Automatic beam correction in a scanning transmission electron microscope

An electron microscope comprising an electro-optical lens system including a stigmator,

• a structured object on an object plane,
• beam scanning means for scanning said object with an electron beam,
• detection means for detecting electrons after passing through said object, said detection means including a plurality of individual detector elements arranged in a detector plane to provide independent electrical output signals, and
• electrical control circuit means connected to said detector elements for comparing output signals from a pair of spaced detector elements in order to determine movement of an electron intensity pattern present in said detection plane of said detection means in a direction from one detector element to the other with reference to a corresponding electron beam scan of said structured object,
• said electrical control circuit including first circuit means for forming a correlation integral of signals from said pair of spaced detector elements with respect to time for different applied time delays, second circuit means for determining a time delay required to cause said correlation integral to become a maximum, and third circuit means for generating a correction control signal dependent on said determined time delay to control energization of a corresponding electro-optical element controlling said electron beam before said object plane in order to provide at least one of an optimum focus condition and a predetermined amount of astigmatism correction.