Feasibility study on immersion system using high-index materials

ArF water immersion systems with a numerical aperture (NA) of more than 1.3 have already been introduced for the node up to 45-nm half-pitch production. For next-generation 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 because it is effective in improving the resolution, there are some issues not encountered in a water immersion system. The issues associated with LuAG are its availability and intrinsic birefringence. Fluid degradation induced by dissolved oxygen or laser irradiation, lens contamination, and residual fluid on a wafer are the specific issues associated with second-generation fluids. In this article, we describe the current status of the above issues and discuss the feasibility of an ArF immersion system using high-index materials.