A design of rotationally symmetric four-mirror optical system with reduction magnification 5 x for deep UV (KrF excimer laser, λ-248 nm) submicron lithography is developed. Initially it is attempted the system to be composed entirely of spherical mirrors, and the analytic solutions of the system is found, which is free from the third order spherical aberration, coma, astigmatism and distortion are obtained. However, the systems obtained are turned out to have sizable amount of residual aberrations which limit the numerical aperture to 0.2, and therefore is not useful in the submicron lithography.
To overcome these shortcomings, we have attempted to introduce the non-spherical system. It is known that the axial and higher order aberrations may be controled by using the aspherical surfaces. As the first step, we have designed the four-spherical-mirror system free from the four off-axial Seidel first order aberrations that are coma, astigmatism, field curvature and distortion. In the next step, the aspherical surfaces are introduced to the tertiary and quaternary mirrors, and then the optimization of the system is carried out by using a simple damped least squares method. The optimized system shows improved performances, in other words, axial and higher order aberrations are significantly reduced as expected. The numerical aperture of this system is 0.38 and field angle is 3˚.
The new type of the aspherized 4-mirror system, which is t < 0 ($t=\Sigma{d_i},{d_i}$ is the distance from i-th mirror to (i+1)-th mirror), is proposed. It is a reflection system useful for submicron lithographic applications. The numerical aperture of this system is as large as 0.40, which gives Rayleigh resolution of 0.38 ㎛.
