Aerial Blank Inspection (ABI): This is the process by which mask blanks for the photolithographic process of producing silicon wafers are inspected. The push towards features sizes that have to be reproduced using EUVL, requires a very low rate of defects on the mask with a correspondingly tight tolerance on the maximum defect sizes permissible. The term aerial arises because the systems resemble the operation of a microscope with the blanks being illuminated with the EUV radiation.