In EBSD analysis, backscattered electrons vary in their intensity and direction depending on the topography and composition of the specimen. The contrast of the backscattered electron image is based on several factors such as the atomic number (Z) of the sample material, the acceleration voltage of the primary beam and the specimen angle (tilt) with relation to the primary beam.
The specimen angle is the main parameter for understanding topographic imaging. The Phenom’s backscattered detector of the Phenom SEM is mounted close to the sample, under the objective lens. The detector is split into four quadrants.
The two types of information contained in the backscattered electron image can be separated by electronically pairing the opposing quadrants. It is possible to obtain information on sample composition and topography. The two images are from the same material, showing the compositional mode as in Figure 1 and the topographical mode as in Figure 2.
Figure 1. BSE imaging in compositional mode
Figure 2. BSE imaging in topographical mode
Subtraction of the quadrants will offer topographic information, and operating the detector quadrants in pairs and then adding the signals will provide compositional information about the sample. Rotating the signal pairing enables the operator to effectively ‘shine the light’ on the sample from two different directions, over-under and left-right.
Figure 3 shows the principle of the compositional full mode, where all quadrants are active.
Figure 4 shows the topographical B mode principle, which indicates the over-under illumination capabilities.
The use of the over-under mode will show a strong shadow effect like the sun shining on an object as can be seen in the two corresponding images. Both images show the letter ‘S’ (Specimen - Spanish Euro coin) taken in compositional (full) mode as shown in Figure 5 and topographical B mode as shown in Figure 6. Switching between the two modes is done via the software.
Figure 3. Schematic representation of the Phenom detection system in compositional (full) mode
Figure 4. Schematic representation of the Phenom detection system in topographic mode
Figure 5. Image of the letter S (Euro coin) in compositional imaging mode
Figure 6. Image of the letter S (Euro coin) in topographical imaging mode
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This information has been sourced, reviewed and adapted from materials provided by Phenom-World BV.
For more information on this source, please visit Phenom-World BV.