Physical Principles of Electron Microscopy: An Introduction to TEM ...

150 Chapter 5
A backscattered-electron image can be obtained in the environmental
SEM, using the detectors described previously. An Everhart-Thornley
detector cannot be used because the voltage used to accelerate secondary
electrons would cause electrical discharge within the specimen chamber.
Instead, a potential of a few hundred volts is applied to a ring-shaped
electrode just below the objective lens; secondary electrons initiate a
controlled discharge between this electrode and the specimen, resulting in a
current that is amplified and used as the SE signal.
Examples of specimens that have been successfully imaged in the
environmental SEM include plant and animal tissue (see Fig. 5-21), textile
specimens (which charge easily in a regular SEM), rubber, and ceramics.
Oily specimens can also be examined without contaminating the entire SEM;
hydrocarbon molecules that escape through the differential aperture are
quickly removed by the vacuum pumps.
The environmental chamber extends the range of materials that can be
examined by SEM and avoids the need for coating the specimen to make it
conducting. The main drawback to ionizing gas molecules during the final
phase of their journey is that the primary electrons are scattered and
deflected from their original path. This effect adds an additional skirt (tail) to
the current-density distribution of the electron probe, degrading the image
resolution and contrast. Therefore, an environmental SEM would usually be
operated as a high-vacuum SEM (by turning off the gas supply) in the case
of conductive specimens that no not have a high vapor pressure.
Figure 5-21. The inner wall of the intenstine of a mouse, imaged in an environmental SEM.
The width of the image is 0.7 mm. Courtesy of ISI / Akashi Beam Technology Corporation.