Basic Principles and Applications of Auger Electron Spectroscopy (AES)

1 Overview of Auger Electron Spectroscopy (AES)

Auger Electron Spectroscopy (AES) is a technique that uses an electron beam (or X-rays) with a specific energy to excite the sample and induce the Auger effect. By detecting the energy and intensity of Auger electrons, AES provides information about the chemical composition and structure of the material's surface. Auger electrons were first discovered and theoretically explained by P. Auger in 1925 using a Wilson cloud chamber. In 1953, J.J. Lander was the first to use an electron beam to excite Auger electrons and explored the potential of applying the Auger effect to surface analysis.

The main components of an Auger Electron Spectrometer include an electron gun, an energy analyzer, a secondary electron detector, a sample analysis chamber, a sputter ion gun, and a signal processing and recording system (as shown in Figure 1).

Figure 1: Auger Electron Spectroscopy diagram

2 Basic Principles of Auger Electron Spectroscopy

2.1 Generation of Auger Electrons

The principle of Auger Electron Spectroscopy (AES) is relatively complex, involving the transitions of two electrons between three atomic orbitals. The process can be described as follows:

1. Excitation: When a particle with sufficient energy (such as a photon, electron, or ion) collides with an atom, it can eject an electron from an inner shell orbital, creating a vacancy and forming an excited state ion.

2. Formation of the Excited State: This results in an excited state positive ion with a core hole in its inner shell. This state is unstable and the atom must relax to return to a stable state.

3. De-excitation Process: During the relaxation process, an electron from an outer shell orbital transitions to fill the inner shell vacancy. The energy released from this transition is transferred to another electron, which is then ejected from either the same shell or a higher shell of the atom. This ejected electron is known as an Auger electron (illustrated in Figure 2).

Figure 2: Auger Electron Transition Process

In Auger Electron Spectroscopy (AES), the analysis area involves excited atoms emitting Auger electrons that are characteristic of specific elements. This emission enables surface analysis of the sample being examined. The typical excitation crater pattern for AES is shown in Figure 3.

Figure 3: AES Excitation Crater Pattern

2.2 Definition and Notation of Auger Transitions

The Auger transition process refers to the non-radiative transition that occurs between different energy levels of the orbitals involved in the excitation and filling electrons, as well as the orbitals of the hole. The Auger electron produced by this transition can be labeled using the energy levels of the three atomic orbitals involved in the transition. As shown in Figures 2 , the Auger electron generated by the Auger transition can be denoted as a WXY transition. In this notation, the orbital energy level of the hole is indicated first, followed by the energy level of the filling electron, and finally, the energy level of the ejected Auger electron.

2.3 Intensity of Auger Electrons

The intensity of Auger electrons forms the basis for quantitative elemental analysis in Auger Electron Spectroscopy (AES). The intensity of Auger electrons depends not only on the quantity of the element present but also on factors such as the ionization cross-section of the atom, the Auger yield, and the escape depth of the electrons.

Auger Transition Probability

During the de-excitation process of an excited atom, there are two distinct pathways for relaxation. One pathway involves the Auger transition process, where an electron fills the hole and another electron is ejected, creating a secondary electron (the Auger electron). The other pathway involves the emission of X-rays, known as the fluorescence process. The probabilities of these two processes are related, such that the sum of the Auger transition probability (PA) and the fluorescence probability (PX) equals one (PA + PX = 1).

Figure 4: The Relationship Between Auger Electron Yield and Atomic Number

As illustrated in Figure 4, for elements with atomic numbers less than 19 (i.e., light elements), the Auger transition probability (PA) exceeds 90%. This high probability remains until the atomic number increases to 33, where PA and PX become equal. As shown in Figure 5, the following principles apply based on the atomic number:

Figure 5: The Relationship Between Auger Transition Probability and Fluorescence Probability with Atomic Number

• For elements with atomic numbers less than 15, KLL Auger electron analysis is used.
• For elements with atomic numbers between 16 and 41, where the L-series fluorescence probability is zero, LMM Auger electron analysis is used.
• For elements with higher atomic numbers, considering the zero fluorescence probability, M-series Auger electron analysis is employed.

These principles guide the appropriate selection of Auger transitions for analyzing elements based on their atomic number and the corresponding transition probabilities.

3. Auger Electron Spectroscopy (AES) Analysis Techniques

AES provides five characteristic quantities: characteristic energy, intensity, peak shift, line width, and line shape. These features of AES can be used to obtain information about the surface characteristics, chemical composition, coverage, charge transfer in bonds, electron state density, and electronic energy levels in surface bonds of solid materials.

3.1 Qualitative Analysis of Surface Elements

Principle: The energy of Auger electrons is solely dependent on the orbital energy levels of the atom itself and is independent of the energy of the incident electrons, i.e., it is independent of the excitation source. For a specific element and a specific Auger transition process, the energy of the Auger electrons is characteristic. Therefore, the kinetic energy of the Auger electrons can be used for qualitative analysis to identify the types of elements present on the sample surface. Since each element has multiple Auger peaks, the accuracy of qualitative analysis is quite high.

Method: The measured Auger electron spectrum is compared with the standard spectra of pure elements. By comparing the positions and shapes of the peaks, the types of elements can be identified.

Considerations in Qualitative Analysis:

1. Peak Shift and Shape Changes: Be aware of peak shifts or changes in line shape caused by chemical effects or physical factors.
2. Contamination Peaks: Peaks may arise from surface contamination or exposure to the atmosphere.
3. Peak Position: The crucial part of verification is the peak position, not the peak height.
4. Overlapping Peaks: Auger peaks of the same element can appear multiple times, and peaks of different elements can overlap or even deform. Peaks of trace elements might be overshadowed, yet the Auger peaks do not exhibit significant variation.
5. Energy Loss Peaks: If there are Auger electron peaks in the spectrum that do not match any standard peaks, these might be energy loss peaks of primary electrons.

While qualitative analysis can be performed using computer software, further manual confirmation is often necessary for overlapping peaks and weak peaks.

By following these principles and methods, qualitative analysis using AES can accurately determine the elemental composition of the sample surface.

3.2 Semi-Quantitative Analysis of Surface Elements

Due to the complexity of the excitation process of Auger electrons in solids, it is challenging to perform absolute quantitative analysis using Auger electron spectroscopy (AES). Additionally, the intensity of Auger electrons is influenced by factors such as the surface cleanliness of the sample, the state of existence of elements, and the condition of the instrument. Contamination levels in the spectrometer, surface contamination of the sample with elements like C and O, and variations in the energy of the excitation source all affect the results of quantitative analysis. Therefore, AES provides semi-quantitative analysis results.

Basis: The intensity of Auger electrons emitted from the sample surface is linearly related to the concentration of the atom in the sample. Based on this relationship, semi-quantitative analysis of elements is performed.

Method: The concentration of elements on the sample surface is determined based on the intensity of the measured Auger electron signal. The concentration of an element, denoted by C, represents the fraction (percentage) of the number of atoms of element X in a unit volume of the sample surface area to the total number of atoms. There are two main methods for quantitative analysis:

1. Standard Curve Method: Construct a calibration curve using standards of known composition to correlate Auger electron intensities with known concentrations. Then, use this curve to determine the concentration of elements in the sample.

2. Relative Sensitivity Factor Method: Use the relative sensitivity factors (RSFs) of elements obtained from standards to calculate the concentrations of elements in the sample based on their Auger electron intensities.

Semi-quantitative analysis using AES provides valuable information about the relative concentrations of elements on the sample surface, despite limitations in absolute quantification due to various influencing factors.

3.3 Chemical State Analysis

1. Chemical Environment of Atoms:

   • The "chemical environment" of an atom refers to its valence state or the electronegativity of other atoms (elements) it combines with when forming compounds. For example, charge transfer (such as changes in valence state) within an atom can cause changes in inner-level energy, thereby altering the energy of Auger transitions and resulting in Auger peak shifts.

2. Changes in Chemical Environment:

   • Changes in the chemical environment of an atom may not only cause Auger peak shifts (referred to as chemical shifts) but may also affect its intensity. The overlap of these two types of changes can alter the shape of Auger peaks.

3. Involvement of Three Energy Levels in Auger Transitions:

   • Auger transitions involve three energy levels. When the chemical state of an element changes, there are minor variations in the energy level states. As a result, these Auger electron peaks may shift by several electron volts compared to peaks in the zero-valent state. Therefore, information about the chemical environment or chemical state of atoms in the sample surface area can be inferred from the positions and shapes of Auger electron peaks.

Understanding the chemical state analysis in Auger Electron Spectroscopy involves recognizing how changes in the chemical environment of atoms influence Auger electron transitions, including peak shifts and changes in intensity, which provide valuable insights into the chemical composition and state of the sample surface.

3.4 Depth Profiling Analysis of Element Distribution

The depth profiling capability of Auger Electron Spectroscopy (AES) is one of its most useful analytical features.

Principle: In depth profiling analysis, a noble gas, typically argon ions with energies ranging from 500 eV to 5 keV, is used to sputter away a certain thickness of the surface layer. The Auger electron signal intensity (I), which corresponds to the elemental content, is measured in situ as a function of sputtering time (t), representing sputtering depth. This measurement provides information about the distribution of elements along the depth of the sample. The sputtering yield is influenced by factors such as the energy and type of ion beam, the direction of incidence, the properties of the solid material being sputtered, and the type of elements present. In materials with multiple components, a phenomenon called preferential sputtering occurs, where elements with higher sputtering yields are preferentially sputtered away, leading to changes in the measured composition.

Modes of Operation:

1. Continuous Sputtering: AES analysis is performed simultaneously with ion sputtering.
2. Intermittent Sputtering: Ion sputtering and AES analysis are performed alternately.

Depth profiling analysis by ion sputtering is a destructive analytical method. The sputtering process is complex and can alter the composition and morphology of the sample surface, sometimes leading to changes in the chemical valence states of elements. Additionally, the surface roughness generated by sputtering significantly reduces the depth resolution of the analysis. Longer sputtering times result in greater surface roughness. One solution to this issue is to rotate the sample to enhance the uniformity of the ion beam.

Depth profiling analysis provides valuable insights into the elemental distribution within the sample, allowing researchers to understand the composition and structure of materials in greater detail.

3.5 Surface Microanalysis

Surface microanalysis is another important feature of Auger Electron Spectroscopy (AES), which includes point analysis, line scanning analysis, and area scanning analysis.

1. Point Analysis:

   • Point analysis aims to understand the presence of elements at different locations. The spatial resolution of Auger electron spectroscopy point analysis can reach the size of the beam spot. Therefore, AES can be used for point analysis in very small areas.

2. Line Scanning Analysis:

   • Auger line scanning analysis can be performed on both micro and macro scales (1 to 6000 micrometers) and provides insights into the distribution of elements along a particular direction.

3. Area Scanning Analysis:

   • Area scanning analysis represents the distribution of a certain element in a specific area in the form of an image. Surface element distribution analysis by Auger electron spectroscopy is suitable for the study of micro materials and technologies, as well as research in surface diffusion and related fields.

These techniques enable researchers to obtain detailed information about the elemental composition and distribution on the surface of materials, making AES a valuable tool for studying microscale materials and surface phenomena.

4. Auger Electron Spectroscopy Experimental Techniques

4.1 Sample Preparation

Auger Electron Spectroscopy (AES) has specific requirements for the analysis samples. Typically, only conductive solid samples can be analyzed. With special treatment, insulating solids can also be analyzed. Since the sample transfer and placement involve vacuum conditions, analysis samples generally require some pretreatment.

Sample Size: For bulk and thin film samples, it's preferable for the length and width to be less than 10 nm and the height to be less than 5 nm. For larger volume samples, they must be prepared to appropriate sizes using suitable methods.

Powdered Sample Handling: There are two commonly used sample preparation methods for powdered samples. One is to fix the powder onto the sample stage using conductive tape, and the other is to compress the powder into a thin film and then fix it onto the sample stage. The former is convenient and requires less sample, but the composition of the adhesive may interfere with the sample analysis, and charging effects may affect the collection of Auger spectra. The latter allows for sample pretreatment in vacuum, such as heating and surface reactions, but it requires more sample and charging effects can directly affect the recording of Auger electron spectra.

Samples with Volatile Substances: Before entering the vacuum system, samples with volatile substances must have these substances removed. Typically, the sample can be heated or cleaned with solvents. For samples containing oily substances, they are generally cleaned with n-hexane, acetone, and ethanol sequentially through ultrasonic cleaning, followed by infrared drying before entering the vacuum system.

Surface Contaminated Sample Treatment: Surface contaminants on samples can be cleaned using oily solvents such as cyclohexane and acetone, followed by ethanol to remove organic solvents. To prevent oxidation of the sample surface, natural drying is usually employed.

Absolutely No Magnetic Samples Allowed: Samples with strong magnetism are strictly prohibited from entering the analysis chamber as magnetization of the starting point and sample holder can occur. When the sample contains magnetic materials, Auger electrons deviate from the acceptance angle under the action of the magnetic field, preventing AES spectra from being obtained. Samples with weak magnetism can be demagnetized to remove the weak magnetism.

4.2 Ion Beam Sputtering Technique

Since samples easily adsorb gas molecules (including elements like O and C) in air, when oxygen, carbon, or cleaning of contaminated solid surfaces is needed, the sample should first be sputtered with an ion beam to remove contaminants. For depth analysis, ion guns with energies ranging from 0.5 to 5 keV and beam spot diameters ranging from 1 to 10 mm, which can be scanned, are typically used. Depending on different sputtering conditions, the sputtering rate can vary from 0.1 to 50 nm/min.

4.3 Sample Charging Issues

Samples with poor conductivity, such as semiconductor materials and insulating thin films, accumulate a certain amount of negative charge on the surface under the action of the electron beam, leading to the charging effect in Auger electron spectroscopy. The surface charge of the sample is equivalent to adding an additional electric field to the free Auger electrons on the surface, increasing their kinetic energy. When charging is severe, Auger spectra cannot be obtained.

For insulating thin films below 100 nm in thickness, if the substrate material is conductive, the charging effect can be eliminated by itself.

For insulating samples, the charging effect can be addressed by coating the area around the analysis point with gold. Alternatively, the sample can be coated with foils such as Al, Sn, or Cu with small windows.

4.4 Sampling Depth of Auger Electron Spectroscopy

The sampling depth of Auger electron spectroscopy depends on the energy of the emitted Auger electrons and the properties of the material. Typically, for metals, it ranges from 0.5 to 2 nm, for inorganic materials from 1 to 3 nm, and for organic materials from 1 to 3 nm. Overall, the sampling depth of Auger electron spectroscopy is shallower than that of X-ray photoelectron spectroscopy (XPS), making it more surface-sensitive.

5. Characteristics of Auger Electron Spectroscopy

5.1 Advantages

1. Surface Sensitivity: The depth of information depends on the Auger electron escape depth (the average electron free path), which is typically 0.4 to 2 nm for Auger electrons with energies in the range of 50 eV to 2 keV. The detection limit is approximately 10^-3 atomic layers. With an electron beam as the excitation source, AES has high spatial resolution, with a minimum resolution of up to 6 nm.

2. Analytical Range: AES can analyze various elements except for H and He.

3. High Sensitivity: It has high sensitivity for light elements such as C, O, N, S, and P.

4. Depth Profiling: It can perform depth profiling of compositions or analyze thin films and interfaces.

5.2 Disadvantages

1. Low Accuracy in Quantitative Analysis: Quantitative analysis accuracy is not high.

2. Limited Sensitivity: The detection sensitivity for most elements is in the range of atomic mole fractions of 0.1% to 1.0%.

3. Limitations in Application: Charging effects and electron beam damage limit its application in organic

6 Examples of Applications of Auger Electron Spectroscopy

An electron beam can scan areas of different sizes or directly focus on specific surface features of interest. This ability to focus the electron beam to a diameter of 10-20 nm makes Auger Electron Spectroscopy (AES) an extremely useful tool for elemental analysis of small surface features. Other techniques to consider are XPS and TXRF. When combined with a sputter ion gun, AES can also perform compositional depth profiling.

Figure 6: Auger analysis shows the thin residue is an Al flake,probably originating from the etch chamber

Analysis of the contaminated sample surface yields the following results, as shown in the diagram below: Position 1 indicates the contaminated area, while Position 2 represents the uncontaminated area.

Figure 7: position 1

Figure 8: position 2