RBS measures, without destroying the sample, the thickness and composition of a thin layer (metal, oxide, semiconductor) over the first two micrometres of its surface, useful for example to check a deposit, a doping profile, or an oxidation.
Rutherford Backscattering Spectrometry (RBS) relies on the elastic Coulomb scattering of incident ions (generally ⁴He⁺ alpha particles with an energy of 1-2 MeV) by the atomic nuclei of the sample being analysed.
Physical principle
When an alpha particle collides with a target nucleus, it is backscattered with a characteristic energy that depends on:
- The depth at which the collision takes place (energy loss in matter)
- The mass of the target atom (kinematic factor K)
Technical characteristics
- Incident beam: ⁴He⁺ or ¹H⁺ (1-2 MeV)
- Detector: silicon surface-barrier detector
- Detection angle: 165° (backscattering)
- Sensitivity: ~10¹³ at/cm² (heavy elements)
- Depth resolution: 10-20 nm (surface)
- Analysed depth: up to ~2 µm (⁴He⁺)
Advantages
- Absolute quantitative technique
- Non-destructive
- Sensitive to medium and heavy elements (Z > 6)
- Depth concentration profiles
- Thin-film thickness measurement
- Fast and reproducible
Limitations
- Low sensitivity to light elements (H, Li, Be…)
- Limited mass resolution for neighbouring elements
- Depth resolution degrades with depth
Typical applications on SAFIR
- Metallic thin films (TiN, TiAlN, CrN…)
- Oxide analysis (Al₂O₃, HfO₂, ZrO₂…)
- Interfacial diffusion in multilayers
- Doping control in semiconductors
- Corrosion and oxidation of surfaces
- Materials for microelectronics
Complementarity with other techniques
RBS is often coupled with ERDA for hydrogen detection, with NRA for light elements (C, N, O), and with ion channelling for crystallographic analysis.
Reference publication
S. Rigo, C. Cohen, A. L’Hoir, E. Backelandt, “Precision absolute thin film standard reference targets for Rutherford backscattering microanalysis”, Nuclear Instruments and Methods, vol. 149, pp. 721–726, 1978. DOI: 10.1016/0029-554X(78)90958-8