Astigmatism in electron beam probe instruments

摘要

Supplies for the objective lens scanning coils and stigmator coils of a scanning electron microscope are arranged for digital control by a small computer which receives a digitized input from an imaging electron collector. For a specimen of suitable structure the computer can be programmed for automatic focusing and astigmatism correction or the operator can intervene to apply a manual correction. Astigmatism is detected by comparing the directional derivatives of intensity for corresponding points in two frames with lens settings above and below focus. By taking the difference between the derivatives for each direction of measurement the effect of structural directional features is eliminated; by summing the differences for all directions a value (S) is obtained which represents the magnitude of astigmatism. The required orientation of the stigmator field can be calculated and the excitation current scanned through a range of values to determine the setting for which S is a minimum.