6 December 2004 Placing assist features in layout using a process model
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Sub-resolution assist features (SRAFs) are non-printing features arranged on a mask layout to “assist” the lithographic performance of the lines intended to be printed on the wafer [1]. SRAFs typically are narrow lines located adjacent to the target figure edges. Current practice is to synthesize SRAFs with a rule-based methodology where the assist feature placement is dictated by combinations of feature width and spacing parameters. Optical behavior with off-axis illumination is complex and requires an elaborate set of SRAF synthesis rules. Creating and maintaining a robust set of placement rules guaranteed to work properly for arbitrary configurations is very difficult. Socha, et al, have demonstrated that the optimum configurations for SRAFs can be derived from the aerial image of the target layout configuration [2]. In this paper we show how SRAF synthesis can be optimally implemented in an OPC tool environment, leveraging lithography simulation.
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Lawrence S. Melvin, James P. Shiely, Michael L. Rieger, "Placing assist features in layout using a process model", Proc. SPIE 5567, 24th Annual BACUS Symposium on Photomask Technology, (6 December 2004); doi: 10.1117/12.569179; https://doi.org/10.1117/12.569179

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