The sputtering crater in TEM specimens forms mainly on the beam-exit surface because the chemical bonds at that surface are weakest due to the correlation between the atom position and the incident beam direction. Figure 4406 shows Schematic diagram of sputtering (also called knock-on) of atoms from the beam-exist surface of the TEM sample. The spheres represent atoms in the materials.
Figure 4406. Schematic diagram of sputtering of atoms from the beam-exist surface of the TEM sample.