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Rutherford Backscattering Spectrometry
Timothy P. Spila, Ph.D.
Frederick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign
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Geiger-Marsden Experiment
Top: Expected results: alpha particles passing through the plum pudding model of the atom undisturbed. Bottom: Observed results: a small portion of the particles were deflected, indicating a small, concentrated positive charge.
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Rutherford Backscattering Spectrometry
He+
He
RBS is an analytical technique where high energy ions (~2 MeV) are scattered from atomic nuclei in a sample. The energy of the back-scattered ions can be measured to give information on sample composition as a function of depth.
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Van de Graaff accelerator
http://archive.thedailystar.net/newDesign/print_news.php?nid=73473
http://cnx.org
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Rutherford Backscattering Spectrometry
2 MeV Van de Graaff accelerator
beam size Φ1-3 mm flat sample can be rotated
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Primary Beam Energy
thin film projected on to a plane: atoms/cm2
Figure after W.-K. Chu, J. W. Mayer, and M.-A. Nicolet, Backscattering Spectrometry (Academic Press, New York, 1978).
(Nt)[at/cm2] = N[at/cm3] * t[cm]
ener
gy lo
ss p
er c
m lo
g(dE
/dx)
1 keV 1 MeV (log E)
1 keV 1 MeV
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Elastic Two-Body Collision
22 22 sin cost i i
i t
MMMKM M
θ θ +−= +
E1 = KEo M1<M2, 0 ≤θ≤180o
RBS: He backscatters from M2>4
0 ≤Φ≤90o
0 50 100 150 2000.0
0.2
0.4
0.6
0.8
1.0
θ = 150oHe4
Ki
nem
atic
fact
or: K
Target mass (amu)
M1vo2 = M1v1
2 + M2v22
M1vo = M1v1 + M2v2
Elastic Scattering
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Rutherford Scattering Cross Section
2 241 2 21
2( , ) sin ( ) 2( )
4 2RZ Z ZME ME E
θσ θ − ∝ − ∝
Coulomb interaction between the nuclei: exact expression -> quantitative method
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Electron Stopping
Figure after W.-K. Chu, J. W. Mayer, and M.-A. Nicolet, Backscattering Spectrometry (Academic Press, New York, 1978).
1 keV
ener
gy lo
ss p
er c
m lo
g(dE
/dx)
1 MeV (log E)
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RBS – Simulated Spectra
Element (Z,M): O(8,16), Al(13,27), Ti(22,48), In(49,115), Au(79,197)
hypothetical alloy Au0.2In0.2Ti0.2Al0.2O0.2/C
Au
In
Ti Al O
C 10 ML 100 ML Au
In
Ti Al O C
1200 8000
Au
In
Ti Al O C
1000 ML 16000
22( , )R
ZEE
σ θ ∝
0 50 100 150 2000.0
0.2
0.4
0.6
0.8
1.0
θ = 150oHe4
Kine
mat
ic fa
ctor
: K
Target mass (amu)
4000 ML 10000 ML 20000 50000
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Calibration Sample
Au
Cu Si
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Cu-Nb-W Alloy on SiO2/Si
Cu
Nb W
Si
O
Courtesy N. Vo and R.S. Averback
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SIMNRA Simulation Program for RBS and ERD
27.5% Hf
1.8% Zr 13% Al N
O
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Thickness Effects
Series 0Series 1
Channel700650600550500450400350300250200150100500
Cou
nts
11,500
11,000
10,500
10,000
9,500
9,000
8,500
8,000
7,500
7,000
6,500
6,000
5,500
5,000
4,500
4,000
3,500
3,000
2,500
2,000
1,500
1,000
500
0
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
Energy [keV]
N surface
300 nm
Ti surface Ti interface
N, O, Mg interface
Series 0Simulated
Channel700650600550500450400350300250200150100500
Cou
nts
14,000
13,000
12,000
11,000
10,000
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
Energy [keV]
Ti surface
400 nm
Ti interface
N, O, Mg interface
N surface
Series 0Simulated
Channel700650600550500450400350300250200150100500
Cou
nts
14,000
13,000
12,000
11,000
10,000
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
Energy [keV]
600 nm
Ti surface
Ti interface
N, O, Mg interface
N surface
TiN/MgO
N
He
DScattered
1515
Incident
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Incident Angle Effects
TiN/MgO
Series 0Simulated
Channel700650600550500450400350300250200150100500
Cou
nts
14,000
13,000
12,000
11,000
10,000
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
Energy [keV]
Ti surface 400 nm
Ti interface
N, O, Mg interface
N surface
Series 0Simulated
Channel700650600550500450400350300250200150100500
Cou
nts
8,500
8,000
7,500
7,000
6,500
6,000
5,500
5,000
4,500
4,000
3,500
3,000
2,500
2,000
1,500
1,000
500
0
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
Energy [keV]
400 nm
N surface
Ti surface
Ti interface
N, O, Mg interface
22.5
52
N
N
He
Scattered
1515
Incident
Surface peaks do not change position with incident angle
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Example: Average Composition
I. Petrov, P. Losbichler, J. E. Greene, W.-D. Münz, T. Hurkmans, and T. Trinh, Thin Solid Films, 302 179 (1997)
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RBS: Oxidation Behavior
TiN/SiO2
Annealed in atmosphere for 12 min at Ta = 600 °C
As-deposited
Experimental spectra and simulated spectra by RUMP
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RBS Summary
N
He
DScattered
1515
Incident
• Quantitative technique for elemental composition • Requires flat samples; beam size Φ1-3 mm • Non-destructive • Detection limit varies from 0.1 to 10-6, depending on Z
•optimum for heavy elements in/on light matrix, e.g. Ta/Si, Au/C… • Depth information from monolayers to 1 µm
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