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Auger Tutorial: Theory

Auger electron spectroscopy (AES) identifies elemental compositions of surfaces by measuring the energies of Auger electrons. Auger electron emission is stimulated by bombarding the sample with an electron beam. The Auger electron energies are characteristic of the elements from which the electrons come. Auger electron spectroscopy is a widespread method for analysis of surfaces, thin films, and interfaces.

Although both are dated, two good reference book are:

  • Photoelectron and Auger Spectroscopy, T. A. Carlson (Plenum Press, New York, 1975)
  • Methods of Surface Analysis, A. W. Czanderna, ed. (Elsevier, New York, 1975)
The Auger Process

The basic Auger process starts with removal of an inner shell atomic electron to form a vacancy. Several processes are capable of producing the vacancy, but bombardment with an electron beam is the most common. The inner shell vacancy is filled by a second atomic electron from a higher shell. Energy must be simultaneously released. A third electron, the Auger electron, escapes carrying the excess energy in a radiationless process. The process of an excited ion decaying into a doubly charged ion by ejection of an electron is called the Auger process. Alternatively, an X-ray photon removes the energy. For low atomic number elements, the most probable transitions occur when a K-level electron is ejected by the primary beam, an L-level electron drops into the vacancy, and another L-level electron is ejected. Higher atomic number elements have LMM and MNN transitions that are more probable than KLL.

The figure illustrates two competing paths for energy dissipation with titanium as an example. The illustrated LMM Auger electron energy is ~423 eV (EAuger = EL2 – EM4 – EM3) and the X-ray photon energy is ~457.8 eV (Ehv = EL2 – EM4).

Electron Beam Effects

When the electron beam strikes a sample surface, it produces a plethora of different interactions. Elastic scattering occurs when a high energy electron (1 to 30 keV) strikes a sample atom and recoils with essentially all of its energy. (The RBS kinematic factor equation applies to this situation and indicates an energy loss of 1 eV for a 25 keV electrons striking a surface iron atom and scattering back at 180 degrees.) The electron beam loses energy as it passes through material, thereby broadening the energy distribution of backscattered electrons. Inelastic scattering occurs by several mechanisms as the primary electron gives up larger amounts of its energy.

  • Plasmon excitation, occurs with high probablility as the free electron gas between ionic cores absorbs energy. Typical plasmon excitations involve transfer of around 15 eV to the solid.
  • Conduction band excitation ejects loosely bound conduction electrons as secondary electrons. The majority leave with 0 to 50 eV kinetic energies.
  • Bremsstrahlung (from the German for “braking radiation”) occurs when a primary electron undergoes deceleration in the Coulombic field of an atom. The bremsstralung consists of X-ray photons with energies spread between zero and the primary beam energy.
  • Excitation of lattice oscillations (phonons) transfers a substantial portion of beam energy to the sample as heat.
  • Inner shell ionization leaves the atom in an a highly energetic state while absorbing a large amount of primary electron energy. Decay of this excited state produces characteristic Auger electrons and X-rays.
Auger Analytical Volumes

Electron beams disperse into small volumes, typically about one cubic micron (1e-12 cc). X-rays are emitted from most of this volume. Auger signals arise from much smaller volumes, down to about 3e-19 cc.

The X-ray analytical volume increases with electron beam energy and decreases for materials with higher atomic numbers. The Auger analytical volume depends on the beam diameter and on the escape depth of the Auger electrons. The mean free paths of electrons depend on their energies and on the sample material. The minimum mean free path (~0.5 nm) occurs at about 80 eV. Under practical analytical conditions the mean free path increases to as much as ~25 nm.

Auger Electron Spectroscopy

Auger electron spectroscopy (AES) identifies elemental compositions of surfaces by measuring the energies of Auger electrons. An Auger spectrum plots a function of electron signal intensity versus electron energy. The Auger energies fall between secondary electron energies on the low end and backscattered electron energies on the high end. Those backscattered electrons that recoil with 100 % of their primary energy form the elastic peak.

The terms secondary and backscattered are sometimes defined in the operational terminology of scanning electron microscopy (SEM). The true secondary electrons have energy less than ~50 eV. They can be detected with the SEM secondary electron detector biased at +50 to +200 V. All electrons with too much energy to be trapped in the secondary electron detector fall in the backscattered category.

The Auger electrons start with narrow energy distributions, but they soon lose energy as they pass through materials. Auger electrons fail to emerge with their characteristic energies if they start from deeper than about 1 to 5 nm into the surface. Thus, Auger analysis is surface specific. Auger electrons that escape from deeper in the sample contribute loss tails to the spectrum background. The secondary and backscattered electrons have broad energy distributions that tail into the Auger region. The sum of these interfering signals is much greater than the Auger signals themselves. Auger display algorithms use differentiation to enhance the signal relative to the interferences.

Uses of Auger

Auger electron spectroscopy provides compositional information for many types of surfaces, thin films, and interfaces. Typical samples include both raw semiconductors materials and finished electronic devices. Many of these devices consist of thin layers. For example, Auger can distinguish between Si, SiO2, SiO, and Si3N4 in a 10 nm layer on a silicon wafer.

Auger analytical volumes down to about 3e-19 cc are possible. It is common to analyze individual small features within finished or partly finished electronic devices. Many other analyses rely on this microanalytical capability for characterizing heterogeneous materials. For example, Auger analyses of failed materials are common. The fractured surfaces of a broken piece of steel might be examined for the presence of unusual elements such as lead at metal grain boundaries. In contrast to Auger, less finely focused microanalytical techniques provide only average concentrations from larger analytical volumes.

Limitations of Auger

Although widely useful, Auger does have limitations. It cannot detect hydrogen or helium. It does not provide for nondestructive depth profiles. It requires that samples be small and compatible with high vacuum. Nonconducting samples sometimes charge under electron beam bombardment and simply canít be analyzed. Elemental quantitation by Auger depends on instrumental, chemical, and sample related factors.

Auger Electron Energies

Qualitative analysis by Auger electron spectroscopy depends on identification of the elements responsible for the various peaks in the spectrum. The Auger electron energies are widely tabulated for all elements in the periodic table. The figure shows the most useful Auger peaks in the KLL, LMM, and MNN parts of the spectrum as well as higher transitions for elements above cesium. The red dots indicate the strongest and most characteristic peaks and the green bands indicate the rough structure of less intense peaks.

Elemental Quantitation

Auger electron peaks are proportional to elemental concentrations. However, it is seldom possible to measure concentrations from first principles. Several instrumental factors influence Auger peak heights. These include primary beam energy, sample orientation, and the energy resolution and acceptance angle of the analyzer.

The chemical states of elements in the sample also influence the process of elemental analysis by Auger. Both peak intensity and peak shape vary, especially as a function of oxidation state. Changes in peak shape are important when the quantitation proceeds from a differential data display.

Sample heterogeneity must be considered for quantitative analysis. The sample should be homogeneous in the lateral directions relative to the primary beam diameter for measurements to be accurate. The Auger signals arise from an analytical volume that depends mainly on the diameter of the primary beam. If the beam is narrower than the scale of heterogeneity, then meaningful analyses can be made on islands within a sample. The thickness of the analytical volume is small because Auger is highly surface sensitive. Therefore, the analyzed surface may not be representative of the bulk material. For example, many metal samples acquire thin oxide coatings when exposed to air. In spite of the above considerations, quantification of elemental concentrations is possible in cases where relative sensitivity factors have been measured in the same sample matrix.

A typical Auger analysis requires quantification of major and minor elements. This concentration range is consistent with Auger analytical detection limits (1 to 0.01 %). (In contrast, SIMS usually provides trace element quantification while the major elements remain essentially constant.) Since concentrations of all elements, (including the matrix) can vary in an Auger measurement, it is necessary to express concentrations as percents (CE%) normalized relative to the sum of all others.

The next part of the procedure uses the same logic as SIMS RSFs.

Substituting the right side of the RSF equation for concentrations (CE and CX) in the top equation (and eliminating the matrix current terms from the numerator and denominator) give the following equation. This format is the most common method for elemental quantification by Auger. However, it should be noted that Auger RSFís are often reciprocals of those defined here. (They must be incorporated by division into the Auger signal rather than multiplication.)

Auger Electron Emission Probabilities

The Auger electron is the final electron in the Auger emission process. The primary excitation beam removes the first electron from a core level of an analyte atom to produce a vacancy. A second electron falls from a higher level into the vacancy with release of energy. The resulting energy is carried off with the Auger electron which is ejected from a higher energy level.

Auger Depth Profiling

To analyze samples in depth, Auger instruments incorporate ion beam sputtering to remove material from the sample surface. One cycle of a typical depth profile consists of sputtering a small increment into the sample, stopping, measuring relevant portions of the Auger spectrum, and using the equation for elemental quantification.