The original inverse lithography technology (ILT) commitment of creating a free-form mask from ex nihilo underwent transformation under pressure of constraints that are imposed by the capabilities of manufacturing and inspection equipment. This pressure became especially noticeable in the dealing with sub-resolution assist features (SRAF). The freeform SRAFs are naturally unmanufacturable and hard to inspect. In addition to this complication, the ILT signal for SRAFs is substantially weaker and often ill-defined in comparison to a strong signal from fidelity objectives to print main features on target. So, it is difficult to automatically produce SRAFs that are simultaneously geometrically stable, do not print, MRC-clean, and deliver the best lithographic quality. To overcome these obstacles, we propose new concept of structured SRAFs by combining ILT ideas with robust geometrical parameterization to introduce well-behaved and MRC-clean by construction mask decorations.
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