Cassegrainian-inverse Cassegrainian four-aspherical mirror system (magnification = +1) derived from the solution of all zero third-order aberrations and suitable for deep-ultraviolet optical li

A four-aspherical mirror system with unit magnification is investigated for use in deep ultraviolet (DUV) optical lithography. It is derived from the solution of all zero third-order aberrations for the four-spherical mirror system with unit magnification. We have first examined the holosymmetric four-spherical mirror system in which all third-order aberrations are zero and all orders of coma and distortion are also zero. However, the system does not have any optical design freedom left for the correction of higher order aberrations, so a new solution of nonholosymmetric system is derived. In this system aspherizations on the spherical surfaces are carried out to reduce the residual aberrations. The aspherization is optimized to give near diffraction-limited performance for DUV wavelengths of 0.193 μm (ArF excimer laser line). The final system we have obtained consists of all aspherized mirrors with a numerical aperture of 0.35. This reflective system is compact in size and expected to be useful in optical lithographic applications.