Rutherford Backscattering Spectrometry

---

Method

Rutherford Backscattering Spectrometry (RBS) is a physical method for the for the determination of the elemental composition of thin films, especially for medium to heavy elements. An ion beam, typically He ions, is used to bombard the samples and the number and energy of ions scattered on the nucleus of sample atoms are detected.

Principles:

• Elastic collisions between ion and nucleus of sample atom:
  conservation of energy and impuls
  → Identification of the elements (via mass)

• Scattering cross-section determined by classical mechanical calculations
  → Absolute quantitative analysis

• Ions are slowed down in matter
  → Provides depth information

• The ion beam can be aligned with a crystal axis
  → Channeling measurements

Advantages

• Quantitative determination of concentrations without the need for (matrix-matched) standards
• Concentration depth profiles can be obtained with a depth resolution of 15-20 nm at the surface
• Especially sensitive for heavy elements
• Channeling measurements provide information on the quality of crystals, e.g. to assess crystal damage after implantation, or the location of dopants in the crystal

Limitations

• Not sensitive to light elements (an alternative for light elements is ERDA).
• Some damage can occur, e.g. creation of defects, amorphisation, elemental loss in polymers, but the sample is not destroyed.
• Standard measurements take place in vacuum with a millimeter size beam but measurements with a micrometer size beam is also possible (see Ion microbeam) or on fluids in a separate chamber (see below).
• The identification of two or more elements with similar Z is not very good, especially for heavy elements, because of limited mass resolution and/or overlap with depth in the spectra.

---

RBS setups at HZDR

At the HZDR, three RBS setups with different specialities are available that are described below.

RBS setup at the 2 MV Van de Graaff accelerator

A general purpose RBS setup is connected to this accelerator and a 1.7 MeV He⁺ beam is used as standard.
Measurement possibilities

• Simple RBS spectra (with fixed angles)
• RBS spectra in so-called random mode (angular scan for two angles)
  • "real" random spectra for crystalline samples or substrates (accidental channeling is excluded)
  • determination of the channeling direction (axis) in a crystalline sample
• RBS spectra in a channeling direction
• Additional PIXE spectra: An X-ray SDD detector can be used for better identification of elements in the sample or determination of concentration ratios for elements with close/similar Z.

Samples

Sample requirements

• Sample size: minimum 5x5 mm², maximum ~25 mm, optimum 10-15 mm
• Sample thickness: max. 7-8 mm, optimum ≤ 1 mm (total thickness for mounting)
• Low roughness and porosity

---

Applications

• Enhanced energy density in epitaxial SmCo₅/Fe/SmCo₅ trilayers

Aim: Permanent magnet films with enhanced energy densities are of great interest for energy applications as engines or generators in miniaturized devices. Multilayers allow for a better control over phase architecture and texture. SmCo₅/Fe/SmCo₅ is a promising trilayer for such devices and in this study the effects of the Fe layer thickness has been investigated on the properties of the film and emphasis is put on the detailed investigation of the multilayer architecture and thickness detemination.
Procedure: Films with Fe layer thicknesses from 0 to 21 nm were prepared by UHV pulsed laser deposition on a Cr buffer layer on a MgO substrate. A Cr capping layer has also been deposited. The stack architecture has been verified by RBS (and EDX).

A sample model of a trilayer with nominal 12 nm Fe and an putative FeCo interdiffusion zone reveals good agreement between experimental and simulated intensities for all elements. The formation of the FeCo interdiffusion layer was important in explaining the unexpectedly strong increase in magnetic remanence.

S. Sawatzki, R. Heller, Ch. Mickel, M. Seifert, L. Schultz, and V. Neu
Largely enhanced energy density in epitaxial SmCo₅/Fe/SmCo₅ exchange spring trilayers
Journal of Applied Physics 109, 123922 (2011)

---

In-situ liquid RBS setup at the 2 MV Van de Graaff accelerator

This setup is especially designed for measurements on fluids or to study the interface between fluids and a solid surface. The liquid under investigation is put into a liquid cell that is separated by a thin Si₃N₄ membrane from the vacuum part of the setup that houses the detector and through which the ion beam enters the cell. The membrane can be coated with a thin film to study different solid-liquid interfaces.

For more details see

Rev. Sci. Instrum. 90, 085107 (2019); doi: 10.1063/1.5100216

Samples

Samples are an integral part of the setup and the measurements require individual planning.

• The cell-membrane can be coated with thin layer to investigate the solid-liquid interaction for this material.
• The electrolyte has to be provided by the user and can be cycled.

For a discussion of the measurement and sample requirements, contact R. Heller

---

Applications

• Studying the formation dynamics of the electric double layer (EDL) of the liquid-solid interface

Aim: A so-called double layer of accumulated ions and orientated water molecules forms as soon as a solid is in close contact with a liquid. The formation dynamics and the properties of the EDL play a key role in many applications but many assumptions underlying the EDL model are still under debate. With the liquid-RBS setup the formation dynamics of the EDL could be investigated in real-time. A low molarity BaCl₂ solution and the thin Si₃N₄ window are used as a model system to study the formation of the EDL. The RBS results showed the dynamics of Ba adsorption on the window for several pH values.

Nasrin B. Khojasteh, Sabine Apelt, Ute Bergmann, Stefan Facsko, and René Heller
Revealing the formation dynamics of the electric double layer by means of in-situ Rutherford backscattering spectrometry
Rev. Sci. Instrum. 90, 085107 (2019); doi: 10.1063/1.5100216

In a further study there results there results were compared with electrochemical impedance spectroscopy (EIS). Both methods are complementary because the RBS results can be used to quantify the adsorption processes in the EDL, and the EIS spectra are helpful in identifying what components define the time behaviour of the interface.
Ute Bergmann, Sabine Apelt, Nasrin B. Khojasteh, René Heller
Solid–liquid interface analysis with in-situ Rutherford backscattering and electrochemical impedance spectroscopy
Surf Interface Anal. 2020; 52:1111–1116; DOI: 10.1002/sia.6835

---

Multi-detektor RBS setup at the 3 MV Tandetron

A multi-detektor setup has been installed at the 3 MV Tandetron for high sensitivity RBS measurements and channeling mapping experiments. The setup consists of 80 detectors, each with its own specially designed and compact electronics. The detectors are arranged in five rings and all detectors in each ring have the same scattering angle, so the spectra for each ring can be added together.

Sample requirements

Sample size: minimum: 5x5 mm², maximum: ~25 mm, optimum: 10-15 mm
Sample thickness: ≤ 1 mm (total thickness for mounting)
Low roughness and porosity

Applications

• Low or very low amounts of medium of heavy elements measured by RBS
• Low amounts of light elements measured by resonant RBS or resonant NRA (Nuclear Reaction Analysis), using resonances in the cross-sections for ion scattering or nuclear reactions, respectively.
• Channeling maps, which show intensity as a function of two incidence angles and give information about channeling planes from which the crystal structure can be deduced.