Sub-Resolution Assist Feature Cleanup Based on Grayscale Map

As an approach of resolution enhancement technique (RET), sub-resolution assist feature (SRAF) is used to meet the pitch requirements for sparse patterns in off-axis illumination (OAI) and supplements the spatial frequencies of main patterns to improve the resolution. Rules-based SRAF (RB-SRAF) is a technique that foundry extracts SRAF placement rules from wafer data or simulation data and then apply these rules to full chip. RB-SRAF often produces a lot of conflicts. Usually, the geometric constrains are the only factors taken into account in cleaning up of these conflicts, which may lead to mistaken cleanup and be counter-productive. Therefore, it is necessary to introduce some physical factors in the course of cleanup. The transmissivity distribution of mask in hotspot region can be obtained by inverse lithography technology (ILT). And the statistical information of the grayscale map from ILT can be used for leading cleanup and keeping the SRAFs which contribute more to process window (PW). The feasibility of this method has been verified by a series of simulations.