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Sub-Resolution Assist Features (SRAFs) are used in optical lithography to improve the manufacturing process window
(PW). They are added to the mask shapes to create a denser environment that improves the printability of the target design
shapes on wafer. As the critical dimensions (CDs) that need to be patterned shrink with every technology generation,
SRAFs have become a critical and key component in enabling processes with manufacturable process windows.
The size and placement of the SRAFs must be carefully optimized to provide the maximum benefit to the main feature
while avoiding any printing on resist that could affect subsequent etching processes. The un-intended printing of assist
features on wafer is a critical yield detractor and is especially pervasive in newer technology nodes, where more aggressive
and more complex SRAF patterns and placement are becoming commonplace. The need for the accurate prediction
of SRAF printing is therefore very important to achieve the maximum main feature process window benefit without any
assist feature printing.
Traditionally, the optimization of SRAF sizing and placement consisted of a set of rules obtained through the extensive
analysis of wafer printability on a variety of assisted mask patterns while using Scanning Electron Microscope images of
the resist surface to monitor unwanted SRAF printing. Recent advances in model-based assist feature optimization methods
allow for the automated adjustment of both main feature and assist feature size and placement through simulation
of the aerial image, but critically rely on the accuracy of the lithography process model to ensure non-printing of the
SRAF. Lithography or Optical Proximity Correction (OPC) models usually comprising an optical and a resist model are
calibrated to measurements of the resist bottom CD. These models are naturally better at predicting the printing of
SRAFS that are lines in resist. When the SRAFs are holes in resist, for eg. assist features supporting main features on a
dark field mask, or SRAFs supporting inverse tone features on a bright field mask, these models do not have the required
accuracy in predicting SRAF printing. SRAF printability prediction has thus far been tackled by large dose adjustments
to the OPC model, to match simulation to wafer results. The drawbacks of this method have been two-fold - simple dose
adjustments do not accurately predict printing across various SRAF configurations and the main feature printability is
compromised
We present in this paper a method to calibrate and predict printing of assist features that appear as a dimpling in the resist
surface, by carefully selecting the calibration data and separately tuning the model parameters for the main feature
and of the SRAF printing models. With this method, we obtain a model that accurately predicts the printing of various
configurations of SRAFs on wafer while still maintaining the accuracy on the main features. An analysis of the implementation
of such a model in the OPC flow and the corresponding supporting results will be presented.
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