Patent ID: 7091486

Claim:
An electron beam apparatus for imaging a substrate surface, the apparatus comprising: an electron source configured to generate an electron beam; a beam-limiting aperture configured to block a portion of the electron beam while at a same time allowing transmission of another portion of the electron beam through the aperture to subsequently impinge upon the substrate surface for said imaging; a first scattered electron (SE) detector for gain adjustment which is configured to detect a first signal from scattered electrons emitted from a surface of the aperture due to impingement onto the surface by the blocked portion of the electron beam; a second scattered electron (SE) detector for generating image data which is configured to detect a second signal from scattered electrons emitted from the substrate surface due to impingement onto the substrate surface by the transmitted portion of the electron beam; and a gain control device configured to use the first signal to adjust a gain level for the second signal, where the first signal is detected at a same time as the second signal.