![background image](http://html2.mh-extra.com/html/zeiss/sigma-series/sigma-series_instruction-manual_4422473050.webp)
3 Product and Functional Description | 3.2 Main Components
ZEISS
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.
If the collector bias is too high, the VPSE detector may receive too much signal and may get satu-
rated. In this case, very bright lines are visible on the image in periodic intervals and proper imag-
ing is no longer possible. Whether a given value for the collector bias is too high, depends on the
specimen, the acceleration voltage, the probe current, and the pressure in the specimen chamber.
Fig. 35: Saturation of the
detector. Bright lines become visible in the microscope image
To eliminate the bright lines in the images, either reduce the collector bias or reduce the chamber
pressure. However, the detector efficiency is reduced by either of these adjustments. You need to
find the optimum parameters for imaging different specimens. It is generally better to reduce the
collector bias, because reducing the chamber pressure can cause new charging effects.
50
Instruction Manual ZEISS SIGMA series | en-US | Rev. 7 | 352102-9344-006