Non-imaging optics for illumination homogeneity in YieldStar

The metrology tool YieldStar measures overlay on-product. Homogeneity and brightness of the illumination limit the accuracy of overlay measurements. Recent trends in the integrated circuit industry forecast a reduction in power throughput of overlay targets of a factor ten. This requires a substantial increase of brightness on the sample to further guarantee sufficient accuracy.The YieldStar uses a light source, which is already optimized for high power output. Further improvements can be made by design changes in the optical track of the illumination. Currently, brightness is often sacrificed for the sake of illumination homogeneity. An example is the Optical Pupil Symmetrizer, which transmits only 40%of the light but greatly reduces pupil asymmetry. Therefore, alternative solutions for improving homogeneity can mean a substantial gain in transmission.Two feasibility studies were performed and are described in this document: Beam homogenization with chaotic mixing rods Beam shaping with non-imaging opticsWe developed techniques to develop beam homogenizers and analyze their performance with respect to homogeneity and asymmetry in the YieldStar. The created mixing rod design maintains pupil asymmetry and outperforms the OPS in terms of field homogeneity (100x) and transmission (2x). It can be produced as an optical fiber, which can be interfaced with an existing YieldStar machine.A software tool for designing optics with the purpose of beam shaping was created. This software enhances the toolbox of optical engineers for devising illumination systems. We demonstrated that it can be used to improve stability in the illumination of the YieldStar.Concerning beam homogenizers, we recommend pursuing tests on a YieldStar system to determine the benefit with respect to Total Measurement Uncertainty.Further development on illumination optics and the software tool is advised. A mature and versatile design tool promises simpler and more efficient optical systems