Abstract

To satisfy the increasing demand for extremely tight overlay accuracy in semiconductor manufacturing processes, all the measurement error factors in alignment systems and overlay measurement tools need be identified and eliminated. The principle of most alignment systems is based on image processing of target marks on the wafer under bright-field illumination. Although the phenomenon that the sensitivity to the alignment error varies with the step height (SH) of the mark has been known and used for evaluating the performance of the alignment optics, no investigation has been made into the origin and the physical mechanism of the phenomenon. We propose a simplified optical model that can account for the origin of the asymmetric image and clarify its relation to the SHs. The model is validated with simulation and experimental results. The improved performance of an alignment system using marks with optimally designed SHs is demonstrated.

© 2007 Optical Society of America

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