INSPECTION SYSTEM WITH IN-LENS, OFF-AXIS ILLUMINATOR

摘要

An inspection system (2) employs a beam of monochromatic light (12) that travels through a Fourier transform lens (16) before striking a specimen wafer (4) at an angle ( theta ) with respect to the normal (26) of the specimen wafer (4) to produce diffracted light (28b and 28c) that has a broad spatial frequency spectrum which can be selectively filtered to produce a dark field image pattern of the various sized defects in an inspection area (22) of the wafer. The nearly collimated beam of monochromatic light strikes the wafer at an angle ( theta ) with respect to the normal of the wafer of between zero degrees and a predetermined maximum angle. For the inspection system disclosed, the predetermined maximum angle is the angle formed when the beam of monochromatic light is as far away from the optic axis as possible yet still within the numerical aperture of the Fourier transform lens (16). Moreover, if a specific range of defect sizes is anticipated, the system can be optimized by setting the angle ( theta ) at which the collimated beam of monochromatic light strikes the wafer to the angle ( theta ) which allows the system to collect those spatial frequencies which are best representative of the anticipated range of defects sizes.