Study on algorithm of automatic alignment compensation of electron gun in a scanning electron microscope

Abstract

Primary electrons are emitted from the cathode in a scanning electron microscope (SEM). The electron gun's cathode diameter is ~60 µm and the anode's internal diameter is ~3 mm. When the cathode and anode centres do not match, the SEM image is distorted. We developed an automatic alignment method for the electron gun. The primary electrons are scanned under the anode hole for secondary electron image acquisition. However, when the primary electrons are scanned from the upper anode hole, secondary electrons on the specimen as big as the size of the anode hole are generated. The alignment of the cathode and anode centres is determined from the image's brightness and position on the screen; if the image is very bright, the alignment is normal. We employ pattern recognition to analyse images to achieve automatic alignment of the electron gun. We designed a scanning device and performed image scale calibration.