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.