499 29
zone axes are clearly visible. In high quality patterns as shown
in. Fig. 29.11, there are many other features that are visible
and may be used to understand the unit cell from which the
pattern was collected.
29.2.2 Cameras for EBSD Pattern Detection
It is relatively easy to collect EBSD patterns in the SEM. The
earliest EBSD images were collected by directly exposing
photographic emulsions inside the specimen chamber of the
SEM. As discussed previously, EBSD patterns are obtained
from bulk samples by illuminating a highly tilted surface
with the electron beam and then collecting the patterns on a
position sensitive detector, sometimes referred to as a cam-
era, that is 1–2 cm from the tilted sample surface. Generally
the detector surface is located normal to the sample tilt axis
so that tilting of the sample is easily achieved but the detec-
tor may also be tilted a few tens of degrees away from the
horizontal position. The first electronic capture of EBSD pat-
terns utilized low-light video rate cameras that produced
useable but somewhat noisy images requiring the use of very
high beam currents in the SEM. Current EBSD cameras con-
sist of a fluorescent screen, either circular or rectangular,
that is about 2 cm in diameter if circular or 2 cm on a side if
square or rectangular formats are utilized. The screen is
coated with a thin Al coating to avoid charging and to
exclude light. The fluorescent screen is kept thin so that it
can be imaged from the opposite side using either a fiber
optic bundle or a lens to transfer the image on the phosphor
to a charge coupled device (CCD) or a CMOS type solid
state imager. Camera designs strive to minimize the loss of
light from the fluorescent screen to increase the speed at
which patterns can be collected or to increase the quality of
the detected patterns. The transfer optics must also be
designed to minimize any optical distortions to the collected
patterns (Schwarzer et al. 2009 ).
The imager should have a sufficient number of pixel ele-
ments so that the angular resolution is adequate for detection.
In addition, it is extremely useful to be able to bin the pixels
in the detector. Binning pixels simply means that adjacent
pixels are summed together (normally in specific patterns
like 2 × 2 or 4 × 4) to increase the signal but with a decrease
in pattern resolution. Currently, detectors are capable of col-
lecting more than 1000 patterns per second when heavily
binned and with sufficient electron beam current.
The entire detector must be mounted on a precision
retractable stage. The precision retractable stage allows the
detector assembly to be moved into the same position each
time the detector is inserted to maintain the geometrical cali-
bration of the system. It is important to position the detector
close to the sample, generally the sample to detector distance
should be on the order of or even slightly less than the fluo-
rescent screen diameter so that a large portion of the EBSD
pattern may be collected. The insertion mechanism is motor-
ized to allow the camera to be moved into position in a mat-
ter of few seconds. A typical arrangement of the sample and
the EBSD camera or detector is shown in. Fig. 29.12. Also
visible at the bottom of the EBSD detector are the forescat-
tered electron detectors. These solid state detectors provide
excellent crystallographic contrast when the sample is highly
tilted and standard backscattered or secondary electron
imaging are not optimal.29.2.3 EBSD Spatial Resolution
It is important to understand the volume from which the dif-
fraction pattern is generated in the SEM. In many applica-
tions the highest resolution is not required to utilize EBSD
for the mapping of the texture of a microstructure. However,
there are applications where high spatial resolution is
required. Examples are the mapping of deformed micro-
structures or the study of fine grained materials.
The formation of EBSD patterns depends on the scatter-
ing and subsequent diffraction of electrons. Bragg’s Law
describes the diffraction of specific wavelengths of electrons
from the planes in a crystalline solid. If there is a large spread
in the energy of the electrons that are exiting the sample sur-
face we would not expect to see the lines that we see in our
EBSD patterns due to diffraction. These lines are in nearly the
exactly correct position as described by the Bragg diffraction
of the electrons based on the beam voltage used to produce
the patterns. From this we must conclude that the electrons
leaving the surface are of nearly the same energy. Electrons
that have lost larger amounts of energy contribute to the
background intensity and do not contribute to the diffraction
features that we want to observe (Deal et al. 2008 ).SampleDetector. Fig. 29.12 An image from the inside of an SEM sample region with
the sample tilted for EBSD and the EBSD camera or detector inserted.
Also note at the bottom of the detector screen there are small rect-
angular solid state electron detectors for imaging the sample while it
is highly tilted. These detectors result in remarkable crystallographic
contrast
29.2 · Electron Backscatter Diffraction in the Scanning Electron Microscope