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Ion Beam Analysis techniques:

NRA, RBS, ERDA

Andrius MartinavičiusEmmanuel Wirth

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Ion beam interaction products

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elastic atomic collisions:very low energiestypically below a few keV

inelastic atomic collisions:ionization of target atomscharacteristic x-ray emission

elastic nuclear collisions:scattering

inelastic nuclear collisions:nuclear reactions

Ion – target interaction

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Ions lose energy, interacting elastically with nuclei and inelastically with electrons

What happens to ions inside the material?

)()( ESESNdx

dEen

N – the number of target atoms per unit volume of the solid;

Si(E) is stopping power (eVcm2)

Ion range in target:

0

00 )()(

1 E

en

R

ESES

dE

NdxR

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Stopping Power of 20Ne on Polyethylene

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(MeV) 3/13/1 Aa

zZE

where E is the ion energy,a and A are the atomic weights of the incident ion and sample nucleus, and z and Z are the corresponding charges

For some reactions sharply defined resonance energy

Condition for nuclear reaction

Energy of the incident particle must exceed the Coulomb barrier

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3He + D → α + p 2H + 12C → 13C + p

Ion beam energy up to 50MeV

The yield of the characteristic reaction products is proportional to the concentration of the specific elements in the sample.

Nuclear Reaction Analysis (NRA)

15N + 1H → 12C + α + γ1H + 27Al → 28Si + γ

non-resonant nuclear reactions resonant nuclear reactions

For profiling energy of reaction product is measured

For profiling energy of incident beam is changed

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Typical NRA spectra

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Elements H – Al

Standard Conditions

~ 1 MeV proton beam (15N, 19F, etc. for H – detection)

NaI-, Ge-detector (Si detector for non-γ reactions)

15 minutes per measurement

5 hours per profile

Precision Composition: 5% relative

Absolute concentrations only by calibration standards

Sensitivity ppm to % depending on element

Depth Resolution

1 to 20 nm

Probed depth ~μm

Resume of NRA

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RBS (Rutherford Backscattering Spectrometry)

Identification of target atom (Conservation of energy and momentum)Thickness determination (Energy loss in target)

with ion channeling, RBS detect crystalline defects in single-crystal materialsEnergetic ion beam aligned along rows or planes in a single crystal

Reduction of scattering events in the direction of aligned atoms

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RBS (2): Energy and dependences

The detection limit depends on the scattering cross section

Backscattered energy Mass resolution low for heavy element Identification of the atoms possible if ≠ of E between

incident ions and target is enough

Number of backscattered ions is prop. to Z2

σp depend on

Z2

Concentration of the element

kinematical factor

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RBS (3): Example of spectra

Light Ions / Heavy Ions

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RBS (4): Advantages/ Disadvantages

Advantagesstandard free, absolute method composition and depth information (and more) Rapid Analysis Typical analysis times are

10 minutes or less

RBS is very sensitive to heavy elements The RBS spectrum is easy to interpret in general

DisadvantagesYou can not detect atoms with a mass inferior than incident ion massless sensitive to light elements ( PIXE)The mass resolution, or ability to distinguish between elements, is very low for high atomic number elements ( use of heavy ion beam)

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ERDA (Elastic Recoil Detection Analysis)

Detection of recoiled atoms

Low angles (for thick sample)

Larger dynamic range in energy (depth)

Identification of target atom and depth profile

SiNx:H layer on Si

Can be used with measurement of the time-of-flight (TOF) of the recoil particles

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ERDA (2): Similiarities and Differences from RBS

Differences When using heavy incident

ions no restriction of the detectable mass range exists

Detection sensitivity is almost the same for all elements

Only for hydrogen the sensitivity is enhanced by a factor of four

Similiarities Composition and depth Standard free, absolute

method Rapid Analysis

Concentration of the element

kinematical

factor

Differential cross section

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ERDA (3): Example Al2O3-C-multilayer-sample

Depth distribution of the layer constituents

simulation of the measured spectra

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Conclusion: comparison between methods

ERDA RBS NRA Sensitivity

depends on matrix and element looked for

ppm for H

10 ppm for others

ppm for heavy elements

0.1% for light elements

100 ppm

Depth

Resolution 10 nm close to

surface 10 nm close to

surface5 nm close to

surface

Max. analytical depth a few μm a few μm a few μm

Elements all M > Mion H – Al