22-nm overlay accuracy of synchrotron radiation stepper using an improved chromatic bifocus alignment system

摘要

Abstract: The overlay accuracy of the synchrotron radiation (SR) lithography stepper `Satellite 600' developed by Sumitomo Heavy Industries, Ltd. (SHI), was analyzed. The stage precision of positioning elements and an alignment system of the stepper were examined. As a result, it was proved that a major factor of the overlay error originated from the focus condition of the alignment optics system. We name this factor `focus dependency of alignment system,' which means that the detected value of the alignment system is changed in the order of 10 nm as the proximity gap width is changed even in the case that both mask and wafer marks are within the depth of focus. Experimental data of the stepper under operation was analyzed by Taguchi's experimental planning method to prove the affect of the focus dependency. The results of this analysis showed the overlay accuracy of the stepper and that the focus dependency was the dominant factor of the total error. The result also showed that the flatness of the reference base was the main factor in controlling the gap width. After numerical correction of the base undulation, 22 nm (mean value $PLU 3$sigma@) overlay accuracy was obtained. !8