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Rutherford Backscattering Spectrometry

Timothy P. Spila, Ph.D.

Frederick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign


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.



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.


Van de Graaff accelerator

http://archive.thedailystar.net/newDesign/print_news.php?nid=73473

http://cnx.org



2 MeV Van de Graaff accelerator

beam size Φ1-3 mm flat sample can be rotated


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


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


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


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)


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


Calibration Sample

Au

Cu Si


Cu-Nb-W Alloy on SiO2/Si

Cu

Nb W

Si

O

Courtesy N. Vo and R.S. Averback


SIMNRA Simulation Program for RBS and ERD

27.5% Hf

1.8% Zr 13% Al N

O


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


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


Example: Average Composition

I. Petrov, P. Losbichler, J. E. Greene, W.-D. Münz, T. Hurkmans, and T. Trinh, Thin Solid Films, 302 179 (1997)


RBS: Oxidation Behavior

TiN/SiO2

Annealed in atmosphere for 12 min at Ta = 600 °C

As-deposited

Experimental spectra and simulated spectra by RUMP


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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