Basics of Ion Beam Analysis

Srdjan Petrović

Laboratory of Physics, Vinča Institute of Nuclear Sciences, University of Belgrade, Serbia

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INTRODUCTION

• Ion Beam Analysis (IBA) of the target material is based on the information obtained from the ion beam – target interaction.

• In general, IBA provides the depth profiling of the material - the material atoms concentration dependence on the target depth and/or the elemental analysis - material atoms composition (stoichiometry).

• Rutherford Backscattering Spectrometry (RBS) - depth profiling and elemental analysis

• Elastic Backscattering Spectrometry (EBS) – depth profiling and elemental analysis

• Elastic Recoil Detection Analysis (ERDA) – depth profiling and elemental analysis

• Nuclear Reaction Analysis (NRA) – depth profiling and/or elemental analysis

• Particle-Induced X-Ray Emission Analysis (PIXE) – elemental analysis• Particle-Induced -Ray Emission Analysis (PIGE) – elemental analysis

and/or depth profiling

• Ion Channeling – RBS/C, EBS/C, ERDA/C and impurity/crystal characterization

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Rutherford backscattering spectrometry (RBS)

• Rutherford scattering – scattering by the pure Coulomb interaction:

, =

Schematic figure of the Rutherford experiment3/32

Rutherford Backscattering Spectrometry (RBS)

Diagrammatic view of the internal configuration of the alpha-scattering sensor head deployed on the surface of the moon, Surveyor V, September 9, 1967 (from Turkevich et al. (1968). This experiment was the first widely publicized application of the Rutherford scattering introduced some 50 years earlier. 4/32

Experimental set up for RBS

Schematic diagram of a typical backscattering spectrometry in use today 5/32

Rutherford Backscattering Spectrometry (RBS)

• RBS is an analytical method, which provides depth profiling and

stoichiometry

• It is suitable for heavier elements in lighter matrix material

• Typical depth profiling accuracy is 10 – 30 nm

• Detection limit range from about a few parts per million for heavy elements

to a few percent for light elements

• RBS is nondestructive method, which is insensitive to the sample chemical

bonding

• From the practical point of view, it is quick and easy experiment, with data

acquisition times of a few tens of minutes

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Kinematics of elastic particle collisions

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• x = - mass ratio

• K = - projectile kinematic factor

• =

Backscattering geometry ()

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• Rutherford differential scattering cross section (the center of mass frame):

=

• Rutherford differential cross section (the laboratory frame):

= ,

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Energy loss and depth scale (depth profiling) - single element target

• - the energy of the incident particle

• - the energy loss of particle from the surface at thickness t

• - the energy loss of particle after the collision from the thickness t to the surface

• - energy of the incident particle just prior to the collision

• - energy of particle measured at the detector

= K - - the dependence of the measured energy on the depth!10/32

RBS spectrum – homogeneous thick one element sample

• - the scattering cross section at the measured angle • – the detector solid angle• - the measured number of incident particles• N – atomic density• the number of target atoms per unit area in the layer thick

Backscattering spectrum for 1.4 MeV He ions incident on thick Au sample

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Compound target – thin film

(A) = -

(B) = -

• Surface area approximation

, ) , )

• Stoichiometry (thin film)

=

- the area of compound A

- the area of compound B

• Depth profiling

A schematic representation of the backscattering process from a free-standing compound film with composition and thickness t (a) Energy loss relation, and (b) predicted backscattering spectrum. 12/32

Energy loss in compounds – Bragg’s rule

• , - stopping power

• - stopping cross section

• Bragg’s rule

= m + n

Stopping cross sections for ions on , , and . The stopping cross section was determined on the molecular basic with 2.3 .

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Compound target – thin film + thick substrate

- kinematic factor of Ni

(0.7624, )

- kinematic factor of Si

(0.5657, )

- the surface height of Ni

- the surface height of Si

=

Stoichiometry from the surface height

Scematic backscattering spectra for MeV ions incident on 100 nm Ni film on Si (a) Before reaction, and (b) after reaction to form 14/32

Examples

RBS data at 2 MeV He ions from two reference film standards that were used to measure the relative cross section of Cu, Y, and O relative to to Ba as a function of He energy

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Examples

The 1.9 MeV He backscattering spectrum of a three-layered film on a carbon substrate. The backscattering signals from the three layers are clearly separated. The Ni and Fe peaks are not resolved.

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Examples

The 1.9 MeV backscattering spectrum of a ceramic glass. The indicated stoichiometry was determined from the step heights.

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Examples

RBS spectrum from the optical glass filter consisting of layers of layers interspersed with . Non-uniformity is clearly visible on the height of the peaks ( content).

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Examples

Energy spectrum of 2 MeV He ions backscattered from a silicon wafer implanted with 250 keV As ions to a nominal fluence of 1.2 ions/. The vertical arrows indicate the energies of He backscattered from the surface atoms of and . 19/32

Non- Rutherford backscattering – homogeneous thick one element sample (a clear need for the use of a special

computational programs, e.g., SIMNRA)

Backscattering spectrum of 1 MeV protons in the random orientation (scattering angle of 170 degre) from a thick diamond (just recently obtained results from the experiment performed in the Rudjer Bošković Institute, Zagreb, Croatia). 20/32

Elastic Recoil Detection Analysis (ERDA)

• ERDA is an analytical method, which provides depth profiling and

stoichiometry

• It is complementary with RBS and suitable for lighter elements in heavier

matrix material

• Depending on the way how the recoil ion(s) are detected, there are for

example: a conventional range-foil ERDA, Time of Flight (TOF) ERDA and

E-E ERDA

• Depth profiling accuracy and detection limit range depend on the way recoil

ion(s) are measured.

• ERDA is nondestructive method, which is insensitive to the sample

chemical bonding

• From the practical point of view, it is quick and easy experiment, with data

acquisition times of a few tens of minutes

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Kinematics of recoil elastic particle collisions

Recoil kinematic factor:

=

22/32Dependence of the kinematic on the recoil angle and mass ratio.

Recoil cross section:

Range-foil ERDA

Schematic presentation of the standard range-foil ERDA

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• Converting a measured energy spectrum, i.e. counts versus recoil

energy into a desired depth profile requires a lot of analytical efforts

that in many cases cannot produce an accurate result.

• In the computational programs, like SIMNRA, one can include the

reflection geometry (ERDA) and a foil, as well as, the energy loss

straggling, multiple scattering energy spread due to specific ERDA

geometry and the non-Rutherford cross sections. Practically, the

only computational programs are used for the ERDA depth

profiling.

• Sensitivity of a standard foil ERDA for hydrogen is around 0.1%, for

1 – 3 MeV He projectile beams.

Ideal versus realistic recoil energy spectrum

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Energy spectra of recoils observed when targets containing are bombarded by a 35-MeV beam. Each target was made of two thin layers of separated by a copper layer of thickness ; corresponds to the first layer and to the second layer. In (a) and (b) the target surface was perpendicular to the beam direction; in (c) it was tilted at with the beam direction.

Examples of standard range-foil ERDA – thin layers

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The ERDA spectrum of by using the Kapton polyimide foil []. It was collected with 1.3 MeV beam under the measurement geometry of the target tilt, = , and the detector tilt, = .

Examples of standard range-foil ERDA – homogenous thick target

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Example of a transmission ERDA experiment

Schematic of a transmission ERDA experiment for hydrogen profiling with an helium beam, with the zero detection angle and a target thick enough to stop completely incident particles.

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Typical range-foil ERDA hydrogen profiling with an helium beam in the case of the hydrogen implantation a target. (The thin peak is due to hydrogen adsorbed on the surface.)

Example of a range-foil ERDA implanted depth profiling of hydrogen

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Time-of-flight ERDA (TOF-ERDA)

• Simultaneous measurements of both the velocity (via time-

of-flight) and energy of recoiled ions.

• Energy measurement – standard solid state detector.

• TOF measurement – a telescope with the start (T1) and stop

(T2) detectors (two ultra thin carbon foils producing

secondary electrons, not disturbing the ion path).

Example of the TOF-ERDA experimental set-up.

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The TOF-ERDA coincidence spectrum for a polyimide sample () measured with a 84 MeV beam. Each recorded data point corresponds to the measured energy, E (abscissa) and the delayed flight time, (ordinate).

Example

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E–E ERDA (solid-state telescope)

• Simultaneous measurements of both the energy loss and

energy of recoiled ions with a E–E telescope.

• Energy loss measurement – very thin solid state detector

• Energy measurement – standard solid state detector

Schematic view of a solid-state telescopeVariation of energy loss in a 10 m thick solid state detector for protons, deuterons and alpha particles

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Example

(a) Three-dimensional plot of hydrogen, deuterium, and tritium distributions in a titanium hydride sample bombarded with 4 MeV He ions. (b) Two-dimensional plot in the interval 470-510 keV.