Four-mirror imaging system (magnification +1/5) for ArF excimer laser lithography

Research was conducted on the axially symmetric four-mirror system (magnification +1/5) for use in optical lithography using an ArF excimer laser beam. The initial design is derived from an extensive numerical calculation that makes the sum of the third-order aberration coefficients very small _j=I ^V |S _j |<10_-6. Using an optimization method (damped least-squares method) the finite aberrations are reduced; then, to obtain the diffraction-limited performance, three surfaces are aspherized. The final system has NA=0.38 for the ArF excimer laser line (=0.193 m) and depth of focus of 1.2 m over a 2.6×2.6 cm² object field. Nearly all rays fall within the Airy disk in the image plane. The resolution is 540 cycles mm^-1 at MTF=0.5 level for axial object point. We consider that the present four-mirror system may be further refined for use in soft x-ray lithography.