Design and tolerancing of ArF excimer laser optics for lithography

Abstract

An ArF excimer laser exposure tool is expected as a work horse in the sub-quarter-micron lithography era. We have been trying to develop such an exposure tool. As a part of this development project, we have designed an optical system that uses an ArF excimer laser as an illuminating light source. The optical system is a catadioptric system and uses a polarizing beam splitter and a quarter-wave plate to eliminate the central obscuration of the system, its specifications are a numerical aperture of 0.5, a reduction ratio of 1/4, and a field size of phi 24 mm (or 22x5 mm(2) slit). The modulation transfer function values at 2500 cycles/mm are more than 40%. We have also analyzed the permissible tolerances of the system parameters. The resultant permissible tolerances are lambda/10 for the mirror surface, lambda/4 for the lens surface, +/-5 mu m for the lens thickness and the spacing between lenses, 0.5 minutes of are for eccentricity, 300 pm for the laser bandwidth, and +/-0.1 degrees C for the temperature.