Design of UV LED illumination system for direct imaging lithography

Abstract

UV LED, which can be 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. In practical application, there are high requirements for output power density, stray light control, and luminance uniformity of DI system to improve the productivity and exposure performance. We present an optical structure of the illumination system for DI lithography to provide uniform illumination low stray light for digital micromirror device (DMD), which combines Kohler illumination and double telecentric imaging. In this design, the high luminance illumination can be obtained by LED etendue analysis, fly’s eye condenser is not needed, and the stray light on the DMD caused by light outside the effective angle of the LED can be mostly filtered out. Based on this idea, a lithographic illumination system with the numerical aperture of 0.1 is designed and fabricated for 0.95-in. DMD. According to experimental measurements, the effective illumination power is up to 10 W, the stray light accounts for 10.7%, and the illuminance uniformity is about 90%, which meet the requirements of DI lithography equipment used in PCB manufacturing.