Brighter regions are visible, indicating regions of higher secondary electron emission. A close examination seems to suggest that these particles are not resting on the top of the surface. In the second image the beam energy is dropped down to 2 kV and these bright regions disappear showing a fairly smooth surface. What is going on?
These images show the difference between SE1 and SE2 secondary electrons. In the first image taken at 15 kV there are secondary electrons coming from the sample surface, but also back-scattered electrons scattering from denser objects just under the sample surface. These elastically scattered electrons have considerable energy as they leave the surface, nearly 15 kV, and can generate secondary electrons themselves. These secondaries are called SE2 and appear in the vicinity of the dense sub-surface particles.
By dropping down to 2 kV these SE2 are no longer generated because the cross section for elastic scattering is greatly reduced, as is the cross section for generating SE2's from lower energy electrons. Thus the lower energy image fails to show these subsurface structures.
The point of this application note is to show that SE2's can reveal subsurface structure. This can be exploited to see objects under the surface of a sample. It can also be a source of confusion-- and imaging at several beam energies may be required to resolve this confusion.
The final images shows a back-scattered electron image (BSED) confirming the presence of higher-density inclusions in this material.