Abstract
A three-aspherical-mirror system for Extreme Ultraviolet Lithography has been developed. The aspherical mirrors were fabricated using the computer controlled optical surfacing (CCOS) process and a phase shift interferometer. The mirrors have figure errors of 0.58nm and surface roughness of 0.3nm. In order to obtain a high efficiency mirror, M1 and M2 were coated with a graded d-spacing Mo/Si multilayer and M3 was coated with a uniform d-spacing Mo/Si multilayer. The peak reflectivity was 65% at the wavelength of 13.5nm. The wavelength matching of each mirror spans 0.45nm. The mirrors were aligned with a Fizeau-type phase shift interferometer, and a final wavefront error of less than 2nm was achieved. Using this system, exposure experiments were performed. 60nm L & S patterns on the exposure area of 10mm×10mm were achieved.
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