Coherence control of illumination optics in mask inspection systems

One annoying problem that degrades high-fidelity pattern images in mask-defect inspection systems is the generation of ghost images in the imaging process. Ghost images arise from spatial coherence periodicity on the mask plane, which is due to periodic and discrete arrangements of fly-eye elements in mask inspection optics. By considering the contrast of ghost images under partial coherence illumination, we can derive the condition that represents the necessary number of fly-eye elements to substantially suppress ghost images in the image field. In addition, we confirm this theoretically derived condition of suppressing ghost images by numerical calculations. As a result, we prove that this suppresing condition is effective, and that the nonuniformity in distribution of image intensity can also be reduced in this way.