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13 March 2012 Binary modeling method to check the sub-resolution assist features (SRAFs) printability
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As modern photolithography feature sizes reduce, the use of sub-resolution assist features (SRAFs) to improve the manufacturing process window has become more prevalent. Beyond the assist features placement based on rules, a model based assist feature (MBAF) flow is needed to optimize the shape and the size of SRAFs, so that the process margin of the main features (MFs) is maximized. In the MBAF flow, a vital component is to build an accurate model that specifically checks the printability of SRAFs, which are supposed to leave no trace on wafer. Compared to the traditional optical proximity correction (OPC) model, the SRAF printability check model faces extra challenges, for example, the small size of SRAFs makes their direct transfer to the mask pattern more difficult, the SRAFs are usually not measurable on wafer and the worst-case SRAFs printability is typically at off-nominal conditions. In this paper, we propose an innovative binary modeling method for SRAF printability check model, which does not require the measurement of SRAFs' size on wafer and yet provides accurate prediction of SRAFs printing on wafer. In this modeling method, the binary determination of whether an SRAF prints/does not print (i.e., clean) on wafer was acquired by inspecting the SEMs taken from real wafer measurements. Then the local extrema of the signal intensity around the SRAFs was simulated and used to classify print/clean groups of SRAFs, and a special cost function was designed to separate the print SRAFs and clean SRAFs as much as possible during model calibration.
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Jianliang Li, Weimin Gao, Yongfa Fan, Jing Xue, Qiliang Yan, Kevin Lucas, Peter De Bisschop, and Lawrence S. Melvin III "Binary modeling method to check the sub-resolution assist features (SRAFs) printability", Proc. SPIE 8326, Optical Microlithography XXV, 83261D (13 March 2012);

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