Electron Interaction with Matter: Backscattered Electrons

2.3.1. Backscattered Electrons

Scattering of beam electrons produces backscattered and transmitted electrons as the strong electrical field of the specimen’s atomic nuclei deflects them; no additional electrons are produced from the sample. Where transmitted electrons pass completely through the material after interacting with it, backscattered electrons are ejected from the top surface of the specimen at high angles. Transmitted and backscattered electrons can have energies from about 50 eV up to the accelerating voltage (E_o).

The number of backscattered electron produced from a material may be quantified by its backscattering coefficient, nb. This coefficient depends strongly on a sample's average atomic number, Z. Neglecting the effects of E_o, the following equation yields a good approximation for the coefficient, n_b (Love & Scott, 1978):

Average Z is calculated using the weight fractions (w) of each element:

Thus, for SiO2, with 0.4674 Si and 0.5326 O by weight:

and n_b= 0.142. About 48% of incident electrons are backscattered by a tungsten target (Z = 74), whereas only about 14% are produced by one of sodium (Z=11).