This work compared the CD-based and image-assistant approaches for calibrating the OPC models. OPC models were
first developed for 65nm-node memory contact layer and calibrated by contact test patterns with various ellipticities. The
image-assistant model is a hybrid one calibrated by SEM contours and 1D measurement results, while the CD-based
model calibration uses 1D measurement results as the sole data source. The fitting errors, model prediction ability and
OPCed results were compared between these two models. Besides, the challenges on calibrating the edge-detection
algorithm of the CD SEM images to the extracted contours of OPC tool were also discussed. Finally, the layouts
corrected by CD-based and image-assistant models were written on a test mask for wafer-level comparison.
The results displayed that the CD-based model showed smaller error on fitting and interpolation, but image-assistant
model got improvement on extrapolation prediction of array-edge contact, unknown contact pattern and long contacts.
The wafer-level comparison also revealed the image-assistant model outperformed to the CD-based model by smaller
correction error on unexpected patterns.