SRAF generation based on SGM/CTM contour line

With the continuous improvement of chip integration level, resolution enhancement techniques (RET) has become one of the important technologies to promote the continued development of process nodes, such as source mask optimization (SMO), optical proximity correction (OPC) and sub-resolution assist feature (SRAF). Model-based SRAFs are generated by the guidance of the Continuous Transmission Mask (CTM) or SRAF Guidance Map (SGM). During operation, a threshold combined with the ridge is used to extract a suitable location for SRAFs. However, this generation method ignores many details, resulting in the need to constantly adjust SRAFs referring to the SGM/CTM in the subsequent optimization process, which will undoubtedly increase the simulation time. Therefore, we propose a contour line based SRAFs generation method. The extreme value region and the gradient of CTM/SGM will be displayed intuitively, so that more precise positions can be obtained at the initial SRAFs extraction. The SRAFs will be extracted from the extreme value region, and have a distribution similar to the final result. The gradient of the contour line can also be referenced in the following steps to guide the SRAF cleanup. During cleanup process, the SRAFs at high altitude region will have higher priorities to ensure the image quality of main patterns. Another advantage of this method is that when extracting rules from the model-based method, different SRAF priorities can be set according to the contour line and be used in the rules, so as to improve the accuracy of rule-based SRAFs.