Puts restrictions for imaging of magnetic specimens, which could be drawn inside lens
With negative consequences for the samples and the SEM. The advantage is that it has
Low aberrations, and the sample size is limited by the lens gap.
The aberrations of the final lens and consequently the resolution are controlled by
A final lens aperture, which affects the beam convergence angle.
The final lens aperture has three important effects on the final probe. First there is
An optimum aperture angle that minimizes aberrations. Second, the current in the final
Probe is controlled by the size of aperture. And third, theprobe convergence angle a
Controls the depth of field. Smaller a produces greater depth of field. Spot size in SEM is
Minimized at the expense of current.
The general approach in SEM is to minimize the probe diameter and maximize
The probe current. The minimum probe diameter depends on the spherical aberration of
The SEM electron optics, the gun source size, the electron optical brightness and the
Accelerating voltage.
The probe size, which directly effects resolution can be decreased by increasing the
Brightness. The electron optical brightness B is aparameter that is function of the
Electron gun performance and design. For all types of electron guns, brightness increases
Linearly with accelerating votage, so every electron source is 10 times as bright at 10 kV
As it is 1 kV. Decreasing the wavelength and the spherical aberration also decreases the
Probe size.
The maximumbeam current depends on several parameters. We can increase the
Brightness by increasing the accelerating voltage, the limiting tendency here is that
Increasing the accelerating voltage too much increases the volume of generation of X-
Rays and thuslimits resolution. As a consequence increasing the accelerating voltage can
Be done successfully up to about 30 kV, above that the resolution is effected negatively.
Decreasing the spherical aberration is another way of increasing the maximum
Current. This is achieved more successfully in the immersion type lenses. The above
Considerations for maximum probe current and minimum probe diameter ignore the
Contribution of chromatic aberration. These cannot be ignored when operating at low
Accelerating voltages in the range of 2 kV or less. The dependence of the probe current
Becomes much more complex due to the chromatic aberration term.