It is very important to understand the effect of varying the microscope operating conditions on the diffraction pattern.
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.
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.
(a) Left: With a tilted sample, the pattern centre position will depend on the sample working distance. Middle: The top and bottom of the field of view may have a different working distance and hence pattern centre positions. Right: If the sample is moved, the working distance and hence pattern centre position will change.
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.