Electron backscatter coefficients

The calculation of backscatter coefficients via the approach outlined above is mathematically complex. Heidenreich 44) developed a simple empirical backscatter model which is applicable to resist exposure being based on the direct observation of chemical changes produced by backscat-tered electrons at different accelerating voltages on several substrates. The model is independent of scattering trajectory and energy dissipation calculations and is essentially a radial exponential decay of backscatter current density out to the backscatter radius determined by electron range. 

For phosphors based on zinc sulfide (ZZn = 30, Zs = 16 hence, ZZnS = 23) this gives a backscattering coefficient ri 0.27. This means that 27 % of the energy supplied is not available for luminescence. The effect of the electron energy on backscattering is described in [5.309],... 

The intensity of the electrons backscattered from a particular region of a specimen depends approximately on the mean atomic number of the material of which that region is composed more precisely, it is represented by the backscattering coefficient, q, which may be calculated from the formulae ... 

Figure 4.15 Backscatter coefficient as a function of atomic number of specimen atoms. (Reproduced with kind permission of Springer Science and Business Media from J.I. Goldstein et al, Scanning Electron Microscopy and X-ray Microanalysis, 2nd ed., Plenum Press, New York. 1992 Springer Science.)...

Backscatter coefficient The number of backscattered electrons emitted from a specimen... 

Figure 39. Backscattering coefficient and secondary electron emission (Eq = 30 keV) as a function of atomic number Z (from Seidel, Wittry).

When rocking an incident electron beam, the backscattering coefficient is modulated by a few percent. This results in an electron channeling... 

The probability of elastic scattering varies strongly with atomic number, as Z, but the probability of backscattering, rj, varies much less. For 20keV electrons it is 0.06 for carbon and increases to 0.5 for silver [68] (shown in Fig. 3.21 of [38]). If the scattering is very efficient, in a short distance into the specimen, half the electrons will be going forward and half backward. Thus the backscattering coefficient saturates near 0.5 there is only a few percent difference between values for silver and uranium. 

The third spectral region is the EXAFS, where the core electron goes into the continuum. As the selected atom absorbs X-rays, an inner shell electron is ejected, producing an outgoing photoelectron wave. This wave is backscattered by the surrounding shell(s) of atoms to yield a new wave with an energy-dependent phase difference with respect to the out-going wave (Fig. 2). This results in either an increase or a decrease of the absorption coefficient of the... 

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