Use gun bias 1 for
20 and greater kv
and bias 2 for 10 –
20 kv.
Use a spot size of 500
and the lowest
magnification to align
the electron gun.
Non-conductive samples may charge with these settings. Biological
specimens that have been coated usually can be viewed with a lower
accelerating voltage, usually 10 – 15 kV. With this setting, set the gun bias
to “2” so as to keep the emissions current in a safe range of 30 – 40
µ
A. A
lower kV setting usually improves the surface image but also adds “noise” to
the detector signal.
With the Wehnelt gun cap fitted with a 200
µ
aperture, and the filament to
Wehnelt distance adjusted 6 small divisions, the following should be
expected at filament saturation:
Bias 1, 25 kv, saturated filament current = 40
µ
A.
Bias 2, 10 kv, saturated filament current = 35
µ
A.
To lower the saturated filament current, increase
the filament to Wehnelt distance.
Select spot size:
Generally speaking, use a large spot size (200 or
500) for low magnification work under 1000 x
magnification. Smaller spot size is used for higher
magnifications. As spot size is reduced, the
detector gain must be increased (and visa versa)
because fewer secondary electrons are generated.
Black level is not affected as much. For 5K to 20K magnification work, use a
10 - 20
µ
spot size.
Stage controls:
Do not adjust the “Z” control. This is set to provide an optimal distance
(10 – 12 mm for secondary electron detector and 20 mm for the EDAX
d e t e c t o r ) .
Do not force any stage control knob.
