Stereo Scan 440 - DCU

Electron Gun and Column
Setting Filament Cu"ent and Aligning the Beam
Filament Setting
Optimum perfonnance of the SEM is achieved only after satUration and
alignment of the electron gun. The gun runup function sets the filament current
at the value it was last set, so it is important to keep a check on this as the
f~nt ages and after large changes to accelerating voltage, so that its working
life is maxirnised.
Saturation and alignment can both be adjusted, quite quickly from the same
operating panel - Gun Alignment. This panel is obtained by pressing the Align
icon with the right mouse button. Adjust as follows:-
For Tungsten:
Press the Emission button. The fll~nt emission profile can now be seen -
adjust the manual brightness. if necessary. or switch on autovideo peak from
the signal adjust panel. The profile should be roughly egg-shaped. depending
on the present value of fil~nt cun-ent.
2. Using the arrows at each end of the Filament current scroll-bar. adjust the
filament current so that the profile jmt condenses down to an egg-shaped
mass. This is the saturation point (or second peak see Figure 2.2) and usually
occurs at about 2.75A for a new filament. If "first peak" operation is
required. reduce the filament current to about 2.25A when a bright cenb'al
emission image will be seen surrounded by an outer "halo". Operation at this
point gives reduced resolution but much longer lifetime.
2nd peak
1 st peak '"
Source
Brightness
\
Filament I
FIgUre 2.2 Seoond Peak
Slereoscan 440 Operator Manual