The application of aggressive Optical Proximity Correction (OPC) has permitted the extension of advanced lithographic
technologies. OPC is also the source of challenges for the mask-maker. Small shapes between features and highly-fragmented
edges in the design data are difficult to reproduce on masks and even more difficult to measure exactly with
CD-SEM, which requires not only tool stability but also better measurement methods. To cope with this problem, we
have been focusing on finding better methods for measuring actual mask Critical Dimension (CD) that would show a
good correlation to wafer CD. In BACUS 2006, we presented an effective measurement for closed patterns, which is
"area measurement". In time paper we are introducing new potential solution, which include a reliable method, distance
measurement, for certain types of unclosed patterns.
For instance, we evaluated an unclosed pattern which couldn't be measured with Region of Interest (ROI) that is large
enough, and found a reliable method, Distance ROI. Though the method has a major drawback of image tilt, we also
found an approach to avoid this. Finally we verified that Distance ROI could be new solution for unclosed patterns by
jointly applying tilt monitoring, beam rotation correction, and area scan.