Download Ion Beam Analysis techniques

Ion Beam Analysis techniques:
NRA, RBS, ERDA
Andrius Martinavičius
Emmanuel Wirth
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Ion beam interaction products
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Ion – target interaction
elastic atomic collisions:
very low energies
typically below a few keV
inelastic atomic collisions:
ionization of target atoms
characteristic x-ray emission
elastic nuclear collisions:
scattering
inelastic nuclear collisions:
nuclear reactions
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What happens to ions inside the material?
Ions lose energy, interacting
elastically with nuclei
and
inelastically with electrons
dE
  N S n ( E )  S e ( E )
dx
N – the number of target atoms
per unit volume of the solid;
Si(E) is stopping power (eVcm2)
1 E0
dE
R   dx 
 S (E)  S (E)
N
0
0 n
e
R
Ion range in target:
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Stopping Power of 20Ne on Polyethylene
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Condition for nuclear reaction
Energy of the incident particle must
exceed the Coulomb barrier
zZ
E  1/ 3
1/ 3
a A


(MeV)
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
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Nuclear Reaction Analysis (NRA)
Ion beam energy up to 50MeV
non-resonant nuclear reactions
+D→α+p
2H + 12C → 13C + p
3He
For profiling energy of reaction
product is measured
resonant nuclear reactions
+ 1H → 12C + α + γ
1H + 27Al → 28Si + γ
15N
For profiling energy of incident
beam is changed
The yield of the characteristic reaction products
is proportional to the concentration of the
specific elements in the sample.
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Typical NRA spectra
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Resume of NRA
Elements
H – Al
Standard
Conditions
~ 1 MeV proton beam (15N, 19F, etc. for H – detection)
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
NaI-, Ge-detector (Si detector for non-γ reactions)
15 minutes per measurement
5 hours per profile
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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 materials
Energetic 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
Backscattered energy


Mass resolution low for heavy element
Identification of the atoms possible if ≠ of E between
incident ions and target is enough
kinematical factor
The detection limit depends on the
scattering cross section
Concentration of
the element
σp
depend
on Z2
Number of backscattered
ions is prop. to Z2
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RBS (3): Example of spectra
Light Ions / Heavy Ions
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RBS (4): Advantages/ Disadvantages
Advantages
standard 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
Disadvantages
You can not detect atoms
with a mass inferior than
incident ion mass
less 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)
SiNx:H layer
on Si
Identification of target
atom and depth profile
Can be used with
measurement of the
time-of-flight (TOF) of
the recoil particles
Larger dynamic range in
energy (depth)
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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
kinematical
factor
Similiarities



Composition and depth
Standard free, absolute
method
Rapid Analysis
Concentration
of the element
Differential
cross section
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ERDA (3): Example Al2O3-C-multilayer-sample
simulation of the measured spectra
Depth distribution of the layer
constituents
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Conclusion: comparison between methods
ERDA
Sensitivity
depends on matrix
and element looked
for
Depth
ppm for H
10 ppm for
others
RBS
ppm for heavy
elements
0.1% for light
elements
NRA
100 ppm
10 nm close to
surface
10 nm close to
surface
5 nm close to
surface
Max. analytical
depth
a few μm
a few μm
a few μm
Elements
all
M > Mion
H – Al
Resolution
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