Characterization of mask CD mean-to-target for hotspot patterns by using SEM image contours and feed forward intra CDU by mask MTT

The mask CD mean-to-target (MTT) has been widely adopted as one of the key metrics for the mask quality control. As more aggressive optical proximity correction (OPC) is applied to push the resolution limit, traditional CDSEM measurement-based metrology is not sufficient to characterize mask CD MTT, especially for complicated 2D patterns. In this paper, we present the method of using SEM image contours for the characterization of mask CD MTT. The full flow includes contour extraction from mask SEM images, contour-to-contour alignment, contour averaging and edge placement error (EPE) measurement of mask image contour against the target. The OPC Verify engine is employed to give fast EPE check at closely packed sampling sites along the target. We apply this method to evaluate mask CD MTT for the hotspot patterns from two masks. The generated mask CD MTT distribution histograms and color maps demonstrate a good correlation with the wafer defect counts. As sub-nanometer feature dimensions are shrunk below 20nm, Mask error enhancement factor (MEEF) values grow rapidly. Thus keeping mask fingerprint good uniformity becomes a significant factor of printed CDU on the wafer. To address these problems, we propose a method to use yield limiting device patterns mask MTT to feed forward mask-based intra field CDU. Through this feed forward method, engineers could obtain preliminary CDU correction earlier than the traditional feed backward, basically before the mask arrived in factory. It will effectively economize manpower and appropriation budget.