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29.2.1 Origin of EBSD Patterns
EBSD patterns are obtained in the SEM by illuminating a
highly tilted specimen with a stationary electron beam. The
beam electrons interact with the sample and are initially
inelastically scattered. There are two current methods that
are employed. The first method (conventional EBSD) uses
the sample in a mode where backscattered electrons are
detected (i.e., from a bulk sample). The second method
(often termed TKD or transmission-EBSD although the use
of the latter implies the impossibility of simultaneous trans-
mission and backscattering of the electrons) relies on a very
thin sample and the patterns are then formed by the trans-
mitted electrons. In the case of the bulk sample and back-
scattered electron detection, the sample is held at a steep
angle with respect to the electron beam. When a thin sample
is used for TKD, the sample does not need to be tilted at
such a high angle, and in fact the electron beam may be used
to illuminate a sample which is not tilted. In either case,
backscatter or transmission, some of these inelastically scat-
tered electrons that have lost little of their original energy
satisfy the diffraction condition with the crystalline planes
within the sample. When this interaction happens near the
surface of the sample these electrons escape and form the
EBSD pattern that is observed. These backscattered elec-
trons appear to originate from a virtual point below the sur-
face of the specimen. These types of patterns were first
described by Kikuchi and are often referred to as Kikuchi
patterns (Randle 2013 ). Some of the backscattered electrons
satisfy the Bragg conditions for diffraction (+θ and −θ) and
are diffracted into cones of intensity with a semi-angle of
(90-θ), with the cone axis normal to the diffracting plane. As
shown by the Bragg Eq. (29.1), and. Table 29.1, the short
wavelength of the electron (at typical accelerating voltages
of 10–30 kV used in the SEM) results in a small Bragg angle
of less than 2°. Each plane yields two cones of intensity. The
cones are quite flat and when they intercept the imaging
plane they are imaged as two nearly straight lines separated
by an angle of twice the Bragg angle. An alternative but
equivalent description is the single event model. In this
model, it is argued that the inelastic and elastic scattering
events are intimately related and may be thought of as one
event. In this case the electron channels out of the sample
and forms the EBSD pattern (Winkelmann 2009 ; Randle
2013 ).. Figure 29.11 are two examples of an EBSD patterns
collected from the mineral hematite at 5 kV and 40 kV. Note
that the Kikuchi bands appear as nearly straight lines. The
effect of the accelerating voltage is clearly seen. As the accel-
erating voltage is increased the Bragg angle decreases result-
ing in more narrowly spaced Kikuchi bands in the patterns.
It is also important to note that the patterns are fixed with
respect to the crystal orientation and so the positions of the
line traces in the patterns have not moved. Fortunately we
need not understand the exact physics of EBSD pattern for-
mation in order to use these patterns for crystallographic
analysis.EBSD patterns consist of what appear to be nearly straight
bands (they are actually conic sections) which may have
bright or dark centers with respect to the rest of the pattern.
These straight bands are the Kikuchi lines discussed previ-
ously. The Kikuchi bands intersect in many locations and
these are called zone axes which are actual crystallographic
directions within the unit cell of the crystal. The angles
between the Kikuchi bands and the angles between the zone
axes are specific for a given crystal structure. These features
can be seen in. Fig. 29.11b where the Kikuchi bands and theab. Fig. 29.11 EBSD patterns acquired from the mineral hematite (trigonal)
that demonstrate the pattern changes that result from the acceleration
voltage change. Note that as expected from the Bragg equation the band
widths decrease with increasing accelerating voltage. a 5 kV, b 40 kV
Chapter 29 · Characterizing Crystalline Materials in the SEM