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Undertaking EBSD experiments
Microscope operating conditions
It is very important to
understand the effect of varying the microscope operating conditions on the
diffraction pattern.
Probe current
Increasing the probe current will
increase the number of electrons contributing to the diffraction pattern and
so allow the camera integration time to be reduced
(Figure 1). However, this must be
balanced with the spatial resolution required, because increasing the probe
current will also increase the electron beam size.
Accelerating voltage
Increasing the accelerating
voltage reduces the electron wavelength and hence reduces the width of the
Kikuchi bands in the diffraction pattern (see equation 2). Also, because
more energy is being deposited on the phosphor screen, this will result in a
brighter pattern which requires a shorter integration time (Figure 4).
Changing the accelerating voltage may require adjustment to the Hough
transform filter size to ensure the Kikuchi bands are detected correctly.
Higher accelerating voltages may be required to penetrate conducting layers,
and lower accelerating voltages for restraining the beam to thin layers, or
for charging samples.
Working
distance and magnification
Because the sample is tilted, the
SEM working distance will change as the beam position moves up or down the
sample, and the image will go out of focus (Figure 5). The image will also
be foreshortened because of the tilt and at low magnifications much of the
field of view could be out of focus. Some EBSD systems can compensate for
the image foreshortening by using different horizontal and vertical image
beam steps and can adjust the SEM focus automatically as the beam is moved
over the sample (Figure 5).
In addition, movements of the
beam will alter the pattern centre position on the phosphor screen and this
can affect the EBSD system calibration (Figure 6) . EBSD systems can compensate
automatically for shifts in the pattern centre by calibrating at two working
distances and interpolating for intermediate working distance values. It is
important to know the range of working distances for which the EBSD system
will remain accurately calibrated.
Pressure
Diffraction patterns can also be
collected from samples at low vacuum in environmental SEMs (Figure 7). This
can be useful with specimens which may otherwise charge, such as ceramic or
geological materials.
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