Patent ID: 8228497

Claim:
A system comprising: an optical unit; wherein the optical unit comprises an illumination optics, a collection optics, and a grey field detector; wherein the illumination optics is adapted to scan a spot of radiation over an object having a repetitive pattern of multiple regularly repeating structural elements such as to generate a diffraction pattern that comprises multiple diffraction lobes; wherein the collection optics is adapted to collect and filter the diffraction pattern, wherein the filtered diffraction pattern comprises a single diffraction lobe while not comprising other diffraction lobes, wherein the collection optics is adapted to collect and filter the diffraction pattern to produce a filtered diffraction pattern that consists of a single diffraction lobe; wherein the grey field detector is adapted to collect a focused radiation pattern from the collection optics and generate detection signals, wherein the detection signals include a first pattern component that results from scanning the repetitive pattern and a second pattern component that results from scanning a defect on the object; and wherein the detection signals include oscillating signals of the first pattern component and oscillating signals of the second pattern component, the optical unit being adapted to optically attenuate at least one of the oscillating signals of the first pattern component and the oscillating signals of the second pattern component to weaken the oscillating signals of the first pattern component so as to be distinguishable from the oscillating signals of the second pattern component.