we introduce the current status for the above issues and discuss the feasibility of ArF immersion system using high-index materials.