EUV mask blank

An EUV mask blank is the molybdenum-silicon multilayer-coated substrate before absorber patterning begins — roughly 40 to 50 alternating Mo/Si bilayers engineered to reflect 13.5nm light at near-normal incidence, capped with a protective layer and later an absorber. Multilayer deposition defects are uniquely unforgiving: a single buried particle or bump beneath the reflective stack distorts the surface above it and cannot be repaired once the multilayer is deposited, so it can print a fatal defect on every exposed wafer. This is why EUV blank qualification relies on actinic (13.5nm) inspection rather than conventional optical inspection, since only light at the actual exposure wavelength reveals how a defect will actually interact with the reflective stack.