First of all, do you know any First of all, do you know any methods to check chemical methods to check chemical

composition?composition?Or how you know what is what?Or how you know what is what?

Surface sensitive?

Auger Electron Spectroscopy (AES)

Fix kinetic energy

Binding energy

Electron-Surface InteractionAuger electrons can be generated by any energetic particles, which are able to and excite electrons and leave holes, such as X-Ray irradiation, ion-beam bombardment and electron beam irradiation. In the sense of AES, it is excited by electrons.

Electrons interaction with surface brings:•X-rays (both continuum and characteristic)•Backscattered and transmitted electrons,•Secondary electrons•Auger electrons•Cathodoluminescence•Heat.

What does electron spectrum do?

Certain energetic particles interact with material, there will be electrons with different energy come out from the material. Spectrum is to record the intensity (number) of electrons as a function of energy (kinetic energy).

Two important information from the spectrum:

Where are the peaks? (peak at certain energy) How intense is the peak? (peak height)

The electron analyzer is device to record spectrum.

x-ray notation

The Auger emission is nominated as x-ray notation: initial core-hole, initial location of the relaxed electron and the second core-hole

AES spectra

I(V+v0sint) = I0+dI/dV*sint+…Normally login technique to measure the dI/dV (dN/dE)

Auger peaks are very broad (several eV)…..

Typical spectra

The electric voltage of the analyzer lens is modulated by AC one, then login technique to detect signal

Experimental aspects 1: e-beam sourceAlthough AES can be generated by both x-ray and high energy e beam, the e beam is

easy to be generated and manipulated (focus, scan) and AES normally use e-beam.

The e-beam generation:

1. Thermionic emission of heated filament with low work function such as W. cheap

2. Filed emission gun (FEG): high electric field gradient remove electrons by tunneling emission material fashioned to sharp point. High flux, good focused.

Thermionic emissionLaB6 Electron Gun

•Single crystal lanthanum hexaboride (LaB6) cathodes provide higher current densities •LaB6 has a lower work function and greater emissivity than tungsten ~100 A/cm2 . •Narrower electron beams, dg=~10-20u •Useful for analyzing smaller features

W Electron Gun

•Wire filament in the shape of a hairpin. •The filament operates at ~2700 K by resistive heating. •The tungsten cathodes are reliable and inexpensive. •Lateral resolution is limited dg=~50u •Current densities are only about 1.75 A/cm2.

How electron analyzer works?Analyzer has certain pass energy (Ep), electrons with this energy in a small energy range (Ep±ΔE) can pass.Energy resolution Δ E is proportional to Ep.

As analyzer is works in small range of pass energies. To measure big energy range, electrons with different energy need to be retarded (or accelerated) by a potential to change the electron energy to be able to analyze.

There are two methods to retard the energy of electrons:

Constant pass energy mode: retard the electron energy to a fixed pass energy by varying the retarding voltage, therefore with fixed ΔE for whole spectrum.

Constant retarding ratio mode: retard the electron energy with a fix ratio to a energy range that the analyzer can use corresponding pass energy to detect. Therefore the ΔE/E is fixed and not the ΔE is fixed.

E

RetardingV=(E-Ep) Ep

Experimental aspects 2a:The electron energy analyzer:

Principally all the electron energy analyzer can be used, however,the Cylindrical Mirror Analyzer (CMA) is common. This analyzer has large angular acceptance and high sensitivity. (AES peak generally broad and with isotropic angular dependence.)

Experimental aspects 2a:The electron energy analyzer:

CMAwith double pass (high resolution)

Energy diagram for AES

Ekin = Ek - EL1 - EL2

Ev

Ef

EL1

EL2

EK

Considering the many-electron relaxation effects (2 holes and 1 electron), there is:

Ezkin = Ez

k-EzL1-Ez

L2 – E(L1L2)

in a simple model with

E(L1L2) = ½ * (Ez+1L2- Ez

L2 + Ez+1

L1 - EzL1)

Z dependence

The strong Z dependence of the kinetic energies of

Auger electrons gives AES elemental sensitivity

AES database

Surface sensitivity

The short free path length of the electron at energies at tens to hundreds eV gives AES surface sensitivity

Theoretical calculation

Check film thickness!

AES needs the primary e beam with energies over several thousand eV to be enough to generate the core holes.

Long free path length

Short free path length

The intensity of Auger electrons

n electrons cm-2 s-1

Auger electrons s-1

cos escape depth

Vacuum

Solid

IA=NI0r(1-)/(cos)Incident e beam current

The acceptance angle Auger backscattering factor (Auger from some secondary)

Correction of x-ray fluorescenceCross section of ionization

Angular resolvedThe shown formula is more for the incidence angle, it is integration over large acceptance angle of electron (for CMA analyzer). When consider the acceptance angle, only the Auger electrons from the depth of cos contribution can come out.

IA proportional to cosThe change of detection angle will change the surface sensitivity. In many case, it is possible to get quantitative analysis of film thickness from the Auger intensity ratios of substrate and the coated material.

Quantitative analysis

Quantitative analysis(1) Required: The same instrumental settings, e.g. resolution,

e-beam energy, for both the determination sensitivity factors and sample analysis.

(2) Needed: The same peak shapes for all peaks; Reduce effect of the peak shape using high energy peaks. Alternatively, different sensitivity factor for different peak shapes.

Sensitivity of AES: ~0.1 atomic% of a monolayer!Error in AES: analysis: < 15%, Error within a few % can be achieved with better standards and calibration. Take care Sensitivities Si for peak to peak height of differentiated Auger peak different from the one for original Auger peak(with background subtraction)

Transition possibility of AES

AES by e beam is simple and cheap, basically there are two steps: creation of core-hole and the following Auger excitation. The first process is described by cross-section :

= C(Ei/EA) / EA2

where C depends on the ratio between the energies of the incident electron (Ei) and the Auger electron (EA). typically is between 10-3 to 10-4.

Maximum about 3

cross section as function of the ratio of the energy of incident electron Ep and core-hole energy Ew

Competition between XRF and AES

There are two processes to fill the core-hole after the 1st step: Auger emission and X-ray fluorescence (XRF). Auger emission favored for light Z atoms and X-ray emission for heavier atoms (different dependences for different core-holes.