Auger Electron Spectroscopy (AES)

Take Home Point:

Surface Composition and Depth Profiles of small areas.

What It Provides:

Provides surface atomic composition and concentrations down to sub-percent levels and in small areas. Can provide 'Bulk' composition via depth profiles. Also provides high quality SEM images showing surface details and the analysis location.

Brief Description:

Electrons In/Electrons Out

The sample is bombarded with electrons. Auger electrons (whose energy is characteristic of the atom from which it originates) are produced. The energies and intensities of the Auger electrons generated within the top few nanometers of the sample are measured to produce the AES spectrum. The instruments have secondary electron detectors allowing for high quality SEM images as well which clearly shows the analysis location. Concentrations versus depth are obtained by sequentially acquiring data and then sputtering with an ion beam to remove a defined amount of material.

What is Detected:

Li-U

Detection Limits:

~0.1-1 Atomic % (element dependent)

Information Depth:

~0.4-5nm

Applications:

Identify surface contaminants and composition
Analysis of submicron defects and particles in semiconductors, disk drives, medical devices etc...
Bond Pad Analysis
Oxide thickness on metal alloy medical devices
Composition of multi-layered film stacks
Corrosion studies
Catalysis studies

Manufacturers:

Physical Electronics

In a Nutshell

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Greater Detail

Overview

In Auger (pronounced 'O.J.') Electron Spectroscopy the sample is hit with a beam of electrons (~25KeV) causing the emission of core level electrons from atoms resulting in 'holes' in the electron orbitals. The atom is now in an 'excited' state which can then relax by an electron from a higher core level dropping into this hole. This process results in the release of energy either in the form of radiation or another electron, which is called an Auger electron. The latter is what is measured in AES.

The Auger electron energies are unique to the atoms (e.g. Fe, Cu, O etc...) from which they are emitted. Thus, by measuring the energy and intensity of the Auger electrons released from material during electron beam bombardment, the composition and concentration (semi-quantitative) can be determined by comparing to references of pure materials analyzed using the same incident beam energy of electrons.

It is important to note that only Auger electrons formed in the surface/near-surface region of the sample escape the material without losing some of their energy by secondary collisions within. This is what makes AES 'surface sensitive'. Those that lose a portion of their energy do not produce characteristic signals and instead appear as background. Sampling depths are typically no greater than 5nm making AES an important surface analysis tool. Surface composition is important as it can have significant influence on surface properties such as adhesion, biocompatibility, reactivity and visual effects. Further, contaminants and residues are often only found on the surface. Using techniques that sample larger volumes of depth may not have the sensitivity to detect such species.

AES is not only relegated to surface analysis. It can also be used to obtain depth profiles by hitting the sample with an ion beam of sufficient energy to erode (i.e. sputter) and expose underlying material. By repeating the analysis step followed by sputtering, depth profiles (i.e. composition versus depth) are obtained.

Because electron beams can be tightly focused down to 10nm, AES is often used for the analysis of very small areas. This ability is a key feature of AES. Further, since hitting the sample with a beam of electrons also generates secondary electrons (just like SEM), their collection during the AES experiment provides high quality SEM images as well. These images are extremely useful when performing analyses on small areas of interest (e.g. defects, particles, specific locations on devices).

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