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

78 Chapter 3
The major advantage of a top-entry design is that the loading arm is
disengaged after the specimen is loaded, so the specimen holder is less liable
to pick up vibrations from the TEM environment. In addition, its axially
symmetric design tends to ensure that any thermal expansion occurs radially
about the optic axis and therefore becomes small close to the axis. However,
it is more difficult to provide tilting, heating, or cooling of the specimen.
Although such facilities have all been implemented in top-entry stages, they
require elaborate precision engineering, making the holder fragile and
expensive. Because the specimen is held at the bottom of its holder, it is
difficult to collect more than a small fraction of the x-rays that are generated
by the transmitted beam and emitted in the upward direction, making this
design less attractive for high-sensitivity elemental analysis (see Chapter 6).
3.5 TEM Imaging System
The imaging lenses of a TEM produce a magnified image or an electrondiffraction
pattern of the specimen on a viewing screen or camera system.
The spatial resolution of the image is largely dependent on the quality and
design of these lenses, especially on the first imaging lens: the objective.
Objective lens
As in the case of a light-optical microscope, the lens closest to the specimen
is called the objective. It is a strong lens, with a small focal length; because
of its high excitation current, the objective must be cooled with temperaturecontrolled
water, thereby minimizing image drift that could result from
thermal expansion of the specimen stage. Because focusing power depends
on lens excitation, the current for the objective lens must be highly
stabilized, using negative feedback within its dc power supply. The power
supply must be able to deliver substantially different lens currents, in order
to retain the same focal length for different electron-accelerating voltages.
The TEM also has fine controls that enable the operator to make small
fractional adjustments to the objective current, to allow the specimen image
to be accurately focused on the viewing screen.
The objective produces a magnified real image of the specimen (M � 50
to 100) at a distance v � 10 cm below the center of the lens. Because of the
small value of f , Eq. (2.2) indicates that the object distance u is only slightly
greater than the focal length, so the specimen is usually located within the
pre-field of the lens (that part of the focusing field that acts on the electron
before it reaches the center of the lens). By analogy with a light microscope,
the objective is therefore referred to as an immersion lens.