Overlay mark sub structure design to improve the contrast

The contrast of image based overlay (IBO) plays an important role during overlay mark metrology. For advanced lithography nodes, it is better to implement sub structures for the IBO mark to make the patterning process friendly. However, the sub structures may decrease the image contrast and lead to metrology errors and challenges of mark recognition for the overlay tools. In this paper, a theoretical analysis was conducted to simulate the IBO marks contrast in relation with different structural designs of the marks. The result provides a potential solution for the IBO contrast enhancement.