Page 64 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology en001f-44 May 7, 2001 15:8

788 Auger Electron Spectroscopy

bibliography will provide the interested reader with a start
in this field.
An obvious question that must be answered is why
Auger electron spectroscopy has achieved this place in
the surface analytical field. First of all, it is a relatively
straightforward technique to use, and high-quality com-
mercial spectrometers are available. The data are easily
interpretable, at least on a qualitative level, and it is also
possible to use an incident beam that is well under 1 µm
in diameter. This diameter is several orders of magnitude
smaller than what is possible for most other surface an-
alytical techniques, and this fact means that Auger spec-
troscopy can be used to probe changes in composition
across a surface at a microscopic scale. Finally, Auger
electron spectroscopy can detect every element except hy- FIGURE 1 Energy level diagram representing de-excitations by
(a) Auger electron emissions and (b) X-ray fluorescence. In the
drogen and helium, so it useful for studying both light and
particular Auger process that is shown, a K-shell hole is first cre-
heavy element. We now wish to consider Auger electron ated and an electron in the L I shell drops down to fill it. In so doing,
spectroscopy and the process on which it is based. it gives off enough energy to knock an electron out of the L III shell,
which becomes the Auger electron.
II. THE AUGER PROCESS AND AUGER
ELECTRON SPECTROSCOPY solid must come from very near the surface of the solid.
Although the yield of Auger electrons will again depend
When an electron beam impinges on a solid surface, it can on the particular element, it is estimated that most come
knock out inner shell electrons if it is of sufficient energy. from the top two to five atom layers.
The holes that are left behind will then be filled by having It had been recognized for some years after their discov-
outershellelectronsfallintothem.Thislastprocessoccurs ery that Auger electrons might provide a chemical analysis
with the emission of energy, since the outer shell electron of a surface, but it was not until the work of L. A. Harris
is changing from a higher to a lower energy. The energy in 1968 that the technique actually became feasible in the
that is given off can cause one of two events to occur. The laboratory. Prior to Harris’ work, the problem had always
energy can simply escape from the solid as an X-ray; in been that the Auger signal was so small compared with
this case, the process of X-ray fluorescence has occurred. that from other types of emitted electrons that it could not
In the other case, this energy can knock out another outer be adequately detected. Harris applied a phase-sensitive
shell electron. These electrons are the Auger electrons that detection scheme in which a small, regularly oscillating
were named for Pierre Auger who discovered this process voltage was superimposed on a larger, constantly increas-
in 1929. Such processes are usually denoted by the shells ing voltage. By measuring the electrons being collected,
of the electrons that are involved. Thus, a KLL Auger he was able to detect a small perturbation in the number
process means that the first electron that was knocked out of electrons at a given energy level, because the increased
was from the K shell, the hole was filled by an electron yield would be in phase with the frequency of the oscillat-
from an L shell, and the Auger or secondary electron that ing voltage. By electronically differentiating the signal in
was emitted was also from an L shell. this way, he could help bring out these small peaks. With
Figure 1 shows the processes of Auger emission and flu- this advance, the use of Auger electrons to measure sur-
orescence schematically. The frequency with which one face composition became a reality. Today, one can simply
of them will occur relative to the other depends on the obtain differentiated spectra such as those shown in Fig. 2
particular element; but as a rule of thumb, it can be re- and identify the elements present on the surface by com-
membered that elements with low atomic numbers tend to paring the energies of the peaks with those in handbooks
favor the Auger process, whereas those with high atomic of elemental standards.
numbers favor fluorescence. The Auger electrons that are Although Harris’ contribution was pivotal in the devel-
emitted have energies that are characteristic of the element opment of Auger electron spectroscopy, there have been
from which they came. Therefore, if they can be detected other advances since his that have helped Auger electron
they will provide a method of chemical analysis. Further- spectroscopy to become such a useful surface analytical
more, since the electrons that arise from such a process are technique. One of the most important was the development
going to be of low energy, those that do escape from the of the cylindrical mirror analyzer by Palmberg, Bohn, and