EUV lithography has been well-established over the past few years, to be the next technological milestone, to achieve lower pattern resolutions i.e. higher technology nodes. Ensuring a lower defect count however is particularly important for its use in mass-production, because of the considerably high cost of EUV masks. EUV wavelength (actinic EUV inspection) is necessary to be able to catch and characterize EUV defects correctly, particularly phase defects, originating due to imperfections in multi-layers. Defects on a patterned mask require EUV wavelengths (higher resolution) for accurate assessment of their CD impact. However, DUV optics are also employed by mask shops, along with SEM review, as a cost-effective option. Emphasizing the importance of defect handling in EUV masks, especially for EUV masks without pellicles in the memory industry, the most difficult part of EUV mask management is in controlling adder. Detecting smaller adder size is required as mask pattern size is getting smaller in higher technology nodes, which accompanies increased sensitivity of patterned mask inspection and unavoidable increase in noise signal that makes it hard to proper classify mask defects. This paper talks about the details of how DUV inspection optics are utilized to achieve an efficient pre-filtering of EUV mask defects. This, coupled with well-tested automatic defect classification algorithms of Calibre® DefectClassify, results in a reliable solution to manage regular classification of mask defects as real or false, under such sensitive conditions.
The impact of mask defects on wafers has been successfully simulated by KLA’s mask SEM-to-Aerial (S2A) image simulator. The high prediction accuracy, high throughput and low cost have been proven by mask shop production users. S2A generates the reference mask SEM images by rendering the post-OPC design to match with measured mask SEM images containing defects. From the two mask SEM images, defect-free reference absorber contour and defective absorber contour are extracted, and the two contours are used for wafer-level aerial image simulation through EUV scanner conditions. Pass/Fail of %CDE, EPE, etc. are reported by automatically generating measurement cutlines. S2A can be used for wafer prediction for pre/post-repair full height absorber defects using top-down mask SEM images, but it cannot be used for EUV multi-layer buried defects or for partial height residues remaining after repair. KLA and SK hynix have been investigating a solution for this use case using AFM images, which can measure the height error from the buried defect or partial height residue in post-repair mask. In this paper, we will show how measured SEM and AFM images are processed to fit with rendered reference SEM and AFM images. We then show how advanced scanner simulation models are used for determining the wafer printability of mask absorber defects, buried defects and absorber residues. Finally, the predicted wafer impact of the mask defect is compared with wafer SEM images for validation. This approach, named S2A-3D, will help reduce loading on EUV actinic metrology and provide a fast, accurate and cost-effective dispositioning of post-repair EUV defects.
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