Application of SEM-based contours for OPC model weighting and sample plan reduction

Continued improvements in SEM contour extraction capabilities have enabled calibrating more accurate OPC models for advanced technology nodes using a hybrid approach, combining CDs for 1D structures and full contour measurements for more complex 2D patterns. Previous work has addressed various components of contour modeling including alignment, edge detection, CD to contour consistency, and image parameter space coverage. This study covers weighting strategies for CDs compared to contours. Additionally the total number of structures in a sample plan can be reduced by incorporating contours for model calibration due to the increased number of evaluation points they provide. Repeated measurements of the same structure at separate locations are used to extract SEM contours across several instances. The average measurements from these locations can then be used for OPC model calibration. Using 14nm process data, it is shown that including more contours in hybrid OPC model calibration leads to improved model verification. Within an appropriate range, higher weight on the contour patterns leads to improved model verification on measurement sites unseen by the calibration set. Calibrating a model with fewer contour structures, but at higher weight shows improvement over standard CD only model calibration.