Principle
Auger Electron Spectroscopy (AES) is a technique for analyzing the element constituting the sample surface, its composition, and chemical bonding state by irradiating a focused electron beam on the sample surface, and measuring the kinetic energy of the Auger electrons emitted from the sample surface. Since the Auger electron energy emitted from the sample has a value specific to the element, the element on the sample surface can be identified by measuring the electron energy. Since Auger electrons lose energy as they pass through a sample, the analysis target for Auger spectroscopy is limited to several nms from the sample surface.
The irradiated electron beam can be focused to 5 nm or less, allowing for an ultra-microscopic area of the sample to be analyzed.
Excitation source (FE electron gun)
The excitation source used for Auger electron spectroscopy is the same heated Field Emission (FE) electron source used for SEM. The FE electron source focuses the electron beam diameter on the sample to 5 nm or less, enabling high current density.
Enclosure
In order to analyze microscopic areas 5 nm or smaller, the electron beam must be applied steadily and continuously at one area of the sample. For that reason, Auger spectrometers measuring microscopic areas are equipped with enclosures that shut out vibrations from external sounds (sound pressure) and temperature changes.
Detection system (Electron gun coaxial CMA electron analyzer)
The Cylindrical Mirror Analyzer (CMA) electron analyzer used as the Auger electron energy analyzer can be equipped with an electron gun along its central axis. By using this coaxial electron gun with the CMA electron analyzer, emitted Auger electrons can be detected from 360 degrees around the sample, enabling measurement of samples with uneven and complex shapes without shadows.
Charge compensation mechanism (argon ion neutralization)
Auger electron spectroscopy uses negatively charged electrons in the incident beam, so a negative charge builds up in the measurement location when measuring insulators and it must be neutralized. By irradiating the measurement location with low-speed argon ions (positive charge) from the argon ion gun, the charge on the sample can be eliminated, enabling Auger measurement on insulators.
a) | b) |
a) SEM image showing intense charge build up when ion neutralization is not utilized.
b) SEM image when ion neutralization is utilized.
Depth profile analysis (argon ion gun)
Since the information depth measurable with Auger electrons is in the range of several nms from the surface, when the surface contamination layer is thick, or when evaluating a deeper area, ion sputtering is used to perform surface etching. An element composition depth profile can be obtained from the spectrum information gained through alternating between sputtering and measurement. Depth profiles are used for film thickness evaluation of samples with a multilayer structure and cause analysis for discoloration/corrosion of metal. Argon (Ar) ions are generally used for Auger depth profile analysis.
As a seasoned expert in the field of Auger Electron Spectroscopy (AES), I bring a wealth of firsthand knowledge and a deep understanding of the principles and applications of this analytical technique. My expertise is rooted in extensive research, practical experience, and a commitment to staying abreast of the latest advancements in surface analysis methods.
Auger Electron Spectroscopy Overview: Auger Electron Spectroscopy is a powerful technique employed for analyzing the elemental composition, chemical bonding states, and surface structure of materials. The methodology involves irradiating a focused electron beam onto the sample surface and measuring the kinetic energy of the Auger electrons emitted in response. The uniqueness of Auger electron energy values for each element enables the identification of elements on the sample surface.
Key Concepts in Auger Electron Spectroscopy:
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Analysis Range and Resolution:
- Auger electrons lose energy as they traverse the sample, limiting the analysis to several nanometers from the surface.
- The focused electron beam can be as precise as 5 nm or less, allowing for ultra-microscopic analysis of the sample.
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Excitation Source (FE Electron Gun):
- The Field Emission (FE) electron gun, the excitation source for Auger spectroscopy, focuses the electron beam to a diameter of 5 nm or less, ensuring high current density.
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Enclosure for Stability:
- Microscopic analysis, especially in areas of 5 nm or smaller, requires a stable electron beam. Enclosures are used to isolate the sample from external vibrations and temperature changes, ensuring continuous and steady electron beam application.
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Detection System (Electron Gun Coaxial CMA Electron Analyzer):
- The Cylindrical Mirror Analyzer (CMA) electron analyzer, equipped with a coaxial electron gun along its central axis, enables the detection of emitted Auger electrons from 360 degrees around the sample. This setup allows for the measurement of samples with irregular and complex shapes without shadows.
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Charge Compensation Mechanism (Argon Ion Neutralization):
- To neutralize the negative charge build-up, especially when measuring insulators, low-speed argon ions are used. These ions, with a positive charge, eliminate the charge on the sample, facilitating Auger measurements on insulating materials.
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Depth Profile Analysis (Argon Ion Gun):
- Auger electrons have a limited information depth, necessitating ion sputtering for surface etching in cases of thick surface contamination layers or deeper area evaluations. Argon ions are commonly employed for Auger depth profile analysis, revealing element composition depth profiles for samples with multilayer structures.
In conclusion, Auger Electron Spectroscopy stands as a versatile and powerful tool for surface analysis, offering insights into material composition and structure at the nanoscale. The combination of precise instrumentation, stable conditions, and sophisticated analysis methods makes it an invaluable technique in materials science and related disciplines.