Page 65 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Auger Electron Spectroscopy 789

dependentonsuchfactorsastracecontamination,thecrys-
tallography of the surface, the roughness of the surface,
and the presence of other alloying elements in the solid
that is being analyzed. Although one can obtain accurate
calibration for a particular system, it is very difficult to
apply a given calibration to a variety of alloys on a rou-
tine analytical basis. Consequently, many researchers fre-
quently rely on relative changes in a normalized signal to
discuss changes in the surface composition. For example,
in the differentiated spectrum shown in Fig. 2, which was
taken from a steel fracture surface, one might choose the
strong iron peak at 703 eV and normalize all other peaks
by it. Then, one could discuss changes in normalized peak
FIGURE 2 Typical differentiated Auger spectra taken from steels.
height ratios to describe changes in the composition of this
The upper spectrum was from a nonembrittled (N.E.) steel, and
the fracture surface that was analyzed was transgranular. The surface.
lower spectrum was from an embrittled steel, and the fracture The previous paragraphs give a brief introduction of
surface that was analyzed was intergranular. [From Stein, D. F., the Auger process and the basic way in which surfaces
Joshi, A., and LaForce, R. P. (1969). ASM Trans. Q. 62, 776.]
are analyzed with it. However, the great power of Auger
electron spectroscopy has been broad in its applicability.
We now wish to discuss these applications in the field of
Tracy. This analyzer provided a much higher signal-to-
materials science.
noise ratio and also allowed for much faster acquisition
of data. It is the analyzer on which most modern sys-
tems are based. Another important improvement was in III. APPLICATIONS IN
vacuum systems. When the early work was performed, MATERIALS SCIENCE
it was almost impossible to obtain a clean surface. The
vacuums were poor enough so that carbon and oxygen Auger electron spectroscopy has become one of the most
were adsorbed rapidly onto the surface. For a technique important tools for the investigation of interfaces in the
that only probes the top two to five atom layers, as does field of materials science. In fact, it is probably safe to say
Auger electron spectroscopy, this adsorption presented a that it has found its most widespread application in this
problem. With the development of ions pumps and tur- field. Because it does not give detailed chemical bonding
bomolecular pumps that provide a background pressure information about the species on a surface, other tech-
of less than 1 × 10 −10 Torr, a clean surface could be pre- niques such as XPS and UV photoemission are more com-
pared routinely, either through fracture or by sputtering monly used for detailed chemical investigations. However,
with rare gas ions, and kept in this condition for 5 to 10 h. in materials science applications the need is more often to
Finally, new and much brighter filaments were developed detect the presence of elements on the surface of the solid,
that allowed a stronger Auger signal to be generated. In and for this application Auger electron spectroscopy is the
particular, the development of the lanthanum hexaboride preferred technique.
filament, which replaced the standard thermionic tung- The types of studies that have been performed can be
sten filament, offered a great increase in filament current. divided into two categories. One set is concerned with
This increase also meant that smaller areas (<1 µmin studies of the external surface. Segregation to the surface
diameter) on the surface could be analyzed, and it led to can cause its composition to be very different from that of
the development of the scanning Auger spectrometer. In the bulk. There has been great interest in understanding
this spectrometer, the Auger beam is first used to form how this difference in composition can affect such pro-
a secondary electron image of the surface that is being cesses as catalysis, corrosion, and oxidation. Also, one can
analyzed. The beam can then be placed on one particular use the kinetics of segregation to the surface to determine
area to obtain a spectrum from that spot. In the most re- the diffusion coefficient of an element in the solid. The
cent machines, field emission filaments are used that allow other type of study is concerned with internal interfaces,
analysis of a spot size of under 20 A. such as grain boundaries and particle–matrix interfaces.
One problem with Auger electron spectroscopy is the The composition of these interfaces can also be quite dif-
difficulty of converting the signal to accurate values of ferent from that of the bulk, as a result of segregation,
atomic percent. The reason for this difficulty is that be- and consequently, they can control various metallurgical
cause the Auger signal is so surface sensitive, it is very properties.