AUGER ELECTRON SPECTROSCOPY

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three electron process

kinetic energy of Auger electron is used to giveelemental info on sample

developed in the late 1960's , -effect first observed by Pierre Auger, a French Physicist, in the mid-1920's.

first ionization process can be initiated soft x-rays ( hν = 1000 - 2000 eV ). -XAES this change in the method has no significant effect on the final spectrum. (see later)

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Auger process can be labeled using the orbitals involved

When the initial vacancy is filled with an electron from the same shell from within a different subshell, the transition is called a Coster-Kronig transition

final state is doublycharged which mayneutralize over time

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X-ray fluorescence is a competing process

so AES is better for light elements but still"works" at high Z

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Energy of Auger Electrons

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

The expression for the energy can also be re-written in the form : 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.

An Auger transition is therefore characterized primarily by :-1.the location of the initial hole

2. the location of the final two holes although the existence of different electronic states (terms) of the final doubly-

ionized atom may lead to fine structure in high resolution spectra.

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as we have seen the transition is described by giving the initial hole location is given first, followed by the locations of the final two holes in order of decreasing binding energy.

so for a KL1L2,3 transition

there are three possible Auger processes

K L1 L1K L1 L2,3 K L2,3 L2,3

In general, since the initial ionization is non-selective and the initial hole may therefore be in various shells, there will be many possible Auger transitions for a given element -some weak, some strong in intensity. AUGER SPECTROSCOPY measures 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.

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Auger peaks for palladium occur between 220 & 340 eV. situated on a high background which arises from secondary electrons generated by a multitude of inelastic scattering processes.

Auger spectra are often shown in differentiated form : the reasons for this are partly historical, and partly because it is possible to measure spectra directly in this mode, giving and by doing so get better sensitivity. The plot shows the same spectrum in such a differentiated form. (see later)

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so in AES each element has a set of characteristic peak positions-usually measured as the first major negative minimum thatserves as its AES "fingerprint"

e.g., C : 272 eV N: 379eV O: 503 eVKE is independent of primary electron beam energy

E(primary) must exceed E(kin)

in practice E(primary) ~ 5 x E(kin)

gives largest signals

2 main considerations1. spatial resolution2. specimen charging

1. for a given probe size max current is prop. to beam energy �desired res os easier toachieve with higher beam energy

but: more energy into sample: heating , decomposition

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In general, the electron flux in an AES experiment can be 106 greater than those from an XPS experiment. This leads to very high sensitivity (~0.1 monolayer).

What elements have no AES signature ?

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LMM

ScN=21

TiN=22

VN=23

L2,3VV L2,3,VVLMM

CrN=24

MnN=25

FeN=26

Things to noteProgression of peak energies with atomic numberIncreased number of bands with increased electronic complexitySplitting of the LMM group due to multiple configurations possible (lower energy bands show less structure)Significant intensity variations among the various ‘bands’and within any multiplet band

In these graphs, the ‘VV’notation (low energy peak) is used to show that these two electrons are in the valenceshell. Here, L2,3VV is equivalent to L2,3MM(some refs have these as M2,3VV)

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The kinetic energies for the Auger electrons across the periodic table show clear and simple trends. In the LMM and MNN series, the peaks show multipletsplittingAlthough in principle the chemical environment of the source atom can lead to peak shift, usually these are small (1-2 eV) and difficult to attribute to specific environments. As such, the chemical shift is not widely used in AES. It is essential in XPS.

Why are the chemical shifts difficult to systematically interpret and analyse?

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Notice that for any given element (e.g. Al) the Sensitivity is in the sequenceS( MNN ) > S( LMM ) > S( KLL )and the Kinetic energies are in the sequenceEK( MNN ) < EK( LMM ) < EK( KLL )

relative sensitivity

Specimen Charging

can be a major limitation

if electron flux leaving sample is larger or smaller than incident flux sample will chargeparticularly bad for insulatorsdistorts spectrum: shifts energies

one solution is to rotate sample

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As we have seen most Auger spectra are recorded in differential mode

differential mode defeats slope of the background . Can then amplify weak features

note S/N is not improved: signal contrast is4-13

Depth profiling

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Because of the relatively high electron emission probabilities by the Auger process, it is possible to simultaneously sputter away the material with an ion beam (e.g. Ar+, 1000 eV) and record the AES spectra. Although difference materials are sputtered (or etched) away at different rates (mass, bonding �), the etching time translates approximately to physical depth into a sample.

Typical etch rates are ~0.1 nm / second.

In this example, brief heating of a thin nichrome film has the effect of

Increasing the Ni content close to the Si substrate

Diffusing oxygen deeper into the filmMaking the Cr content more uniform

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In this example, exposure of a Pb / Snsolder alloy to gaseous oxygen leads to partial oxidation.

It is also possible to observe that the Pb/Sn ratio varies considerably as a function of the depth. This has enormous consequences for microelectronics industries, where the trend is to make increasingly thin solder connections between components, and controlling this to be at or near the eutectic composition is a significant issue. If the composition varies too much, too high a temperature is required to melt the solder, and the components can be damaged.

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Argon gas introduced and ionized: "fired at sample"

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Scanning Auger , Imaging and Quantitative Auger

next lecture

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