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3 Product and Functional Description | 3.5 Optional Components and Accessories
ZEISS
Fig. 26:
images, eye shadow (left), kitchen sponge (right)
On the surface of the specimen, secondary electrons are created. The secondary electrons acceler-
ate towards the VPSE detector due to the bias voltage that is applied to the collector of the VPSE
detector. On their way to the detector, the secondary electrons collide with residual gas mole-
cules (nitrogen, air molecules, or water). The collisions ionize the residual gas molecules and cre-
ate additional electrons. These additional electrons also accelerate towards the detector and col-
lide with further gas molecules, which are ionized.
The result is an “ion cascade” that amplifies the original SE signal and that creates photons as a
secondary product. The VPSE detector does not use the SEs themselves as a signal, but it uses the
photons that are created by the “ion cascade”.
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.
The optimum chamber pressure depends on the specimen and the operating parameters. It is usu-
ally in the range between 20 Pa and 60 Pa.
Info
If the chamber pressure rises, then the scattering of electrons is increased and the resolution of
the microscope is reduced.
Try to find the optimal chamber pressure for each individual application.
Dwell Time
The dwell time is the amount of time that the electron beam stays at one position on the speci-
men before it moves to the next position.
If the dwell time is too short (i.e. the scan rate is too fast), then there is not enough time for an
“ion cascade” to develop and to create the imaging photons. This reduces the efficiency of the
detector.
If the dwell time is too long (i.e. the scan rate is too slow), then the electron beam delivers a large
amount of energy to each individual spot on the specimen. This may result in charging artifacts on
the images.
The optimal dwell time depends on the specimen and needs to be determined by experiment.
Info
To reduce charging effects, use the Frame Averaging function of SmartSEM. Use fast scan
speeds and increase the number of frames (N).
Collector Bias
The collector bias corresponds to the voltage that is applied to the VPSE collector. The collector
bias accelerates the secondary electrons from the specimen surface towards the VPSE detector.
Typical VPSE collector bias values are between 50 % and 80 %.
If the collector bias is too low, the efficiency of the detector is reduced.
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Instruction Manual ZEISS EVO | en-US | Rev. 10 | 354706-0780-006
Summary of Contents for EVO
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