For micro-lithography using KrF excimer laser beam(λ=0.248μm) a mirror system consisting of four spherical surfaces with reduction magnification 5× is designed. During the process of designing the computer aided optimization technique is extensively employed.
Initially the aplanat condition of the mirror system is considered. And for the further improved performance of the system the distortion free condition and flat field condition within Seidel 3rd order aberration are added to the above condition.
The optimized four-spherical mirror system satisfying the above conditons and normalized to focal length f'=-1cm has N.A. 0.25 and its spherical aberration, coma, field curvature, distortion are removed to the diffraction limit. But due to the residual astigmatism and off-axial vignetting this normalized system has field angle 1.5° and the image field is too small(0.495mm in diameter) to be practical.
In order to enlarge the field size to the practical one we have scaled up the normalized system to that with focal length f'=-10cm. But due to the increased ray aberrations in this process, we have re-optimized this scaled-up system with reduced N.A. 0.15 and field angle 1°. As a result, though the residual astigmatism and off-axial vignetting still remain uncorrected, this new system shows better optical performances except for astigmatism than the previous one and the image field diameter is increased to the practical value of 3.3mm(object field diameter=16.5mm). This system has N.A. 0.15, the practical resolution 1.3μm (R=0.8λ/N.A.) and DOF(depth of focus) of 4μm.
