Abstract
where R is the half-pitch resolution of the image, k1 is a constant that depends on the resist process and exposure method, ┣ is the exposure wavelength, and NA is the numerical aperture of the projection optic. According to Rayleigh's equation, there are three ways to enhance the resolution. The first is to shorten the exposure wavelength such as extreme ultraviolet lithography (EUVL). The second is to improve the k1 value, for example, using the double-patterning technique. The third is to increase the numerical aperture (NA) as ArF immersion lithography. It has already realized the NA up to 1.35 and moreover can increase the NA using high-index materials. In this chapter, high-index immersion lithography with the NA over 1.45 is focused. The NA is actually determined by the acceptance angle of the lens and the refractive index of the medium surrounding the lens and is given by eq. (2).
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