Immersion lithography based on water has been very successful. The maximum numerical aperture of 1.3 or 1.35 has allowed this technology to be widely used in the production of 45-nm half-pitch devices. Encouraged by this success, researchers are interested in further boosting NA through the use of fluids and lens materials with higher indices of refraction. In theory, these second-generation immersion technologies could be used in the 32-nm half-pitch node. Table 11.1 shows the resolution requirements for the 45- and 32-nm nodes as described by ITRS 2007. Although DRAM, Flash, and Logic have slightly different specifications, the fundamental challenge for the 32-nm node is to print 32-nm dense lines and 35-nm contact holes. According to the Rayleigh equation (resolution=k 1 Î»âNA ), a numerical aperture larger than 1.55 will be needed in order for k 1 to be larger than 0.25 for the 32-nm half-pitch node. Detailed simulations suggest that numerical apertures of at least 1.65 will be required to print 32-nm patterns with reasonable process windows.
As discussed in Chapter 1, the maximally effective NA of an immersion system is limited by the refractive index (RI) of the lens, the fluid, and the resist. Thus, high refractive index materials are the key to further increases in NA. Currently, the last lens element is composed of fused silica, which has a refractive index of 1.55 at 193 nm. Conventional 193-nm resists have refractive indices around 1.70.
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