Lithographic comparison of assist feature design strategies

In this work the effects of sub-resolution assist features (SRAF) on the process window and the target CD control are investigated for the 100nm node gate level. Using 2-dimensional lithographic simulations the process windows of critical isolated and dense structures are determined and the overlapping process window with a second, dense feature is computed. This is achieved using a novel scheme of simulations over a wide range of line widths for a large pitch range. This approach allows us to explore systematically and simultaneously the impact of line width bias, number of placed assists, spacing to the main feature and spacing between the assist features as well as the assist feature width over a large parameter space. The overlapping process window is optimized following two different strategies: The first strategy places the assists only considering the space between two features independent of their width while reaching the target values for the different feature width using line biases. The second strategy under investigation defines the assist feature parameters based on both the line width of the target feature as well as the space from the target feature to the neighboring feature. For both approaches the implication for target CD control is discussed.