29 September 2010 Natural EUV mask blank defects: evidence, timely detection, analysis and outlook
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A combination of blank inspection (BI), patterned mask inspection (PMI) and wafer inspection (WI) is used to find as many as possible printing defects on two different EUV reticles. These multiple inspections result in a total population of known printing defects on each reticle. The printability of these defects is first confirmed by wafer review on wafers exposed on the full field ASML Alpha Demo Tool (ADT) at IMEC. Subsequently reticle review is performed on the corresponding locations with both SEM (Secondary Electron Microscope) and AFM (Atomic Force Microscope). This review methodology allows to separate absorber related mask defects and multi layer (ML) related mask defects. In this investigation the focus is on ML defects, because this type of reticle defects is EUV specific, and not as evolutionary as absorber defects which can be mitigated in more conventional ways. This work gives evidence of critical printing ML defects of natural origin, both pits as shallow as 3nm and bumps just 3nm high at the surface. Wafer inspection was the first inspection technique to detect these ML-defects with marginal surface height distortion, because both state-of-the-art PMI and especially standard BI on the Lasertec M1350 had failed to detect these defects. Compared to standard BI, the more advanced Lasertec M7360 is found to have much better sensitivity for printing MLdefects and our work so far shows no evidence of printing ML defects missed by this tool. Unfortunately it was also observed that this required sensitivity was only achieved at the cost of an unacceptable nuisance rate, i.e., with a too high number of detections of non-printing defects. Optical blank inspection is facing major challenges : It needs not only to find ML defects with height distortions of 3nm and less (and in theory maybe even 0nm), but also it must be able to disposition between such likely-printing and non-printing defects.
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Dieter Van den Heuvel, Dieter Van den Heuvel, Rik Jonckheere, Rik Jonckheere, John Magana, John Magana, Tsukasa Abe, Tsukasa Abe, Tristan Bret, Tristan Bret, Eric Hendrickx, Eric Hendrickx, Shaunee Cheng, Shaunee Cheng, Kurt Ronse, Kurt Ronse, } "Natural EUV mask blank defects: evidence, timely detection, analysis and outlook", Proc. SPIE 7823, Photomask Technology 2010, 78231T (29 September 2010); doi: 10.1117/12.865812; https://doi.org/10.1117/12.865812

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