Scanning Electron Microscope
EM-30PLUS Operating Manual
100
6
Wobble Operation for Aperture
6.1 Structure of Aperture
EM-30PLUS equipment is equipped with Variable Aperture. Variable Aperture reduces
spherical aberration and increases resolution by making electron beam pass through the
center of aperture. Moreover, it is possible to control Spot size of electron beam by
changing aperture size as there are 4 Stages in Variable Aperture.
What is Spherical Aberration?
As it is farther from optic axis, electron is bent stronger due to magnetic field and focus is
formed at the shorter distance.
It is impossible to remove spherical aberration completely. Minimize it by using lens with
high condensing or decreasing convergence angle.
If the electron beam does not pass through the center of aperture, image is out of focus or
screen gets dark. What is worse, if electron beam faces aperture, there are no images on
screen. In this case, Wobble operation for aperture is needed.
