Design of UV LED illumination system for direct imaging lithography

Abstract

UV LED, which is used as the illumination source in the direct imaging (DI) exposure equipment, has the advantages of rich wavelengths and low cost. It has a good application prospect in the field of printed circuit board (PCB) manufacturing. An optical structure of the illumination system for DI lithography, which combines Kohler illumination and double telecentric imaging, is presented. First, the shape of the spot matched with Digital Micromirror Device (DMD) is obtained by setting an aperture at the imaging plane of LED chips to filter out the stray light. Then the aperture is imaged onto the working plane of DMD by the posterior double telecentric lens. The uniform illumination spot without stray light can be achieved on the DMD in the end. In this design, fly eyes are not needed, and the stray light on the DMD surface caused by light outside the effective angle of the LED can be filtered out completely. Based on this idea, a lithographic illumination system with the numerical aperture (NA) of 0.1 is designed and fabricated for 0.95 inch DMD. According to experimental measurements, the effective illumination power is up to 10W, and the illuminance uniformity is more than 85%, which meet the lighting requirements of direct imaging lithography equipment used in PCB manufacturing.