TERM PAPER(PH 308)

Auger Electron Spectroscopy(AES)

Submitted By:-Manvendra Singh NarwarRoll no. 1301210163rd year Undergraduate

To:-Prof. Dr. P. K GiriDepartment of Physics

Introduction :-

Auger Electron Spectroscopy (AES) was developed in the late 1960's , deriving its name from the effect first observed by Pierre

Auger, a French Physicist, in the mid-1920's. It is a surface specific technique utilising the emission of low energy electrons in

the Auger process and is one of the most commonly employed surface analytical techniques for determining the composition of

the surface layers of a sample.

AES provides quantitative elemental and chemical state information from surfaces of solid materials. The average depth

of analysis for an AES measurement is approximately 5 nm. Physical Electronics Auger instruments provide the ability to obtain

spectra with a lateral spatial resolution as small as 8 nm. Spatial distribution information is obtained by scanning the micro

focused electron beam across the sample surface. Depth distribution information is obtained by combining AES measurements

with ion milling (sputtering) to characterize a thin film structure.

The information AES provides about surface layers or thin film structures is important for many industrial and research

applications where surface or thin film composition plays a critical role in performance including: nanomaterials, photovoltaics,

catalysis, corrosion, adhesion, semiconductor devices and packaging, magnetic media, display technology, and thin film coatings

used for numerous applications.

AES is accomplished by exciting a sample’s surface with a finely focused electron beam which causes Auger electrons to be

emitted from the surface. An electron energy analyzer is used to measure the energy of the emitted Auger electrons. From the

kinetic energy and intensity of an Auger peak, the elemental identity and quantity of a detected element can be determined. In

some cases chemical state information is available from the measured peak position and observed peak shape.

Physical Electronics AES instruments function in a manner analogous to SEM/EDS instruments that use a finely focused

electron beam to create SEM images for sample viewing and point spectra or images for compositional analysis. In contrast to

SEM/EDS which has a typical analysis depth of 1-3 µm, AES is a surface analysis technique with a typical analysis depth of less

than 5 nm and is therefore better suited for the compositional analysis of ultra-thin layers and nanoscale sample features.

Auger Electron Spectroscopy Setup :-

AES experimental setup using a cylindrical mirror analyser (CMA). An electron beam is focused onto a specimen and emitted

electrons are deflected around the electron gun and pass through an aperture towards the back of the CMA. These electrons are

then directed into an electron multiplier for analysis. Varying voltage at the sweep supply allows derivative mode plotting of the

Auger data. An optional ion gun can be integrated for depth profiling experiments.

When a high-energy electron knocks out the inner electron at the K shell of an atom, an Auger process is initiated. In the

Auger process, the inner K shell vacancy is filled by a second electron at a higher L1 shell, together with a third electron at

the L2 shell, the Auger electron, leaving the atom. The excessive energy is deposited to the Auger electron in the form

of kinetic energy. This Auger transition is labelled as: KL2L3.

Nomenclature for Auger Transitions :-

Auger spectroscopy can be considered as involving three basic steps :-

• Atomic Ionization(by removal of core electrons)

• Electron Emission

• Analysis of the Emitted Auger Electrons

Ionization :-The Auger process is initiated by creation of a core hole - this is typically carried out by exposing the sample to a beam of high

energy electrons (2 KeV-10KeV). Such electrons have sufficient energy to ionise all levels of the lighter elements, and higher core

levels of the heavier elements.

In the diagram below, ionisation is shown to occur by removal of a K-shell electron, but in practice such a crude

method of ionisation will lead to ions with holes in a variety of inner shell levels.

Auger Emission/Process :-

The ionized atom that remains after the removal of the core hole electron is, of course, in a highly excited state and will rapidly

relax back to a lower energy state by Electron(Auger) Emission.

Illustrated Example :-In this example, one electron falls from a higher level to fill

an initial core hole in the K-shell and the energy liberated in

this process is simultaneously transferred to a second

electron ; a fraction of this energy is required to overcome

the binding energy of this second electron, the remainder is

retained by this emitted Auger electron as kinetic energy. In

the Auger process illustrated, the final state is a doubly-

ionized atom with core holes in the L1 and L2,3 shells.

We can make a rough estimate of the KE of the Auger electron from the binding energies of the various levels involved. In this

particular example, KE = ( EK - EL1 ) - EL23 i.e. KE = EK - ( EL1 + EL23 )

It should be clear from this expression that the latter two energy terms could be interchanged without any effect, i.e. it is actually

impossible to say which electron fills the initial core hole and which is ejected as an Auger electron, they are indistinguishable.

;here the transition illustrated is a KL1L2,3 transition

An Auger transition is therefore characterized primarily by :-

• the location of the initial hole

• the location of the final two holes

Analysis of Auger Electron :-

Auger Spectroscopy is based upon the measurement of the kinetic energies of the emitted electrons. Each element in a sample

being studied will give rise to a characteristic spectrum of peaks at various kinetic energies.

This is an Auger spectrum of Pd metal - generated using a 2.5 keVelectron beam to produce the initial core vacancies and hence to stimulate the Auger emission process. The main peaks for palladium occur between 220 & 340 eV. The peaks are situated on a high background which arises from the vast number of so-called secondary electrons generated by a multitude of inelastic scattering processes.

although the existence of different electronic terms of the final doubly-ionized atom may

lead to fine structure in high resolution spectra.

• There are a number of uses of AES which can be electron microscopes that have been specifically designed for use in Auger

spectroscopy, these are termed Scanning Auger Microscopes (SAM) and can produce high resolution, spatially resolved

chemical images.

• Sputtering is sometimes used with Auger spectroscopy to perform depth profiling experiments.

• Precise depth milling through sputtering has made profiling an invaluable technique for chemical analysis of nanostructured

materials and thin films.

• AES is also used extensively as an evaluation tool on and off fab lines in the microelectronics industry, and as a standard

analytical tool in research labs.

• Theoretically, Auger spectra can also be utilized to distinguish between protonation states.

• Auger electron spectroscopy is a widely used surface analysis technique that has been successfully applied to many diverse

fields ranging from gas phase chemistry to nanostructure characterization.

Uses:-

• Despite the advantages of high spatial resolution and precise chemical sensitivity attributed to AES, there are several

factors that can limit the applicability of this technique, especially when evaluating solid specimens. One of the most

common limitations encountered with Auger spectroscopy are charging effects in non-conducting samples. Charging

results when the number of secondary electrons leaving the sample is different from the number of incident

electrons, giving rise to a net positive or negative electric charge at the surface. Both positive and negative surface

charges severely alter the yield of electrons emitted from the sample and hence distort the measured Auger peaks.

Several processes have been developed to combat the issue of charging, though none of them is

ideal and still make quantification of AES data difficult. The most common setup to minimize charging effects

includes use of a glancing angle (~10°) electron beam and a carefully tuned bombarding energy (between 1.5

keV and 3 keV). Control of both the angle and energy can subtly alter the number of emitted electrons vis-à-vis

the incident electrons and thereby reduce or altogether eliminate sample charging.

Limitations and Solutions:-

• In addition to charging effects, AES data can be obscured by the presence of characteristic energy losses in a sample and

higher order atomic ionization events. Electrons ejected from a solid will generally undergo multiple scattering events and lose

energy in the form of collective electron density oscillations called Plasmons. If Plasmon losses have energies near that of an

Auger peak, the less intense Auger process may become dwarfed by the Plasmon peak. As Auger spectra are normally weak

and spread over many eV of energy, they are difficult to extract from the background and in the presence of Plasmon losses,

deconvolution of the two peaks becomes extremely difficult.

• Sometimes an Auger spectrum can also exhibit "satellite" peaks at well-defined off-set energies from the parent peak. Origin

of the satellites is usually attributed to multiple ionization events in an atom or ionization cascades in which a series of

electrons is emitted as relaxation occurs for core holes of multiple levels. The presence of satellites can distort the true Auger

peak or small peak shift information due to chemical bonding at the surface. Several studies have been undertaken to further

quantify satellite peaks.

Conclusion/Summary :-Auger Electron Spectroscopy is a surface-sensitive spectroscopic technique used for elemental analysis of surfaces, it offers :-

• high sensitivity (typically ca. 1% monolayer) for all elements except H and He.

• a means of monitoring surface cleanliness of samples

• quantitative compositional analysis of the surface region of specimens, by comparison with standard samples of known

composition.

In addition, it has been used in :-

• Auger Depth Profiling : providing quantitative compositional information as a function of depth below the surface

• Scanning Auger Microscopy (SAM) : providing spatially-resolved compositional information on heterogeneous samples.

Sensitivity, quantitative detail, and ease of use have brought AES from an obscure nuisance effect to a functional and practical

characterization technique in just over fifty years. With applications both in the research laboratory and industrial settings, AES

will continue to be a cornerstone of surface-sensitive electron-based spectroscopies.

References:-

• E N Kaufmann, Characterization of Materials(Wiley, 2003).

• J. C. Vickerman, Surface analysis – the principal techniques, John Wiley & Sons (1997).

• Wikipedia

• Analysis Forums like UK Surface, etc. and Published Papers on AES.