Area measurements for simulation-based dispositioning of masks

Abstract

The dispositioning of mask defects must also heed the increasing gap between the lithography wavelength and wafer-feature widths. For the larger technology nodes, where printed-wafer shapes are similar to those on the mask, mask-level analysis is sufficient. However, for smaller nodes, wafer-level scoring is useful since every defect does not significantly impact the wafer. Wafer-level analysis often relies on measurements of critical dimension (CD). However, as reticle enhancement technology proliferates, there are increasingly more curved edges where CD cannot be used. For example, false alarms can result from measuring CD near line ends because slight variations in measurement position may produce large CD changes. More importantly, a killer defect may be missed if cutlines are forbidden near line ends because the CD measurements are too far from the defect. Wherever CD measurements are not advisable, we advocate the use of Area scoring by computing the difference in printed feature area. We are not abandoning CD scoring but rather combining it with Area scoring, and using the more pessimistic score. For Area scoring, we use (Defect area - Reference area)/(Reference area). In general, the reference area is difficult to define since many shapes are not easily parsed into primitive shapes. Therefore, we use a square of side equal to the target CD. This square defines the window for a sliding-window average of the area difference. The maximum average value is then chosen from the entire image. Unlike Edge Placement Error, Area is sensitive to long, thin difference regions. Unlike Flux or Maximum Intensity Difference, Area is threshold-aware; it measures what prints and shows process-window variation.