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

TEM Specimens and Images 119
From the velocity triangle of Fig. 4-19a, the angle of deflection of the
electron (due to the magnetic field inside the specimen) is
�� tan� = vx /vz � vx /v = (e/m)(Bt/v) (4.26)
Taking B = 1.5 T, t = 100 nm, v = 2.1 � 10 8 m/s, and m = 1.3 �10 -30 kg (for
E0 = 200 keV), Eq. (4.26) gives �� 0.1 mrad. The deflection is therefore
small but can be detected by focusing on a plane sufficiently far above or
below the specimen (see Fig. 4-19a) so that the magnetic domains become
visible, as in Fig. 4-19b.
With the objective lens (focal length f ) turned on, this deflection causes
a y-displacement (of each diffraction spot) equal to �� f at the objectiveaperture
plane, due to electrons that have passed through adjacent domains.
This can be sufficient to produce an observable splitting of the spots in the
electron-diffraction pattern recorded from a crystalline specimen. If the TEM
objective aperture or illumination tilt is adjusted to admit only one spot from
a pair, a Foucault image is produced in which adjacent magnetic domains
appear with different intensity. Its advantage (over a Fresnel image) is that
the image appears in focus and can have good spatial resolution, allowing
crystalline defects such as grain boundaries to be observed in relation to the
magnetic domains (Shindo and Oikawa, 2002).
4.10 TEM Specimen Preparation
We have seen earlier in this chapter that electrons are strongly scattered
within a solid, due to the large forces acting on an electron when it passes
through the electrostatic field within each atom. As a result, the thickness of
a TEM specimen must be very small: usually in the range 10 nm to 1 �m.
TEM specimen preparation involves ensuring that the thickness of at least
some regions of the specimen are within this thickness range. Depending on
the materials involved, specimen preparation can represent the bulk of the
work
involved in transmission electron microscopy.
Common methods of specimen preparation are summarized in Fig. 4-20.
Practical details of these techniques can be found in books such as Williams
and Carter (1996) or Goodhew (1985). Here we just summarize the main
methods, to enable the reader to get some feel for the principles involved.
Usually the material of interest must be reduced in thickness, and the initial
reduction involves a mechanical method. When the material is in the form
of a block or rod, a thin (< 1 mm) slice can be made by sawing or cutting.
For hard materials such as quartz or silicon, a “diamond wheel” (a fastrotating
disk whose edge is impregnated with diamond particles) is used to
provide a relatively clean cut.