AF fixer: new incremental OPC method for optimizing assist feature

Due to shrinking design nodes and to some limitations of scanners, extreme off-axis illumination (OAI) required and its use and implementation of assist features (AF) to solve depth of focus (DOF) problems for isolated features and specific pitch regions is essential. But unfortunately, the strong periodic character of OAI illumination makes AF's print more easily. Present OPC flows generate AFs before OPC, which is also causes some AF printing problems. At present, mask manufacturers must downsize AF's below 30nm to solve this problem. This is challenging and increases mask cost. We report on an AF-fixer tool which is able to check AF printability and correct weak points with minimal cost in terms of DOF after OPC. We have devised an effective algorithm that removes printing AF's. It can not only search for the best non-printing AF condition to meet the DOF spec, but also reports uncorrectable spots, which could be marked as design errors. To limit correction times and to maximize DOF in full-chip correction, a process window (PW) model and incremental OPC method are applied. This AF fixer, which suggests optimum AF in only weak point region, solves AF printing problems economically and accurately.