AUTOMATIC BEAM CORRECTION IN A SCANNING TRANSMISSION ELECTRON MICROSCOPE

摘要

This invention relates to a scanning electron microscope with a periodic structure in the object plane used for the detection of the focus condition of a spot focused electron beam scanning an object in order to correct defocussing and astigmatism in the scanning electron beam spot. To achieve this the detector comprises a plurality of individual elements which can be pair-wise read and an electronic circuit for forming a control signal for controlling the excitation of a spot-forming lens and a stigmator, respectively, from signals representing movement of the electron interference pattern at the detector, relative to the object scan, due to an out of focus condition, the signals being derived from corresponding pairs of detector elements which are situated at a fixed distance from each other in order to correct the focus and compensate for the astigmatism in the electron beam respectively. In the case of astigmatism, signals from at least two pairs of detector elements spaced in directions at right-angles to one another, are used. The invention overcomes the drawback associated with other known methods wherein the lens current must be frequently and continually readjusted.