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.