Abstract

As wavelength decreases and numerical aperture (N.A.) increases, the bandwidth of available excimer lasers can most easily be matched using lenses that are catadioptric. Beam splitters allow such lenses to maintain high N.A. without obstructing the pupil or compromising the field size. We present a compact 0.6-N.A. 248-nm design with 30-mm field diameter. The most challenging element in this system is the 170-mm SiO₂ cube beamsplitter. The dichroic beam splitter coating supports alignment at 364 nm, and operates in converging light over a 36° range without introducing aberration, apodization or image displacement. The cube is a good thermal insulator, and any index gradients arising from residual absorption in the prisms or coating will be traversed in a nonrotationally symmetric way, due to the folded path. Thermal modeling and gradient-index ray-tracing suggest an absorption tolerance of 1 % for the coating and 10% m^-1 for the SiO₂. We have obtained material meeting these tolerances. The beam splitter prisms can be strategically cut to minimize aberrations arising from residual SiO₂ index inhomogeneities, which amount to 2 ppm in the best material we have obtained. An initial study suggests that optimal positioning of the cut surfaces can meet λ/5 wavefront and 20-nm distortion tolerances in a half-scale cube. Further correction can be obtained with external compensation. Other beam splitter-related issues are vulnerability to stray light in the presence of reflective surfaces at mask, wafer, beam splitter and mirror, and the effect of polarizing beam splitter coatings of lithographic imaging.

© 1991 Optical Society of America