Spatial coherence properties of a LED-based illumination system for mask-aligner lithography

A high-power LED-based illumination system has been developed as a replacement for the mercury arc lamps used in mask-aligner lithography. LEDs are arranged in a grid array and placed in the entrance aperture of individual reflectors. Those reflectors decrease the angular extent of the light. With this multisource approach, different groups of LEDs can be switched on independently. The illumination patterns created determine the illumination angles and the spatial coherence in the mask plane. The spatial coherence is measured in the mask plane by using a circular double slits approach. The interference pattern for different illumination patterns are measured, showing the effect of the asymmetry and size of the angular extent of the illumination light. The effect of the different illumination patterns on the quality of the prints are also illustrated with print tests.