Generally, scanning electron microscopy uses the secondary electrons generated by bombarding a sample with an electron beam to generate an image. This secondary electron image can be used with a certain degree of interpretation as an implied topographical shot - things like surface roughness and uniformity can be qualitatively examined. Additionally, a secondary electron image will often show differences in the conductivities of materials, due to the charging effects observed as the beam scans across the surface of the part; this can be useful for identifying contaminants on the surface of a device. In addition to these techniques, there are other applications for the SEM that can provide contrast based on the materials present on a device.
When backscattered electrons are collected instead of secondary electrons, the scanning electron microscope can provide an entirely different view of the same sample. When generating a backscatter image, contrast is determined elementally; the further apart two elements appear on the periodic table, the higher the contrast will be between them on the SEM image. Using backscatter images to identify elemental changes can be a useful precursor for more in-depth chemical analysis like energy dispersive spectroscopy; being able to visually see material changes can greatly influence the sites chosen for a more in-depth look.
By no means are these two rudimentary scanning electron microscope operating modes the only techniques available for examining devices or materials. Changing accelerating voltages and currents, applying outside stimuli (like voltage pulses), or examining a material under different levels of vacuum can all create vastly different SEM images. Being able to generate an image is only half of the battle, however; it is still crucial to know which images are the most valuable, and how to select a given imaging mode based on the needs of the analysis.