A EM microscopy

8/4/2019 A EM microscopy

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Introduction and Applications ofAuger Electron Microscopy AEM


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IntroductionThe Auger effect was discovered by Pierre

Auger in 1925 while working with X rays andusing a Wilson cloud chamber. Tracks

corresponding to ejected electrons wereobserved along a beam of X rays.

The aim of Auger electron imaging is to obtainquantitative surface elemental distributionmaps at high spatial resolution


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WorkingPrinciple-1 When an atom is struck by a high energy electron

(typically in the range 1 to 25 keV) there is some

probability of a core level electron being ejected.

The atom is then in an energetic ionic state with anelectron missing from a core level.

One mechanism by which the atom can relax into alower energy state is for another electron

one electron falls from a higher level to fill an initial

core hole in the K-shell and the energy liberated in thisprocess is simultaneously transferred to a secondelectron ; a fraction of this energy is required toovercome the binding energy of this second electron,the remainder is retained by this emittedAugerelectronas kinetic energy.


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WorkingPrinciple -2


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WorkingPrinciple -3Two-step process, generates Auger electrondiffers from XPS

1) generate excited ion as before:

A + hn A+* + e-or using an electron beamA + ei- A+* + ei- + eA-

where:ei-is the beam electron after interactionwith A and loss of kinetic energy.eA-is ejected from A inner orbital

2)excited ion may relax by emitting anAuger electron (eA-) with kinetic energy (Ek)A+* A++ + eA-or by fluorescence (X-ray fluorescence)A+* A+ + hn fKinetic energy of emitted electron (Ek) is independent ofthe energy of photon or electron that

Kinetic energy (EK) of the Auger electron is:EK= (EbEb) Eb= Eb2Eb

where:(Eb-Eb) energy released in relaxation of the excited ionEb energy required to remove the second electron from its

orbit


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WorkingPrinciple -4The Auger electron has a characteristic energy whichdepends upon a number of factors:

The chemical element involved The energy level within which the initial hole was formed The energy level of the electron which eventually fills the

hole

The initial energy level of the electron which eventuallybecomes the the Auger electron

- Auger emissions are described in terms of the typeof orbital transitions involved in the production ofan electron

KLL:1. removes a K electron2. transition of an L electron to the K orbital3. ejection of a second L electron


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AEM: Efficiency of Auger Electron Production

Two competingprocesses:

X-rayfluorescence

Auger electronemission

Auger electronspredominate atlower atomtomicnumber(Z)

createdvacanciesshellKofnumber

producedphotonsKofnumber

K

KAuger 1


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AES: InstrumentationAES instruments are designed like an

SEM often they are integrated with

an SEM/EDXA system

Unlike an SEM, AES instruments aredesigned to reach higher vacuum (10-8torr)

Helps keep surfaces clean andfree from adsorbed gases, etc

Basic components:

Electron source/gun

Electron energy analyzer Electron detector

Control system/computer

Ion gun (for depth profiling)


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Primary Electron SourcesThree kinds of primary electron sources are in commonuse in Auger electron spectrometers

A tungsten cathode source consists of a wire filamentbent in the shape of a hairpin.

Lanthanum hexaboride (LaB6) cathodes provide highercurrent densities

Field Emission electron sources consist of very sharptungsten points at which electrical fields can be >10E7V/cm.

Auger instruments have primary electron beamcolumns similar to electron microscopes. Thecolumns may include bothelectrostaticandmagneticlenses for beam focusing
http://www.evanseast.com/training/tutorials/rbs_instrumentation_tutorial/index.phphttp://www.evanseast.com/training/tutorials/aes_instrumentation_tutorial/mlens.phphttp://www.evanseast.com/training/tutorials/aes_instrumentation_tutorial/mlens.phphttp://www.evanseast.com/training/tutorials/rbs_instrumentation_tutorial/index.php


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Electron Energy Analyzers Electron energy analyzers measure the number of ejected electrons as afunction of the electron energies.

The analyzers must be located in a high vacuum chamber and isolatedfrom stray magnetic fields (including the earths) that deflect electrons

The schematic shows a cross section of a cylindrical mirror analyzer inred.


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The primary electron beam hits the sample surfaceat the source point of the analyzer. Auger electronsmove outward in all directions and some passthrough the grid covered aperture in the inner

cylinder.

A variable negative potential on the outer cylinderbends the Auger electrons back through a secondaperture on the inner cylinder and then through anexit aperture on the analyzer axis.
http://www.evanseast.com/training/tutorials/aes_instrumentation_tutorial/electron.phphttp://www.evanseast.com/training/tutorials/aes_instrumentation_tutorial/electron.php


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Electron DetectorElectron MultipliersDiscrete dynode detector An electron multiplier consists of a series of

electrodes called dynodes

each connected along a resistor string. Thesignal output end of the resistor stringattaches to positive high voltage. The otherend of the string goes to the electronmultiplier case and ground.

When a particle electron strikes the first

dynode it produces secondary electrons.The secondary electrons are acceleratedinto the next dynode where each electronproduces more secondary electrons. Acascade of secondary electrons ensues. Thedynode acceleration potential controls theelectron gain.


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Ultra High Vacuum environment The surface analysis necessitates the use of a UHV

environment

Contamination of the specimen surface is critical for highlyreactive surface materials, where the sticking coefficientfor most residual gases is very high (near unity).

the background pressure is reduced to the low 1010-torr range in order to minimize the influence of

residual gases in surface analysis measurements


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AEM: Image Surface Analysis

ANALYTICAL INFORMATION Spatial resolution is approximately 0.3 microns.

Depth resolution is dependent upon sample and sputteringparameters (less than 100 resolution is not unusual). Typicalsputtering rate is ca. 30 /min.

SAMPLE REQUIREMENTS

Maximum of "x" (18 mm x 12 mm). Height should not exceed" (12 mm). must be conductive or area of interest must be grounded

properly. Insulating samples including thick insulating films(ca. 3000 ) cannot be analyzed

SUPPLEMENTAL INFORMATION: Minimum area of analysis - 0.3 microns 10% relative error (i.e. estimated error in repeated analyses),

20% absolute error (i.e. error between analysis and knownstandard).

hr per sample .Depth profiles may take longer depending on

the total depth being sputtered.


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AEM VS SEM For true surface analysis, AES is

better than SEM/X-ray emission(electron microprobe) because it is

much more surface sensitive

AEM can be easily madequantitative using standards.

The major advantages of AeMover SEM are greater lateral anddepth analytic resolution and easeof light-element detection.

The Spatial resolution of AEM is0.030 pm

The Spatial resolution of SEM is1 - 3 pm


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Advantages of Auger Electron Microscopy

Sensitivity to atoms of low atomic number

Minimal matrix effects High spatial resolution

Detailed examination of solid surfaces

Electron beam more tightly focused than X-raybeam

Almost any solid can be analyzed.

Sample can be analyzed as it is.

Estimated time to obtain the survey spectrumfrom a sample varies from 1 to 5 min.


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Disadvantages of Auger Electron Microscopy

not used to provide structural and oxidative state

information (XPS)

quantitative analysis is difficult

It does not provide for nondestructive depth profiles

It requires that samples be small and compatible with highvacuum

Analyzes conducting and semiconducting samples.

Special procedures are required for nonconductingsamples. Only solid specimens can be analyzed.

Samples that decompose under electron beamirradiation cannot be studied.

G l U


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General Uses

Identification of elements on surfaces ofmaterials

Quantitative determination of elements onsurfaces

Depth profiling by inert gas sputteringPhenomena such as adsorption, desorption,

and surface segregation from the bulk

Determination of chemical states of elements

In situ analysis to determine the chemicalreactivity at a surface

Auger electron elemental map of the system

li i


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Common Applications

Qualitative analysis through

fingerprinting spectral analysis

Identification of different chemical states ofelements

Determination of atomic concentration ofelements

Depth profiling

Adsorption and chemisorption of gases onmetal surfaces

Interface analysis of materials deposited in

situ on surfaces


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A EM microscopy

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