Recently, optical proximity correction model calibration techniques that use SEM contours have enabled possibly
significant improvements in complex mask design. However, compared to conventional CD-based calibration, contour-based
calibration results in increased errors in 1D features. In fact, our research shows that there is a ~1-nm gap, which
we call "CD-gap," between CD measurements directly calculated from a SEM image and CD measurements calculated
from SEM contours. To achieve accurate calibration, SEM contours must match the corresponding CD measurements.
We have developed a CD-gap-free contour extraction technique in response to this problem. In our technique, a mask
edge is classified into shape structures and an optimized SEM contour extraction method is prepared for each shape
structure to reduce the CD-gap. Experimental results show that the CD-gap can be decreased to sub-nm, which clearly
demonstrates the potential of our proposed technique to play a vital role in the lithography process.