Stereo Scan 440 - DCU

Resolution
Resolution can be defmed as the minimum distance by which two adjacent
features can be seen to be separated. as two discrete points. The best resolution is
usually obtained at high accelerating voltages since the shorter wavelength of
electrons at high voltages minimises any diffraction.
Other instrument parameters also significantly affect the resolution achievable
including Probe Current, Working Distance and Aperture Size.
Probe Current
The probe current is defined as the total electron current reaching the specimen
surface. The value can be adjusted from the user interface in the range I pA to
I~. In most cases, a value of lOOpA gives a good video signal without over
exposing the sample to electron bombardment. Other settings would be as
follows:-
I
I
I
I
I
High Resolution:
Backscattered electron imaging:
X-Ray analysis:
Charge or beam sensitive samples:
5pA to 25pA
200pA to 500pA
600pA to loA
lOpA to 2OpA
The improved resolution performance at low probe currents is achieved because
the diameter of the beam on the specimen is reduced. A larger beam diameter is
acceptable for X-ray analysis because of the larger volume of interaction (see
Figure 3.1).
Working Distance
Working distance is defmed as the distance in millimetres between the bottom of
the lens and the sample surface. In common with many of the other SEM
parameters, choice of working distance for the specimen may have to be a
compromise. The factors to be considered are:-
l.'
As the working distance is decreased the available resolution increases.
The optimum position is in the range 3 to lOmm.
Stereoscan 440 Operator Manual
2. If a large sample is to be examined and there is a danger of contact with
the BSD or lens, it is wise to use a longer working distance.
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