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with the beam’s size not being larger than the features of
interest within the sample. Modern EBSD cameras have been
developed to be more sensitive and capable of dealing with
lower beam currents. This in practice means that one should
try settings that generate 1–2 nA as a starting point. Higher
beam currents will be desirable for faster EBSD map acquisi-
tion. Once these conditions are set, a few patterns should be
collected. If patterns are of poor quality it may be necessary
to revisit the sample preparation steps used and modify them
in order to reduce the surface damage. Although some fine
tuning of the microscope operating conditions may improve
the pattern quality, microscope operating conditions cannot
make up for poor quality patterns due to inadequate sample
preparation.
If good quality patterns are obtained, it now an excellent
time to determine that appropriate match units have been
selected from a database of crystallographic structures. For
the highest-quality EBSD work, one should try and calibrate
the EBSD camera geometry for each experiment. Modern
systems will use one of the known phases from the sample (if
you have a known phase) and the crystallographic data file
selected to try and index one or multiple diffraction patterns.
Once a match has been found, the software will attempt to
vary the calibration parameters to select the parameters that
give the best fit to the observed EBSD pattern. Once this is
done one can assume the system is adequately calibrated. It is
recommended though that multiple points be tried to make
sure calibration is optimal.
As discussed, EBSD is not a method that is routinely used
to determine the crystal structure of the sample, although
there has been work in this area, but it is a method that
requires suitable match units to successfully index the EBSD
patterns. Excellent quality EBSD patterns will not be indexed
if an incorrect candidate unit cell has been selected. One
should attempt to index a few of the patterns obtained from
the sample with the selected candidate unit cells. If indexing
is not possible, then it may be necessary to change the candi-
date unit cell and attempt indexing. In the case of a multi-
phase sample it is important to collect patterns from all of the
phases and insure that they can be indexed from the list of
candidate unit cells.Adjust SEM and Select EBSD Map Parameters
The analysis is now at the point where the quality of the EBSD
patterns and therefore the sample preparation has been
assessed and found to be adequate. The sample has been cor-
rectly positioned within the microscope with respect to col-
lecting data that is referenced to the sample and the
microscope operating conditions have been optimized. The
EBSD collection is nearly ready to proceed with a few other
details.
First, due to the high tilt of the sample it will be apparent
at lower magnifications that the areas of the sample away
from the focus point will be out of focus. The best way todeal with this issue is the use of dynamic focus as discussed
elsewhere in this book. In brief, the dynamic focus adjusts
the focus so that the electron beam focus is changed with
respect to the position of the electron beam on the sample.
This is of most importance for low magnification detailed
scans but may still provide some advantages at higher mag-
nifications as well. Once the dynamic focus has been prop-
erly set up, one can collect the desired SEM images of the
sample.
EBSD is useful for a range of samples that include elec-
trical conductors, like metals through electrically insulating
oxide. For insulating samples or metal samples mounted for
metallography, methods must be employed to reduce charg-
ing of the sample. Sample charging is an important issue as
charging in the extreme can result in pattern distortions to
the point that the patterns are no longer useful and if only a
minor effect sample drift may result causing the resulting
images and maps to be distorted. It is sometimes adequate
to sputter coat samples to ensure conductivity. However,
this coating should be kept as thin as possible and it is best
to use metal films rather than carbon coating as the conduc-
tivity of metal coating are usually much higher. If the sam-
ple cannot be coated there is another approach that has been
found to be quite useful and that is the use of variable pres-
sure in the sample chamber. The introduction of a low pres-
sure, generally only a few 10–20 Pa, is required to reduce
charging effects for EBSD. If a pressure that is too high is
utilized there will be a noticeable degradation of the pattern
quality due to scattering of the electrons between the sample
and the phosphor screen. Thus, some trial and error may be
required to set the conditions correctly (El-Dasher and
Torres 2009 ).
There is no absolutely correct way to set up EBSD map
parameters due to the variety of sample types and the
information that may be required. There are a few simple
things that can be done to ensure that quality orientation
data are collected. One, the most important is to determine
the step size that will be used for the EBSD map. If too
large of a step size is selected, important details of the
microstructure may be missed; and conversely if too fine of
a step size is used, the resulting maps may be of high qual-
ity but will have taken much longer to acquire than if a
larger step size had been selected. One strategy for an
unknown sample is to start with a coarse step size so that a
rather quick map can be obtained and allow better sam-
pling strategies to be developed. The large step size map
will allow the user to determine if the candidate match
phase or phases are appropriate, get an estimate of the
grain size and to begin to visualize the microstructure of
the sample. The large-step-size image will then allow a bet-
ter selection of step size for more detailed studies. One rule
of thumb is that at least 10 points across the diameter of a
grain are required for a reasonable assessment of the
grain size.Chapter 29 · Characterizing Crystalline Materials in the SEM