Scanning electron microscope settings for EBSD
The SEM conditions used to collect your EBSD data depend upon the type of SEM being used, resolution of the camera, phases of interest, desired speed of data collection and desired quality of data. Conventional SEMs utilize thermionic emitters such as tungsten or lanthanum hexaboride (LaB6) whereas field emission guns (FEG) can use cold-cathode, thermal or Schottky type emittters. The most important difference is that FEGs produce an electron beam which has a smaller diameter and therefore has a higher resolution, which is important if working on fine-grained samples. The specifications of the charge-coupled device (CCD) camera available will affect the quality of the patterns collected and therefore affect the optimum operating conditions. EBSD utilises a geometry with an angle of 70° between the incident beam and the normal to the sample surface. EBSD can be performed at lower angles (such as where the sample stage can only be tilted to 60°) but the quality of the patterns will be reduced. The distance between the phosphor screen (mounted in front of the CCD camera) and the sample is important, the closer the screen is to the beam interaction point, the larger the angular range of the collected EBSP which improves indexing. The downside of this close geometry is the increased risk of collision between the sample and detectors. The spacing between the pole piece and the sample surface, the working distance, also needs to be matched to the best projection of the EBSP on the phosphor screen. Typical distances are between 14 and 33mm.
Typical accelerating voltages used for EBSD are between 10 and 30kV with most systems using 20kV (Bestmann et al., 2011; Ebner et al., 2010; Okudaira et al., 2010; Peternell et al., 2010; Schwarzer et al., 2009). It is important to note that if the carbon coat is a little too thick a higher accelerating voltage can be used to generate a useable EBSP or if the sample is heavily deformed a lower accelerating voltage may be more appropriate. The beam current is controlled by the chosen aperture size and spot size. A high beam current (10-100nA depending upon the SEM) can be used if mapping very quickly (30-50 points per second in minerals or 300 points per second in metals), but if running more slowly (3-6 points per second) then a lower beam current should be used to avoid unacceptable levels of electron damage. The settings given here are guidelines which will need to be adjusted based upon the hardware and software available as well as the phase(s) to be measured. For an in depth discussion of the factors which affect EBSD patterns see (Britton et al., 2010; Mingard et al., 2011).