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2015-4
Joint ICTP/IAEA Workshop on Advanced Simulation and Modellingfor Ion Beam Analysis
F. Schiettekatte
23 - 27 February 2009
Universite de MontrealCanada
IBA intro II
RBS, EBS, ERD & NRA
Universitede Montreal
Measure the atomic concentration and distribution in a targetDepth information comes from electronic energy loss (dE/dx)Types of interactions:- Elastic collisions
• Energy/particles conserved in collision• Rutherford / non-Rutherford cross sections
- Nuclear Reactions• Energy not conserved, possible creation of new particle
- Ray emission: X, y• Element identification
backscattering Nuclear reactions
elastic recoils
ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
Outline
Kinematics and stopping powerRutherford Backscattering Spectrometry (RBS)Elastic Backscattering Spectrometry (EBS)Elastic Recoil Detection (ERD)Nuclear Reaction Analysis (NRA)
ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
In the ideal case:
- Strait-line, schematic trajectories
• point source, point detector
• uniform energy loss:energy-to-depth correspondence
- Classical kinematics
• Usually applies, even for NRA
- Probability of collision
• Rutherford cross section- Assumed in RBS, ERD
- If minimal approach distancenot too small or too large
- Known at all angles and energies
> quantitative n = qNt\ — AQ[da
ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
Depth profiling
Beam (£, Scattered ion / recoil / reaction product (E2)
ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
- » • » —
2 MeV He ->50 nm Au /200 nm SiO2 / Si
Ion backscattered
- Same ion in & out- most of the time 1 -2 MeV He
Advantages
- easy to set up• Simple detectors & electronics
can achieve <10 nm depth resolution- -0.5-2 nm depth resolution
• with sophisticated detectors (e.g. TEA)• at grazing incidence• with beam energy near maximum dE/dx
- ppm sensitivity to heavy elements is lightmatrices
Inconvenience
- not very sensitive to light element in heaviermatrix
- bad mass resolution for elements muchheavier than the beam
- single spectrum, no elemental separationother than the kinematics
• Simulation
ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
\lnP InGaAs -RBS examples:
- InGaAs/lnP/QW/lnP• InGaAs: 150 nm• analysed using 3 MeV He+
• good sensitivity to heavy atoms• good separation of In vs Ga,As• Ga, As indistinguishable• light atoms (P) barely visible• composition must be extracted
by comparison to a simulationHeavier ions help to separate masses
- here: 5 MeV O3+ on GaAsN• Ga, As separable despite
horrible energy resolution- or use PIXE
ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
• Still elastic collisions
- energy conserved- cross-section (probability of detection)
affected by nuclear interactions 104
• Advantages
- higher cross-sections• here, normalized to Rutherford 10
• resonances g• more sensitivity, especially to light ~B 0
elements 10
- Solves an inconvenient of RBS• Examples: _2
- x25 for a-> O 3.04 MeV 1 0
•Inconvenience - x125 for a-> C 4.26 MeV
- unobvious energy & angular dependence• have to be measured reliably
2 4 6Energy (MsV)
- but many useful cross-sections available theoretically- e.g. SigmaCalc
- good only for thin layers:• resonance width usually small
- still piled-up spectrum• extraction of depth profile by comparison to a (sophisticated) simulation
ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
Channel
RBS: InZnO/glass
- He 4.5 MeV @ 170° (
i ?i Zn
•examples:- Ge1.xCx/Ge with x=1 %
• (plus surface contamination)• problem:
- Zc«ZGe.:aGe(Ec)~50ac(Ec)- [C]~1%:YGe~500Yc
• solution:- 4.26 MeV a resonance- a12C~125aR
- similar solution for 16O• a0 ~ 25 aR with a 3.04 MeV
- not so good for 14N• 9.2 MeV resonance with a• 3.2 MeV protons
- Many people around in yourspectrum
- Can't stay next to your target!
Still: we haven't detected H, He,ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
Elastic Recoil Detection—> Detection of collision partner+ Energy conserved
• Simple expression for K+ Rutherford cross-section
• Simple expression for a• (small exception: MeV H e ^
- Need to filter out beam oridentify recoiled atoms
- Limitations in experimentalgeometry
• grazing incidence (roughness)• limited depth of probe• can't easily do channeling
H) _
Nuclear reactions—> Detection of reaction product
+ No particular geometry
+ If narrow resonance: best depth
resolution achievable (e.g. 15N -> 1H)
Non-Rutherford cross-sections
• Usually relatively small
• Known only at certain angles,otherwise must be measured
- Theoretical models for some of them• Unobvious shape & amplitude:
uncertainty
a IICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
Elastic Recoil Detection
incident ions4M,M2
Elastic: energy conserved (ziZ2 /E)Direct detection of the atoms from which the target consistsBut mass separation/identification required at detectionTwo possibilities- Filter: only let the ion of interest reach the detector (e.g. H)
• Absorber: dE/dx of heavier ions much higher than for light ones- Requires thick foil: energy straggling badly affects depth resolution
• Electrostatic filter• Kinematics: for beam scattering, 9 < arcsin M,/M2
- Identification: measure M or Z of each detected ion• Time of Flight (TOF)• E-AE: Energy loss in different zones of a gas or solid-state detector
depending on dE/dx —> Z
11 ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
+ Cheap• ExB filter
• also only E or B
• surface barrier detector (SBD)
+ Small accelerators• here 400 kV, near dE/dx max
+ Pretty good energy resolutionfor H^SBD
• 2-3 nm depth resolution for H
- Small detector solid angle- Charge fraction- Depth of probe- Scattering on electrodes
ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
•» c«
.will be discussed further tomorrow
ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
" p, d, 3He, a, y
notation: 15N(p,a)12C
• Four types of nuclear reactionsp,d, He, a, IN... _ R e s o n a n c e s j n cross-section (EBS)
- Broad reactions producing a "new" particle• Exothermic:
- energy increase- one of the reaction products often has a
higher energy than the backscattered ions:easily distinguishable
- High energy = small dE/dx :bad depth resolution
• Endothermic:- less energy than scattering, lost in
background- but "new" particle produced, distinguishable
- Reactions producing a "new" particle andfeaturing a sharp resonance
- Nuclear excitations or reactions producinga photon (e.g. p,y)
14 ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
Useful exothermic NRA reactions
IS.3K 0-MISSSI MOJ3.'S: O.1C4.OJ
2.L514S6
1B9MI9JBP5
17 SIS1-5S?
I.«[?1,11
Table from Guy Demortier,J. Electr. Spectr. 129 (2003) 243
15 ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
M. B. Huang, L. J. Huang, I. V.Mitchell, W. N. Lennard, W. M. Lau,J. P. Noel, Nucl. Instr. and Meth.B100 (1995) 149
- 660 keV p -> 5-doped Si
- 8.3 urn Al foil to stop H• broad peak• bad energy resolution
- but high cross-section:• precise total amount
- excellent depth resolutionachieved by using beveledsamples or successiveetching
• here, 0.7 nm
Fig. 2. Chjrged particle spflctnjm observed for The "ifCp.unuclear reaction.
i: I
i16
Fig. J . E r̂n>n depth profile for H fi-dopcc) Tuyer in Si: # th i* w
ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
Endothermic reaction: 14N(a,p)17o
absorberexample:
- GaAs.,.xNx/GaAs, x=0.01 - 0.03
- problem:• light atom, low concentration in
heavy substrate• 14N: no non-Rutherford cross-
section available• Can't use 2H beam in the lab
- solution:• endothermic 14N(a,p)17O reaction
- 3.7 MeV He- -1 MeV p
• advantage over ERD:- no geometric constraint
» channeling- better depth of probe
• foil to stop a (i resolution)
17 ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
1a in'
r1i
F =F
Extremely narrow>104 contrastAs the beam energy increase, the ion entersdeeper into the material before reactingDepth resolution:- 5-7 nm at normal incidence- 2-3 nm at gazing angle !
- Doppler broadening
W. A. Lanford, H. P. Trautvetter,J. F. Ziegler, and J. KellerAppl. Phys. Lett. 28, 566 (1976)
18 ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
Conclusion
Ion beam analysis give- quantitative depth profiles if you carefully
know/measure all the parameters
- depth resolutions -10 nm, better if you work a bit
RBS: ~ppm sensitivity for heavy atoms- Spectrum components usually not separated:
need to compare to a simulation
ERD/EBS/NRA for lighter atoms- Not as simple as RBS
Next sessions will introduce all the complications when welook behind the schematic principles
19 ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009
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