Sub-resolution Assist Feature Modeling for Modern Photolithography Process Simulation

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In modern photolithography, as the feature size becomes smaller and smaller, it becomes more and more popular to include sub-resolution assist features (SRAFs) to improve the robustness of the lithography process. Hence, it is vital to simulate the process precisely with SRAFs placement. However, for computational reasons, it is necessary to model features with SRAFs placed using specially developed approximation algorithms. This need arises because of the inherent differences between SRAFs and main features. First, SRAFs are usually hard to verify physically because of their small sizes and, therefore, there is relatively large mask measurement uncertainty on the layout of SRAFs compared to main features. Second, unlike the well-defined main features, the shape of SRAFs is relatively poor, e.g., line edge roughness and critical dimension (CD) variation and, hence, the effective transmission of SRAFs may not be the same as for the main features. Third, the thickness of the mask is comparable to the feature sizes of SRAFs and the three-dimensional (3D) mask effect may play an important role in process simulation. In wafer measurements, the data are usually influenced by all the above effects, and it is practically impossible to identify them individually. In this study we propose a lumped treatment specially designed for SRAFs. The results show significant improvement in the simulation accuracy and reduce the RMS of fitting errors down to the sub-nanometer level for all features with or without placed SRAFs.
Published by the Japan Society of Applied Physics through the Institute of Pure and Applied Physicsの論文
2008-06-25

Sub-resolution Assist Feature Modeling for Modern Photolithography Process Simulation

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