escape depth

The escape depth means the depth from the upper surface a specimen, from which secondary electrons, Auger electrons, X-rays excited by incident electrons and backscattered electrons can escape.
The energy of a secondary electron is very small to be a few 10 eV at maximum. Thus, secondary electrons excited at a deep region in the specimen lose their energy only by a few times of scatterings, and they do not reach the specimen surface. The escape depth of the secondary electron is approximately 2 to 5 nm for most metals.
Backscattered electrons mean electrons which reach the upper specimen surface without losing their energy. The escape depth of the backscattered electron is about one half of the penetration depth of the incident electrons and is two orders of magnitude larger than the escape depth of the secondary electron. The escape depth of X-rays, which are electromagnetic waves with a high transmission power, is as large as approximately 1μm.

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