Sub-Resolution Assist Features (SRAFs) have been extensively used to improve the process margin for isolated and
semi-isolated features. It has been shown that compared to rule-based SRAFs, model-based placement of SRAFs can
result in better overall process window. Various model-based approaches have been reported to affect SRAF placements.
Even with model-based solutions, the complexity of two-dimensional layouts results in SRAF placement conflicts,
producing numerous challenges to optimal SRAF placement for each pattern configuration. Furthermore, tuning of
SRAF placement algorithms becomes challenging with varying patterns and sources [1-3].
Recently, pixelated source in optical lithography has become the subject of increased exploration to enable 22/20 nm
technology nodes and beyond. Optimization of the illumination shape, including free-form pixelated sources, has shown
performance gains, compared to standard source shapes [4-6]. This paper will demonstrate the influence of such
different free-form sources as well as conventional sources on model-based SRAF placement. Typically in source
optimization, the selection of the optimization patterns is exigent since it drives the source solution. Small differences in
the selected patterns produce subtle changes in the optimized source shapes. It has also been previously reported that
SRAF placements are significantly dependent on the illumination . In this paper, the impact of changes in the design
and/or source optimization patterns on the optimized source and hence on the SRAF placement is reported. Variations in
SRAF placements will be quantified as a function of change in the free-form sources. Lithographic performance of the
different SRAF placement schema will be verified using simulation.