Physical Principles of Electron Microscopy: An Introduction to TEM ...

Analytical Electron Microscopy 171
6.8 Auger-Electron Spectroscopy
Both XEDS or XWDS techniques have problems when applied to the
analysis of elements of low atomic number (Z < 11) such as B, C, N, O, F.
Ultrathin-window XEDS detectors make these elements visible, but the
characteristic-peak intensities are reduced because of the low x-ray
fluorescence yield, which falls continuously with decreasing atomic number;
see Fig. 6-8. In addition, the K-emission peaks (which must be used for low-
Z elements) occur at energies below 1 keV. In this restricted energy region,
the bremsstrahlung background is relatively high, and XEDS peaks from
different elements tend to overlap. Also, the low-energy x-rays are strongly
absorbed, even within a thin specimen, requiring a substantial (and not
always
accurate) correction for absorption.
One solution to the fluorescence-yield problem is to use Auger electrons
as the characteristic signal. Because the x-ray yield � is low, the Auger yield
(1 ��) is close to one for low-Z elements. Auger electrons can be collected
and analyzed using an electrostatic spectrometer, which measures their
kinetic energy. However, the Auger electrons of interest have relatively low
energy (below 1000 eV); they are absorbed (through inelastic scattering) if
they are generated more than one or two nm below the surface of the
specimen, just as with SE1 electrons. In consequence, the Auger signal
measures a chemical composition of the surface of a specimen, which can be
1
0
30
x-ray yield
Auger yield
Figure 6-8. X-ray fluorescence yield, Auger yield and their sum (dashed line) for K-electron
excitation, as a function of atomic number.
Z