Patent ID: 7842933

Claim:
A method for detecting overlay errors, comprising: directing a primary electron beam to interact with an inspected object, wherein (i) the inspected object has a first feature formed on a first layer, a second feature formed on a second layer, and an intermediate layer positioned between the first layer and the second layer, (ii) the second feature is buried under the first layer, affects a shape of an area of the first layer, and does not overlap with the first feature, and (iii) the area includes a protuberance positioned above the second feature and that is smaller than the first feature, said directing comprising propagating the primary electron beam along an optical axis, diverting the primary electrical beam to propagate along a secondary optical axis that is parallel to but spaced apart from the optical axis, and subsequently diverting the primary electron beam so as to again propagate along the optical axis; detecting electrons reflected or scattered from the protuberance of the area of the first layer, wherein said electrons are scattered or reflected at angles less than eighty degrees with respect to a surface of the inspected object; receiving detection signals from a first in-lens detector and a second in-lens detector, the second in-lens detector positioned to detect those of the electrons that pass through an aperture in the first in-lens detector; and determining the overlay errors according to the detection signals.