ZEISS
3 Product and Functional Description | 3.2 Main Components
Parameter
Description
Aperture
30 μm
The standard aperture is recommended for many applications
7.0 μm to 20 μm
Limitation of the probe current for the compensation of charges, or
for the analysis of beam-sensitive specimens
60 μm and 120 μm
Only recommended for analytical purposes
Specimen tilt
Tilting the specimen towards the detector increases collection effi-
ciency
Operation mode
Only suitable in high vacuum
3.2.5.5 VPSE Detector
Purpose
The Variable Pressure Secondary Electron (VPSE) detector is a specific type of SE detector for use
in Variable Pressure mode where a standard SE detector cannot be used. The VPSE detector is not
usable and will not operate in HV mode.
The Variable Pressure mode enables analyzing and imaging of non-conducting specimens without
charging artefacts. This is possible, because positively ionized gas molecules stabilize local charg-
ing. Variable Pressure mode can also be used for strongly gassing or moist specimens without any
need for specimen preparation.
Position
The VPSE detector is attached to the MP-Port 1.
1
2
3
4
5
Fig. 34: Schematics of the
detector
1
Preamplifier
2
Photomultiplier
3
Objective lens
4
Light guide with collector electrode
5
Specimen
Function
The collector electrode mounted on the light guide
4
is held on a positive potential forming a
collecting field for SEs. Thus, SEs move towards the detector. In VP mode gas molecules are
present in the specimen chamber. The accelerated SEs excite gas molecules, which emit a photon
when they de-excite to the ground state. Although BSEs also cause collisions, their contribution is
less than 1 % because of the lower ionizing cross section.
The emitted photons are detected by the rod-like light guide pointing at the specimen. The photo-
multiplier
2
amplifies the light signal and converts it into an electron current. The degree of
amplification is depending on the photomultiplier voltage, which regulates the contrast. The
preamplifier
1
amplifies the signal and regulates the brightness.
Chamber Pressure
For an optimum use of the VPSE detector, the pressure in the specimen chamber must be high
enough. If the pressure is too low, then too few gas molecules are present and the collision prob-
ability is too low. This reduces the efficiency of the detector.
Instruction Manual ZEISS SIGMA series | en-US | Rev. 7 | 352102-9344-006
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