FEI Quanta FEG 250, User'S Operation Manual

Page: 155 / 228

System Options: Optional Detectors
9-7
Optional Detectors
GASEOUS BACK SCATTERED ELECTRON
DETECTOR (GBSD)
The GBSD is used in place of the standard Gaseous SE (GSED)
detector and Large Field Detector (LFD). It was specifically designed
to image at pressures above 4 Torr (534 Pa) (predominantly in the
range of 6 to 9 Torr (800 to 1200 Pa)). This makes the GBSD best
suited for use with the Peltier when both secondary and back
scattered images are required.
This detector is integrated into a flexible PC board and plugs into the
signal connector behind the conical lens.
FIGURE 9-3 GBSD OPERATIONAL SCHEMA
The grid on the bottom of the GBSD board is used to collect all SE
signals from the gas. This grid, the surface of the board around the
grid and the PLA1 are connected to high voltages up to ± 600 V during
ESEM operation.
Installing and Settings the GBSD
Follow the same procedure as described for GSED (see Chapter 5).
Note:
the GBSD or other gaseous detectors do not need to be removed to
pump to HiVac at the end of a day. They may be left in the place.
However, stages with water lines inside the chamber must be removed.
At the Detector Settings module the following options are possible:
• Mode list box with choices:
The Secondary Electrons (SE) mode: the converter plate voltage
is reversed and a second positive voltage is applied to the SE
collection grid. SE from the sample are collected directly and the
SE image is similar to the GSED image. All SE generated by BSE
at the converter plate are drawn back to the converter plate.
The Backscatter Electrons (BSE) mode: BSE generated by the
primary beam, strike the BSE converter plate and generate SE,
which are accelerated (repelled) by a negative voltage placed on the