SE2

Secondary electron(s) which are excited by backscattered electrons at a region distant from the incident electrons (primary electrons). Thus, the emission region of SE2(s) depends on the backscattered-electrons.
For a higher accelerating voltage of the incident electrons, the backscattered electrons are emitted from a larger region distant from the incident electrons. Thus, SE2 degrades the resolution of a secondary electron image, especially, of an uneven surface.
The emission rate of SE2 is proportional to that of the backscattered electron. This indicates that SE2 possesses the information similar to that backscattered electrons provide. For a specimen that has a flat surface and has only the compositional difference, a secondary electron image contributed by SE2 provides contrast dependent on the atomic number (though the contrast is low) as is seen in the backscattered-electron image, where a specimen region with a large atomic number appears bright. The schematic of the S2 generation is illustrated below.

Fig. Generation process of SE2