Rutherford Backscattering Spectrometry (RBS)

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RBS is an ion scattering technique that is used for RBS is an ion scattering technique that is used for the surface layer analysis of solids.the surface layer analysis of solids.

A target is bombarded with ions at an energy in A target is bombarded with ions at an energy in the MeV-range (0.5 – 4 MeV)the MeV-range (0.5 – 4 MeV)

The energy of the backscattered projectiles is The energy of the backscattered projectiles is recorded with an energy sensitive detector, recorded with an energy sensitive detector, typically a solid state detector.typically a solid state detector.

RBS allows the quantitative determination of the RBS allows the quantitative determination of the composition of a material and depth profiling of composition of a material and depth profiling of individual elements.individual elements.

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RBS is:RBS is:• quantitative without the need for reference quantitative without the need for reference

samples, samples, • nondestructive, nondestructive, • has a good depth resolution of the order of has a good depth resolution of the order of

several nm, several nm, • and a very good sensitivity for heavy elements and a very good sensitivity for heavy elements

of the order of parts-per-million (ppm).of the order of parts-per-million (ppm).

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Chamber for RBS MethodChamber for RBS Method

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If a particle of mass MIf a particle of mass M11 is scattered in electrical field is scattered in electrical field of nuclear of mass Mof nuclear of mass M22 at angle at angle θθ, its energy is K-, its energy is K-part of initial energy Epart of initial energy E00. Applying the principle of . Applying the principle of conservation of energy and momentum, one can conservation of energy and momentum, one can obtain next equation for kinematic factor K:obtain next equation for kinematic factor K:

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20 40 60 80 100 120 140 160 180 200 2200,0

0,2

0,4

0,6

0,8

1,0K

inem

atic

fact

or

K

Target mass M2

4He

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The yield of scattered particle is calculated using The yield of scattered particle is calculated using the differential scattering cross section which is the differential scattering cross section which is given by Rutherford’s formula:given by Rutherford’s formula:

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If the ion is scattered at If the ion is scattered at the depth X, it will the depth X, it will losses energy as well losses energy as well along the inward path along the inward path ΔΔEEinin as outward path as outward path

ΔΔEEoutout : :

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Energy loss depends on the atomic number Energy loss depends on the atomic number both of incident ion and target atoms, their both of incident ion and target atoms, their atomic density N and on energy of the incident atomic density N and on energy of the incident ion. For the calculations of RBS spectra, ion. For the calculations of RBS spectra, usually is used such named stopping cross usually is used such named stopping cross section section εε::

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the thickness of layers can calculate according to the energy the thickness of layers can calculate according to the energy width of the channel width of the channel δEδE in the multichannel analyzer. The number in the multichannel analyzer. The number of particles getting to the channel from of particles getting to the channel from QQ incident particles is incident particles is ::

where ΔΩ – solid angle of the detector.

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Element Ti Nb

Minimum Thickness, nm 30 10

Maximum Thickness, nm 1800 1500

Real Thickness, nm 264 169.5

Sensitivity, at/cm2 2x1016 1x1016

Possibilities of RBS MethodPossibilities of RBS Method

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