Small Imaging Overlay Metrology Targets for Advance Nodes

Scribe line width reduction and high-order scanner correctibles are driving overlay (OVL) target size reduction. Shrinking imaging-based overlay (IBO) target size for standard target types, such as AIM (R) or BiB, is not possible without a performance impact. In this paper, new target layouts and supporting OVL tool setup methods will be explored to enable small OVL targets to meet performance, accuracy, and robustness requirements. Two approaches have been explored: (1) measuring smaller grating pitches utilizing oblique illumination, and (2) measuring targets not obeying 180 degrees rotational symmetry requirement (half-targets). Each of these approaches allows shrinking conventional imaging OVL target area by a factor of two. Evaluation results are reviewed in this paper and further target size reduction is presented.