Patent ID: 6861660

Claim:
An optical system for detecting defects on a wafer that includes at least one pattern; the system comprising: a source of light to produce a beam; optics directing the beam along a path onto the wafer, producing an illuminated spot thereon; at least one detector for detecting light; an ellipsoidal mirrored surface, said mirrored surface and the at least one detector having an axis of symmetry about a line perpendicular to the wafer surface, said mirrored surface defining an input aperture positioned proximate to the wafer surface to receive scattered light therethrough from the wafer surface; said mirrored surface further defining an exit aperture and being substantially rotationally symmetric about said axis of symmetry, so that the mirrored surface reflects and focuses rotationally symmetrically about said axis of symmetry light that passes through the input aperture to the at least one detector; said exit aperture being located opposite to the input aperture; and at least one filter located between said exit aperture and said at least one detector, and being configured to substantially block a portion of the scattered light corresponding to the at least one pattern of the wafer and to substantially pass to said at least one detector another portion of the scattered light substantially other than the scattered light corresponding to the at least one pattern of the wafer.