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

126 Chapter 5
specimen
scan
coils
objective
lens
z
x
specimen
electron gun
condenser
lenses
detector
x
y
magnification
control
scan
generators
signal
amplifier
display
device
image
scan
Figure 5-1. Schematic diagram of a scanning electron microscope with a CRT display.
Above the specimen, there are typically two or three lenses, which act
somewhat like the condenser lenses of a TEM. But whereas the TEM, if
operating in imaging mode, has a beam of diameter � 1 �m or more at the
specimen, the incident beam in the SEM (also known as the electron probe)
needs to be as small as possible: a diameter of 10 nm is typical and 1 nm is
possible with a field-emission source. The final lens that forms this very
small probe is named the objective; its performance (including aberrations)
largely determines the spatial resolution of the instrument, as does the
objective of a TEM or a light-optical microscope. In fact, the resolution of an
SEM can never be better than its incident-probe diameter, as a consequence
of the method used to obtain the image.
Whereas the conventional TEM uses a stationary incident beam, the
electron probe of an SEM is scanned horizontally across the specimen in two
perpendicular (x and y) directions. The x-scan is relatively fast and is
generated by a sawtooth-wave generator operating at a line frequency fx ; see
Fig. 5-2a. This generator supplies scanning current to two coils, connected in
series and located on either side of the optic axis, just above the objective
lens. The coils generate a magnetic field in the y-direction, creating a force
on an electron (traveling in the z-direction) that deflects it in the x-direction;
see Fig. 5-1.
The y-scan is much slower (Fig. 5-2b) and is generated by a second
sawtooth-wave generator running at a frame frequency fy = fx /n where n is
an integer. The entire procedure is known as raster scanning and causes the
beam to sequentially cover a rectangular area on the specimen (Fig. 5-2d).