Classification and Printability of EUV Mask Defects from SEM images

EUV lithography is starting to show more promise for patterning some of the critical layers at the 5nm technology node and beyond. However, there still are many technical challenges to overcome before it can be implemented into high volume manufacturing (HVM) and one of them is the production of defect-free EUV masks. Mask shops today typically use their cutting-edge 193nm inspection tools to detect defects on patterned EUV masks, since no EUV actinic pattern inspection or even e-beam mask inspection tools are available. The 193nm inspection tools have limited resolution on mask dimensions targeted for EUV patterning. The theoretical resolution limit for 193nm mask inspection tools is about 60nm HP on 4X masks, which means that main feature sizes on EUV masks will be well beyond the practical resolution of 193nm inspection tools. Nevertheless, 193nm inspection tools with various illumination conditions to maximize defect sensitivity and/or main-pattern modulation are being explored for initial EUV defect detection. Due to the generally low signal-to-noise in 193nm inspection imaging of EUV masks, these inspections often need to be run "hot" resulting in hundreds or thousands of defects getting detected. Each one of these detections then need to be accurately reviewed and dispositioned. Manually reviewing each defect is difficult due to poor 193nm resolution. In addition, the lack of a reliable aerial image dispositioning system makes it even more challenging to disposition for printability. In this paper, we present the use of SEM images of EUV masks for higher resolution review and disposition of defects. In this approach, most of the defects detected by the 193nm inspection tools are first imaged on a mask SEM tool. These images together with the corresponding post-OPC design clips are provided to KLA-Tencor's Reticle Decision Center (RDC) platform which provides a comprehensive SEM ADC (Automatic Defect Classification) analysis of every defect. First, a defect-free or reference mask SEM image is rendered from the post-OPC design, and the defective signature is determined from the difference image. The defective signatures help assess the true nature of the defect as seen under e-beam imaging; for example, excess or missing EUV absorber, line-edge roughness, contamination, etc. Next, the defect and reference contours are extracted from the grayscale SEM images and fed into the simulation engine with an EUV mask and scanner model to generate corresponding EUV defect and reference aerial images. These are then analyzed for printability and dispositioned using RDC's Aerial Image Analyzer application to automatically measure and estimate the impact of the mask defect to wafer CDs. By integrating the SEM ADC application into the EUV inspection and review flow this way, every defect is characterized for its type and printability. Such defect characterization is essential not only for determining which defects are nuisance or critical, but also for monitoring the performance of EUV mask process tools. With EUV lithography progressing towards volume manufacturing and progress being made in the area of e-beam based mask inspectors, the EUV SEM ADC software solution will continue serving an essential role of dispositioning defects off e-beam imaging.