Additional evidence of EUV blank defects first seen by wafer printing

First experimental evidence is given that a second generation blank inspection tool has missed a number of printing reticle defects caused by an imperfection of its EUV mirror, i.e., so-called multi-layer defects (ML-defects). This work continued to use a combination of blank inspection (BI), patterned mask inspection (PMI) and wafer inspection (WI) to find as many as possible printing defects on EUV reticles. The application of more advanced wafer inspection, combined with a separate repeater analysis for each of the multiple focus conditions used for exposure on the ASML Alpha Demo Tool (ADT) at IMEC, has allowed to increase the detection capability for printing ML-defects. It exploits the previous finding that ML-defects may have a through-focus printing behavior. They cause a different grade of CD impact on the pattern in their neighborhood, depending on the focus condition. Subsequent reticle review is done on the corresponding locations with both SEM (Secondary Electron Microscope) and AFM (Atomic Force Microscope). This review methodology has allowed achieving clear evidence of printing ML defects missed by this BI tool, despite of a too high nuisance rate, reported before. This establishes a next step in the investigation how essential actinic blank inspection (ABI) is. Presently it is the only known technique whose detection capability is considered independent from the presence of a (residual) distortion of the multi-layer at the top surface. This is considered an important asset for blank inspection, because the printability of a ML-defect in EUV lithography is determined by the distortion throughout the multilayer, not that at the top surface.