AES Auger Electron Spectroscopy

AES was developed in the late 1960s, and in this technique electrons are detected after emission from the sample as the result of a non-radiative decay of an excited atom in the surface region of the sample. The effect was first observed in bubble chamber studies by Pierre Auger (1925), a French physicist, who described the process involved. 

The methods of sample preparation for AES are identical to those used in XPS (see Chapter 2.1), the objective again being to ensure that the surface to be analysed has not been contaminated or altered prior to analysis. AES must be carried out in UHV conditions. 

An electron gun (see Section 5.1.3) provides the requisite electron source for AES, and may consist of a tungsten or a LaB6 cathode, or a Field Emission source. The latter provide the brightest beams, and beam widths as narrow as lOnm permit 

Auger analysis of small features. The primary electron beam column is similar to that in electron microscopes, and it may contain both electrostatic and magnetic lenses for beam focussing as well as quadrupole deflectors for beam steering and octopole lenses for beam shaping. 

The physical basis of AES involves three basic steps, namely atomic ionization (by the removal of a core electron), electron emission (the Auger process), and analysis of the emitted Auger electrons. 

An electron or photon incident at a surface with sufficient energy may result in the ejection of an electron from a core level (X) of an atom in 

A warning note should be sounded in the use of AES for kinetic or coverage measurements. It has long been understood that an electron beam can give rise to significant desorption of an adsorbed layer [102, 

The AES technique provides rapid surface analysis, but is not widely used on fracture surfaces due to beam damage and charging of nonconductive adhesives. On the other hand, the use of AES in the analysis of adherend surfaces has been widespread. An advantage of AES is 

FIGURE 19. A completed lETS device consisting of five tunnel junctions and a single junction shown in more detail. Reprinted with permission from Reference 28. Copyright 1985 Butterworth Scientific Ltd. 

AES can yield chemical information about the surface composition of pretreated adherends as well as compositional information below the surface with depth profiling. Correlations of the surface composition with bond performance can be made. 

After XPS, AES is the next most widely used surface-analytical technique. As an accepted surface technique AES actually predates XPS by two to three years, because the potential of XPS as a surface-specific technique was not recognized immediately by the surface-science community. Pioneering work was performed by Harris [2.125] and by Weber and Peria [2.126], but the technique as it is known today is basically the same as that established by Palmberg et al. [2.127]. 

Considering that heavy elements have more levels than just K and L, Eq. (2.2) also indicates that the heavier the element, the more numerous are the possible Auger transitions. Fortunately, there are large differences between the probabilities of different Auger transitions, so that even for the heaviest elements, only a few intense transitions occur, and analysis is still possible. 

The reasons AES is a surface-specific technique have been given in Sect. 2.1.1, with reference to Fig. 2.2. The normal range of kinetic energies recorded in an AES spectrum would typically be from 20 to 1000 eV, corresponding to inelastic mean free path values of 2 to 6 monolayers. 

The nomenclature used in AES has also been mentioned in Sect. 2.1.1. The Auger transition in which initial ionization occurs in level X, followed by the filling of X by an electron from Y and ejection of an electron from Z, would therefore be labeled XYZ. In this rather restricted scheme, one would thus find in the KLL series the six possible transitions KLiLi, KL1L2, KL1L3, KL2L2, KL2L3, and KL3L3. Other combinations could be written for other series such as the LMM, MNN, etc. 

For the primary energy range 5-10 keV used in conventional AES, the electron emitter is thermionic, usually a hot tungsten filament, and focusing of the electron beam is carried out electrostatically. Typically, such an electron source would be able to provide a spot size on the specimen of about 0.5 pm at 10 keV and a beam current of about 10 A. The beam can normally be rastered over the specimen surface, but such a source would not be regarded as adequate for SAM. Sources for SAM may be of either the thermionic or the field emission type, the latter being partic- 

The electron energy analyzer found to be most suitable for AES, the cylindrical mirror analyzer (CMA), has already been described in Section 

An incident electron, with an energy in the range of 2-50 keV, is sufficiently energetic to excite a core electron from the heavier elements and all 

The Auger process involves transition of electrons between shells. Consequently, e technique cannot be used to detect H or He. However, it is sensitive to all other elements. For heavier elements, the X-ray yield becomes greater than the Auger yield and so AES is most sensitive to those elements with low atomic number. 

Auger electrons have low energies (50eV-3keV). Consequently, their mean free path in a solid is only a few nm and so Auger electrons can only be detected from atoms near the surface of the material of interest. AES is therefore a surface technique. 

The intensity of the Auger peaks may be small compared to the noise level of the background and consequently the spectra obtained are often differentiated to emphasise the structural peaks. Note that the detection sensitivity is 0.1 % at best. 

The electron beam can be focused to a diameter of 10 nm or less and is therefore suited to the analysis of small surface features. Alternatively, the electron beam can be rastered over the surface to enable characterisation 

This method uses a low-energy electron gun, with a power less than 5 keV, to lessen the heating and decomposition of the surface. The number of electrons and their energies are detected by a counter and energy analyzer. The energy of the electron identifies the element, whereas the number of emitted electrons indicates the surface concentration of the element. 

Polymer Treatment Surface Chemical Analysis (%) by ESCA  

In XPS a vacancy is created in an electronic level close to the nucleus by photon bombardment or, in certain cases, by electron bombardment. It is probable that this vacancy is filled by an electron coming from a higher electronic level further from the nucleus (Fig. 12.11). The excess energy 

C(1 s) photoelectron spectrum of a reactively sputtered TiC electrode on a glass substrate, (a) As prepared (b) After 5 min at +1.25 V vs. SCE (passive region) in 0.5 m H2S04 (from Ref. 39 with permission). 

In AES applied to electrochemical studies, normally electron bombardment is employed as it furnishes higher intensities of incident radiation, increasing the number of Auger electrons and facilitating their 

Radiation detected Input Radiation Electrons Ions Photons/X-rays 

Note The number of sampling depth monolayers of methods are given in.  

Source Reprinted with permission from Briggs D, Seah MP, Practical Surface Analysis, 2 ed., Vol 1, Auger and X-ray Photoelectron Spectroscopy, 2nd ed., Vol. 1, John Wiley Sons, Chichester, 4, 1990. Copyright 1990, John Wiley Sons Ltd. 

AES can be used to analyze the top 2-20 atomic layers and in conjunction with ion beam sputtering, can be used for depth profiling. The process must be undertaken in UHV 1.3 X 10 Nm and surfaces must be sputter cleaned by directing a beam of ions such as Ar at 0.5 5 keV onto the surface. This cleaning process can also erode the sample and expose surface structure. AES can analyze areas as small as 100 mn and up to 2.5 cm and is most sensitive when analyzing elements with low atomic number. However, it cannot detect H2 or He. Resolution is down to 250 nm (cf SEM at 100 mn). 

Hopfgarten [44] has measured the atomic composition of Types I, II, and III carbon fibers and found that for Types I and II, the oxygen content increased till down to a depth 

The principal use of Auger spectroscopy is in the determination of surface composition, although peak positions are secondarily sensitive to the valence state of the atom. See Refs. 2, 82, and 83 for reviews. 

Experimentally, it is common for LEED and Auger capabilities to be combined the basic equipment is the same. For Auger measurements, a grazing angle of incident electrons is needed to maximize the contribution of surface 

ENERGY DISTRIBUTION OF SCATTERED ELECTRONS FROM A c(4x2) MONOLAYER OF C2H3 ON Rh(lll) AT 300K 

Madey and co-workers followed the reduction of titanium with XPS during the deposition of metal overlayers on TiOi [87]. This shows the reduction of surface TiOj molecules on adsorption of reactive metals. Film growth is readily monitored by the disappearance of the XPS signal from the underlying surface [88, 89]. This approach can be applied to polymer surfaces [90] and to determine the thickness of polymer layers on metals [91]. Because it is often used for chemical analysis, the method is sometimes referred to as electron spectroscopy for chemical analysis (ESCA). Since x-rays are very penetrating, a grazing incidence angle is often used to emphasize the contribution from the surface atoms. 

Auger electron spectroscopy (AES) is a technique used to identify the elemental composition, and in many cases, the chemical bonding of the atoms in the surface region of solid samples. It can be combined with ion-beam sputtering to remove material from the surface and to continue to monitor the composition and chemistry of the remaining surface as this surface moves into the sample. It uses an electron beam as a probe of the sample surface and its output is the energy distribution of the secondary electrons released by the probe beam from the sample, although only the Ai er electron component of the secondaries is used in the analysis. 

Auger electron spectroscopy is the most frequently used surface, thin-film, or interface compositional analysis technique. This is because of its very versatile combination of attributes. It has surface specificity—a sampling depth that varies 

The complete description of the number of Auger electrons that are detected in the energy distribution of electrons coming from a surface under bombardment by a primary electron beam contains many factors. They can be separated into contributions from four basic processes, the creation of inner shell vacancies in atoms of the sample, the emission of electrons as a result of Auger processes resulting from these inner shell vacancies, the transport of those electrons out of the sample, and the detection and measurement of the energy distribution of the electrons coming from the sample. 

Once an inner shell vacancy is created in an atom the atom may then remrn toward its ground state via emission of a characteristic X ray or through a radiationless Auger transition. The probability of X-ray emission is called the fluorescence yield. 

The inelastic collision process is characterized by an inelastic mean free path, which is the distance traveled after which only 1/e of the Auger electrons maintain their initial energy. This is very important because only the electrons that escape the sample with their characteristic Auger energy are usefrd in identifying the atoms in 

Auger electron spectroscopy (AES) was used in combination with secondary ion mass spectrometry (SIMS) to distinguish between four types of carbonaceous deposits, on metal foils (rhodium, iridium and platinum). The foils were coked by exposing to ethylene at low pressure. Auger spectroscopy can distinguish between molecular or carbidic on the one hand, and graphitic or amorphous carbon on the other. The Auger spectrum of carbonaceous deposits on a metal is 

The process on which AES is based is similar to that of ESCA, but it is a two-step process instead of one step. As with ESCA, the sample is irradiated with either accelerated electrons or X-ray photons. An inner shell electron is ejected, leaving a vacancy in the inner shell. An electron from an outer shell falls into the inner shell as in the ESCA process. 

The Auger process and XRF are competitive processes for the liberation of energy from bombarded atoms. In practice, it is found that the Auger process is more likely to occur with low atomic number elements this probability decreases with increasing atomic number. In contrast, XRF is unlikely with elements of low atomic number but increases in probability with increasing atomic number. This is illustrated in Fig. 14.22. 

The energies involved in Auger spectroscopy are similar in all respects to those of ESC A, since the same atomic shells are involved. A graphical plot of Auger electron energies is shown in Fig. 14.23 and tabulated values of the prominent lines are available in the literature. 

Plots of KE(a) vs. BE(p), with AP + hv shown on the other ordinate (called Wagner plots) are found in the literature and in the NIST XPS database. An example for tin is presented in Fig. 14.25. The Sn 3ds/2 photoelectron and the Sn MNN Auger electron binding energies are tabulated and plotted for the element Sn and a variety of tin compounds, showing how the chemical environment of the Sn affects the electron energies. 

The instrumentation used in AES is very similar to that used in ESCA. The major difference is that the source used is a focused beam of electrons from an electron gun or a field emission source, not X-ray photons. A schematic diagram of an AES instrument is shown in Eig. 14.26. Many instrument manufacturers provide instruments that permit both XPS and Auger spectra to be collected on one instmment. 

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Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and... 

A popular electron-based teclmique is Auger electron spectroscopy (AES), which is described in section Bl.25.2.2. In AES, a 3-5 keV electron beam is used to knock out iimer-shell, or core, electrons from atoms in the near-surface region of the material. Core holes are unstable, and are soon filled by either fluorescence or Auger decay. In the Auger... 

Figure 8.1 Processes occurring in (a) ultraviolet photoelectron spectroscopy (UPS), (b) X-ray photoelectron spectroscopy (XPS) and (c) Auger electron spectroscopy (AES)...

Figure 8.1(c) illustrates schematically the kind of process occurring in Auger electron spectroscopy (AES). The process occurs in two stages. In the first, a high-energy photon ejects an electron from a core orbital of an atom A ... 

X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (xps) and Auger electron spectroscopy (aes) are related techniques (19) that are initiated with the same fundamental event, the stimulated ejection of an electron from a surface. The fundamental aspects of these techniques will be discussed separately, but since the instmmental needs required to perform such methods are similar, xps and aes instmmentation will be discussed together. 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

Laser ionization mass spectrometry or laser microprobing (LIMS) is a microanalyt-ical technique used to rapidly characterize the elemental and, sometimes, molecular composition of materials. It is based on the ability of short high-power laser pulses (-10 ns) to produce ions from solids. The ions formed in these brief pulses are analyzed using a time-of-flight mass spectrometer. The quasi-simultaneous collection of all ion masses allows the survey analysis of unknown materials. The main applications of LIMS are in failure analysis, where chemical differences between a contaminated sample and a control need to be rapidly assessed. The ability to focus the laser beam to a diameter of approximately 1 mm permits the application of this technique to the characterization of small features, for example, in integrated circuits. The LIMS detection limits for many elements are close to 10 at/cm, which makes this technique considerably more sensitive than other survey microan-alytical techniques, such as Auger Electron Spectroscopy (AES) or Electron Probe Microanalysis (EPMA). Additionally, LIMS can be used to analyze insulating sam-... 

Roughness from sputtering causes loss of depth resolution in depth profiling for Auger Electron Spectroscopy (AES), X-Ray Photoelectron Spectroscopy (XPS), and SIMS. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

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