1. An apparatus configured to detect a buried defect in a target microscopic metal feature, the apparatus comprising: a charged-particle beam column configured to generate an incident beam of charged particles, wherein the landing energy is at a level such that the charged particles, on average, reach a depth of interest, and the depth of interest is less than a maximum depth of the buried defect; a detector configured to filter out secondary electrons and detect backscattered electrons; and a data processing system configured to compare a backscattered electron (BSE) image of the target microscopic metal feature with a BSE image of a reference microscopic metal feature to detect and classify the buried defect, wherein the data processing system is further configured, if said target feature is a plug, to classify the buried defect as a hollow bottom if the charged particles reach, on average, a depth near a bottom of the plug, and if pixels in a middle region of said target feature have lower grey levels when compared against pixels in the middle region of said reference feature.