PURPOSE: To obtain sufficient imaging performance when fluorite is used as an optical member for photolithography, such as a stepper, by respectively specifying a refractive index difference, RMS of wave front aberration and optical path difference by double refraction.
CONSTITUTION: The fluorite for photolithography to be used in a specific wavelength band of ≤350nm is so formed as to satisfy the following conditions: Firstly, the refractive index Δn is ≤5×10-6. Secondly, the value of the RMS (root-mean-square) of the wave front aberration is ≤0.015λ after power component correction. Thirdly, the optical path difference by the double refracting power in any direction of three coordinate directions is ≤10nm/cm. The optical system using quartz glass as a single blank for the optical system using a KrF excimer laser (248mn) as a light source 1 is most popularly used at present as a projecting lens 4 for the photolithography. However, there is a significant effect in removing chromatic aberrations by building the fluorite into such optical system. The latitude of the monochromaticity of an exposure light source of the stepper is, therefore, widened by using this fluorite and the drastic cost reduction of the illumination system is attained.
MIZUGAKI TSUTOMU
SHIOZAWA MASAKI
TAKANO SHUICHI
NISHIKAWA HIDEMI
OYO KOKEN KOGYO KK