Illustrative Example: Capabilities of the SEM Technique: Secondary and Backscattered Electron Imaging
The wire specimen shown below was created to illustrate SEM imaging by secondary electrons and backscattered electrons. An incident electron beam drives many forms of emission from the surface it is illuminating. Secondary electrons are emitted by atoms near the surface of a sample material when their electrons become excited and have sufficient energy to escape the sample surface. Secondary electron imaging, being more surface sensitive, has greater resolution. Rather than losing energy by exciting electrons within the sample material electrons from the incident beam can also undergo backscattering such that they re-emerge from the sample surface. The escape depth of backscattered electrons can be greater than that of secondary electrons, consequently resolution of surface topographical characteristics can suffer. However, backscattered electrons have the advantage that they are sensitive to the atomic mass of the nuclei they scatter from. As a result, heavier elements which backscatter more efficiently appear brighter than lighter elements in a backscattered electron image.
To illustrate the capabilities of SEM secondary and backscattered electron imaging a relatively thick Ni wire was wrapped with a twisted pair of relatively thin Ti wires. In the secondary electron image (Figure 1 left) a texture of low rounded bumps is visible on the Ni wire surface while the Ti wire surfaces are rougher and have a flakey appearance. The secondary electron image has good resolution and provides more detail in terms of understanding surface morphology. The higher magnification images of the individual wires better display the characteristic surface features of each type of wire (Figure 2).
Looking back to Figure 1, in the backscattered electron image of the same specimen (right) the surface texture is less obvious, however a difference in contrast is now visible between the Ti and the Ni wires. The heavier Ni nuclei are more effective electron scatterers than the lighter Ti nuclei and much lighter O nuclei of the thick oxide layer on the Ti wire; hence the Ni appears brighter. The combined use of both secondary and back scattered electron imaging modes provides valuable information not typically available through either imaging method alone.
Figure 1: (Top) Secondary electron image of a twisted pair of Ti wires wrapped around a thicker Ni wire is shown. (Bottom) The same specimen is shown in back scatter mode. The two metals can now be differentiated based on contrast; elements of greater atomic mass (Ni in this case) appear brighter in a back scattered electron image. Both images were collected at magnification 70x and show the advantage of great depth of focal field in a SEM.
Figure 2. Secondary electron images were collected at higher magnifications. The Ni wire surface (top) was collected at 310x and shows the relatively smooth surface with a wire drawing line and some elongated pores in the direction of the wire length. Some surface debris is also present. At much higher magnification the surface of the Ti wire (bottom, collected at 2800x) appears to be flakey or scaly, with the features again elongated in the direction of drawing (wire length). The scale is primarily a thick oxide layer.