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Abstract

Photolithographic processes require alignment of substrate to mask prior to exposure. Alignment accuracy becomes critical as feature size diminishes. For alignment tolerances above 1 .mu.m, off-axis camera-based techniques are acceptable, but require frequent calibration. Through the lens (TTL) on-axis alignment is much preferred, not requiring frequent calibrations and having superior alignment accuracies, even though usually more complicated and expensive. This invention provides a simple, easy to incorporate TTL, on-axis alignment technique for alignment accuracies of 1 .mu.m, such as scan-and-repeat patterning systems that employ a unitary mask-substrate stage and a folded projection imaging system. There is a master alignment mark adjacent to the mask, which is imaged onto a phosphor screen with the same radiation as is to be used for patterning. The resulting fluorescent image becomes a virtual master mark, visible to the alignment camera through a lens/compensator assembly after passing through the final fold mirror and not being reflected back towards the mask. The final fold mirror is optimized to pass radiation at the virtual master mark wavelength and reflect the exposure wavelength. Coordinates are developed and used thereafter to align the mask and substrate to one another.

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First Claim