FEI Quanta FEG 250, Руководство пользователя

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Operating Procedures: Detector Types and Usage
5-20
DISCHARGES BETWEEN THE GASEOUS
DETECTORS AND THE SAMPLE
Excessive voltage may cause a “breakdown” between the detector
and the sample, chamber, pole-piece etc. This could damage the
sample (at ground) but does not damage the detector. This condition
is indicated by white flashes or streaks across the image, and on
some systems a large discharge could make the system unstable or
cause the chamber to vent and switch off the HV. There are several
factors that could cause detector voltage breakdown:
• Contrast is set too high.
• Sample is too close to the detector.
• Gas pressure is low (depending on the detector and the
environment).
• Air in the chamber (water vapour purge cycle not complete).
• Sample is not grounded to the stage, or the stage is not fully
grounded (BNC plug not connected).
OBTAINING IMAGE PROCEDURE
FOR GASEOUS DETECTORS
1. After the pumping and purge process click the Beam On button.
2. The Contrast (detector voltage) adjuster in the Detector module is
automatically adjusted to 70% of maximum value. Adjust it if
necessary to increase a signal or to stop possible discharges. The
Enhanced Contrast adjuster (electronic gain) is set to the value last
used.
3. At the lowest possible magnification, adjust the Enhanced Contrast
and Brightness if necessary, until the bright circle of the used cone
can be seen in the centre of the image.
4. Increase magnification and adjust Contrast / Enhanced Contrast
and Brightness more precisely.