Considering the usage extension of ArF immersion lithography down to the node of hp 22nm, EUVL
should be able to cope with at least the 16 nm and preferably the 11 nm nodes. However, numerical
aperture (NA) of projection optics in EUVL exposure tools needs to be around 0.4 which requires the
oblique angle of around 8 degrees for the illumination. When one considers that the thickness of
multilayer is around 280 nm, the reflectivity drop area at the edge of absorbers has the width of
approximately 40 nm that corresponds to the width of 10 nm on wafer. There is another serious problem.
Particle check will become extremely difficult for a pellicle-less EUVL mask with smaller feature size.
Killer particles cannot be detected by an optical inspection system any longer at 22 nm and smaller. In
consideration of all these serious problems, the only simple solution is to increase the mask
magnification factor from the conventional 4X to 8X.1-4) Although is has been reported that the
throughput of an 8X exposure too is 30-70% compared with that of 4X,5X the throughput can be increased
to be equal or even higher than that of 4X when 450 nm wafer is applied.