Freeform source optimization for improving litho-performance of warm spots

As lithography moves into lower k1imaging, traditional illumination "source" shapes may perform marginally in resolving complex layouts. Subsequently hot-spots or warm-spots can result, leading to yield loss in production. Typically, lithographers solve such problems by modifying the local layout instead of optimizing the DOE (diffractive optical element) illumination shape. FlexRayTMcan easily implement freeform source shapes and allows a high degree of freedom in source optimization. Therefore, it becomes practical to use pixelated freeform sources to resolve hot spots or warm spots. In this paper, we investigate the use of freeform source optimization (SO) on a critical dynamic random access memory (DRAM) layer with warm spots to verify the effectiveness of a SO only flow using Tachyon SMO. In order to improve the warm spots without changing baseline performance for other patterns, we optimized not only the warm spot patterns of concern but also the critical reference patterns. Since the optimization minimizes EPE (edge placement error) and maximizes imaging quality for all enclosed patterns, the final optimized source shape performs similar to the baseline source for the base patterns while improving the performance of the warm spot pattern areas. Although the SO source is similar in shape to the baseline source, the optimized source provides enhanced depth of focus (DoF) for all warm spot patterns without suffering degradation in the normalized image log-slope (NILS) performance. Evaluation of the optimized SO source shows no obvious negative impact on modeled CDs across an array of L/S pattern combinations which cover all the pitches appearing in the periphery. Finally, the optimized source is demonstrated using ASML's FlexRayTMfor on-wafer evaluation. According to the observations from on-wafer experiments, consistent results to simulation are verified. Overall DoF for the identified warm spot patterns is definitely improved and no obvious pattern shape changes are found, as well. From the positive demonstration in simulation and on-wafer verification, the vast flexibility of the freeform source enables the SO flow with more powerful capability to improve local hot spot or warm spot problems without negatively impacting the other patterns.