Patent ID: 7759642

Claim:
A method for focusing a scanning microscope, comprising; scanning a primary charged particle beam across a plurality of first sites of a reference die of a wafer; detecting a secondary beam of charged particles emitted from the plurality of first sites in response to the primary charged particle beam; computing first focus scores for the plurality of first sites based on the detected secondary beam; scanning the primary charged particle beam across a plurality of second sites of a given die of the wafer while modulating a focal depth of the primary charged particle beam, the reference die and the given die having congruent layouts, and wherein the second sites respectively correspond vectorially in location with the first sites of the reference die; detecting the secondary beam of charged particles emitted from the plurality of second sites in response to the primary charged particle beam; computing second focus scores for the second sites based on the detected secondary beam emitted therefrom, and determining an exact focus of the primary charged particle beam for the second sites in response to the first and the second focus scores.