Practical Electron Microscopy and Database

In STEM, there is only a condenser system, which is a lens used to form a fine probe. Sometimes this lens is also called objective lens. The correction of the spherical aberration of the pre-field lens is much cheaper comparing to post-field corrections. The main advantage of such correction is to reduce the beam tails so that a fine beam can be positioned at a specified column of atoms and does not spread its intensity into neighboring columns significantly. This tail spilling is critical for high resolution Z-contrast (HAADF) imaging and EELS analysis.

The attainable STEM resolution has improved by about 2.5 × and the current available in a given small probe has increased by about 10 ×. Many modern aberration-corrected STEMs have electron probes less than one atom dimension in diameter so that atom column resolution for composition analysis can be obtained.

Note that the dependence of wave aberration function (W_x,y) on the image plane can be neglected in STEM mode because of the small probe dimensions.