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As lithography moves into lower k1 imaging, 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. FlexRayTM can 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 FlexRayTM for 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.
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