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Source mask optimization for advanced lithography nodes

Source mask optimization is becoming increasingly important for advanced lithography nodes. In this paper, we present several source mask optimization flows, with increasing levels of complexity. The first flow deals with parametric source shapes. Here, for every candidate source, we start by placing model-based assist features using inverse mask technology (IMT). We then perform a co-optimization of the main feature (for OPC) and assist feature (for printability). Finally, we do a statistical analysis of several lithography process metrics to determine the quality of the solution, which can be used as feedback to determine the next candidate source. In the second flow, the parametric source is instead approximated by a pixel based source inverter, providing a fast and efficient way of exploring the source solution space. The final flow consists of pixilated source shapes realizable via DOEs or programmable illumination.