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

The Scanning Electron Microscope 149
Typically, E2 is in the range 1 � 10 keV. Although there are tables giving
values for common materials (Joy and Joy, 1996), E2 is usually found
experimentally, by reducing the accelerating voltage until charging artifacts
(distortion or pulsating of the image) disappear. E1 is typically a few hundred
volts, below the normal range of SEM operation.
The use of low-voltage SEM is therefore a practical option for imaging
insulating specimens. The main disadvantage of low E0 is the increased
chromatic-aberration broadening of the electron probe, given approximately
by rc = Cc�(�E/E0). This problem can be minimized by using a fieldemission
source (which has a low energy spread: �E < 0.5 eV) and by
careful design of the objective lens to reduce the chromatic-aberration
coefficient Cc. This is an area of continuing research and development.
5.8 The Environmental SEM
An alternative approach to overcoming the specimen-charging problem is to
surround the specimen with a gaseous ambient rather than high vacuum. In
this situation, the primary electrons ionize gas molecules before reaching the
specimen. If the specimen charges negatively, positive ions are attracted
toward it, largely neutralizing the surface charge.
Of course, there must still be a good vacuum within the SEM column in
order to allow the operation of a thermionic or field-emission source, to
enable a high voltage to be used to accelerate the electrons and to permit the
focusing of electrons without scattering from gas molecules. In an
environmental SEM (also called a low-vacuum SEM), primary electrons
encounter gas molecules only during the last few mm of their journey, after
being focused by the objective lens. A small-diameter aperture in the bore of
the objective allows the electrons to pass through but prevents most gas
molecules from traveling up the SEM column. Those that do so are removed
by continuous pumping. In some designs, a second pressure-differential
aperture is placed just below the electron gun, to allow an adequate vacuum
to be maintained in the gun, which has its own vacuum pump.
The pressure in the sample chamber can be as high as 5000 Pa (0.05
atmosphere), although a few hundred Pascal is more typical. The gas
surrounding the specimen is often water vapor, as this choice allows wet
specimens to be examined in the SEM without dehydration, provided the
specimen-chamber pressure exceeds the saturated vapor pressure (SVP) of
water at the temperature of the specimen. At 25�C, the SVP of water is about
3000 Pa. However the required pressure can be reduced by a factor of 5 or
more by cooling the specimen, using a thermoelectric element incorporated
into the specimen stage.