Optical alignment system for submicron x-ray lithography

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Abstract
The alignment system described in this paper is applicable to all proximity mode replication systems. The inherent high accuracy of the method makes it particularly attractive for submicron lithography applications requiring positioning accuracy of better than 0.1 μm. The basic alignment mark on the mask is a one dimensional Fresnel zone whose primary focal length is equal to the proximity gap spacing. Helium–Neon laser illumination is used. Elimination of unwanted diffraction orders is achieved by incorporating a transverse grating structure on the alignment mark on the wafer. The alignment signal sensitivity to gap variation is such that it allows very precise setting of the gap spacing in an unambiguous fashion. Three pairs of such alignment marks are necessary to provide x–y–ϑ registration as well as gap spacing and parallelism settings. A key feature of the alignment system is the time varying incidence angle of the laser beam upon each Fresnel zone mark, which results in a small amplitude scanning of the wafer surface and allows continuous registration throughout the entire exposure duration. A description of the alignment system designed for a submicron x-ray lithography system is given, together with some preliminary results.

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