The actual beam diameter results from the diameter of the
original beam leaving the electron gun, dg
broadened by the effect of spherical aberation in the lenses
ds
and diffraction at the aperture,
dd.
These depend on the current, i, convergence
angle, a, brightness,
b, spherical aberation coefficient,
Cs,
and wavelength, l, via
The total beam diameter is found by adding
these three effects in quadrature i.e.
For best resolution in many applications we
need to use the smallest beam diameter.
In what situation would the diffraction limited beam diameter
dd become more important than the gun-limited
diameter dg ?
Use
the simulation to investigate the effect of the key parameters
on the minimum beam diameter. Unfortunately, in a real microscope
you cannot change Cs, nor can you reduce l
below the minimum set by the maximum available kV.
