1. Ion Beam Analysis techniques:
NRA, RBS, ERDA
Andrius Martinavičius
Emmanuel Wirth

2. Ion beam interaction products

3. 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

4. What happens to ions inside the material?
Ions lose energy, interacting
elastically with nuclei and inelastically with electrons
N – the number of target atoms per unit volume of the solid;
Si(E) is stopping power (eVcm2)
Ion range in target:

5. Stopping Power of 20Ne on Polyethylene

6. Condition for nuclear reaction
Energy of the incident particle must exceed the Coulomb barrier
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

7. Nuclear Reaction Analysis (NRA)
Ion beam energy up to 50MeV
non-resonant nuclear reactions resonant nuclear reactions
3He + D → α + p
2H + 12C → 13C + p
15N + 1H → 12C + α + γ
1H + 27Al → 28Si + γ
For profiling energy of reaction product is measured
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.

8. Typical NRA spectra

9. Resume of NRA 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

10. 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
A  high energy beam of He+ ions ( energy > 2 MeV) is partly backscattered by the near surface region of the sample. These ions are analysed by a solid state detector. Both composition and depth distribution of elements in the sample can be deduced. Also, quantitative measurement of crystal damage can be obtained.   Although its sensitivity is not comparable to that of SIMS, RBS provides very valuable complementary informations :   Direct quantitative analysis (no standards needed) with detection limits down to 0.1% for heavy impurities in light matrices. Crystalline damage measurement in crystals. Analysis of non-conducting organic or inorganic solids and powders.  Below is an example of a RBS spectrum from a TiN layer on Si substrate.

11. 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

12. RBS (3): Example of spectra
Light Ions / Heavy Ions

13. RBS (4): Advantages/ Disadvantages
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)

14. ERDA (Elastic Recoil Detection Analysis)
Detection of recoiled atoms
Low angles (for thick sample)
Identification of target atom and depth profile
Can be used with measurement of the time-of-flight (TOF) of the recoil particles
SiNx:H layer on Si
Larger dynamic range in energy (depth)

15. ERDA (2): Similiarities and Differences from RBS
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
kinematical factor
Concentration of the element
Differential cross section

16. ERDA (3): Example Al2O3-C-multilayer-sample
simulation of the measured spectra
Depth distribution of the layer constituents

17. 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
5 nm close to surface
Max. analytical depth
a few μm
Elements
all
M > Mion
H – Al