Five mirror systems with a reduction magnification( M=+$\frac(1)(5)$) are designed for deep-UV optical lithography. Initially, numerical solutions of the five spherical mirror systems have been obtained from the conditions of all zero 3rd order aberrations, and all zero 3rd order aberrations and zero 5th order spherical aberration.
For the five spherical mirror system with all zero 3rd order aberrations, the 3-dimensional solution-domain is obtained. In these solutions, we select solutions which have as less residual aberrations and smaller central obscuration as possible and the aspherization is carried out to the last two spherical mirrors to obtain a system that has as higher NA as possible and, for the telecentricity of the image space, the stop is shifted to the first focal plane. By these considerations we have obtained a final system whose NA is 0.45, the focal length is -100mm, the object field diameter is 26mm, the central obscuration is about 25% and the resolution is about 650 cycles/mm at the 50% MTF value criterion and the depth of focus of 0.8㎛ for the nearly incoherent illumination(δ=1.0) and the wavelength of 0.193 ㎛(ArF excimer laser line). But, the system length is about 200cm and the largest mirror diameter of the system is about 93cm.
Further, works on the five spherical mirror system with all zero 3rd order aberrations and zero 5th order spherical aberration give the two dimensional distribution of the solutions. Similarly, the most suitable solution is selected and the aspherization and the shift of the stop are carried out. In this way, we have obtained a final system whose NA is 0.45, the focal length is -100mm, the object field diameter is 26mm, the central obscuration is about 3%. The resolution of this system is about 600 cycles/mm at the 50% MTF value criterion and the depth of focus of 0.8 ㎛ for the nearly incoherent illumination(δ=1.0) and the wavelength of 0.193 ㎛(ArF excimer laser line). And the system length is about 138cm and the largest mirror diameter is about 80cm.
As the conclusion, the latter five mirror system is more suitable for the optical lithography than the former five mirror system in the system length and the largest mirror size, although the MTF of the latter is slightly inferior to that of the former.
