In scanning electron microscopy (SEM), samples are imaged using a focused electron beam that is rastered across a surface. Different types of electrons are emitted from samples. A backscatter electron detector (BSD) detects elastically scattered electrons. These electrons are higher in energy from atoms below the sample surface. Using a BSD allows for lower vacuum levels, reducing sample preparation requirements and minimizing beam damage.
Backscattered electrons vary in their amount and direction due to the composition and topography of the specimen. The contrast of the backscattered electron image depends on multiple factors, including 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. Materials with elements composed of higher atomic number (Z) yield more backscattered electrons than lower Z elements. For the Phenom SEM, a four-quadrant solid-state backscatter electron detector provides both topography and materials contrast (composition) imaging.
Schematic of a four-quadrant BSD
By pairing the detector quadrants and adding the signals, the Phenom SEM displays material contrast using the composition (full) mode. Heavier elements are brighter in BSD images as shown here for a nickel-based superalloy imaged at 1000x magnification using Full mode for material contrast.
BSD SEM image at 1000x magnification of a nickel-based superalloy
Operating the BSD quadrants in pairs and then subtracting the information yields topographic images with the Phenom SEM. Compositional and topographic images can be acquired in the same location to provide insight to correlate material properties to topography, grain size or morphology.