Eiji Sakamoto

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.

Hyper-NA imaging in ArF immersion lithography

ArF immersion lithography (Lin, 2004) has emerged as the primary solution for the manufacturing of semiconductor device for 65-nm half-pitch node and beyond. The immersion technique allows the design of projection optics with a numerical aperture that exceeds unity. Pure water is the preferred immersion fluid for the first generation of the immersion exposure tool. The water has good transmissivity and relatively high refractive index of 1.44 at ArF wavelength of 193 nm (Burnett et al., 2004). An NA of up to about 1.3 seems feasible with water. Moreover, high-index materials have recently been proposed for the immersion fluid (Miyarnatsu et al., 2005; Peng et al., 2005) as well as for the lens material (Burnett et al., 2004). With the high-index materials, the immersion technique may ultimately allow an NA that is even larger than the refractive index of water. In this paper, we present analytical results on imaging properties of the ArF immersion lithography in hyper-NA region to provide insight on the potential and challenges of the immersion technique.