Abstract

An imaging system to be used in photolithography must provide high resolution images over large fields that must be flat and free of distortion. An additional requirement is the image- space telecentricity, in order to prevent magnification changes due to wafer defocus. In the soft x-ray region only mirrors can be used for imaging. This imposes several specific requirements for reflective system design: to insure high system throughput, the number of mirrors used in the system should be a minimum; the aperture stop should be accessible (e.g., it may be placed at one of the mirrors) to prevent any possibility of creating differences in image across the field; and finally, elimination of the obscuration in the pupil is desired since it may reduce the image contrast. Various system configurations that use two, three, and four mirrors have been investigated. It was found that in order to satisfy all the above requirements, the system design naturally evolves into the scanned ring-field configuration. Four mirrors were found to be the minimum number to satisfy these requirements simultaneously. In reduction ring-field scanned systems, image distortion must be corrected not only across the ring width in the radial direction, but also in the cross-scan direction.

© 1992 Optical Society of America