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Rutherford Backscattering Spectrometry Tool

Photo of the spectrometer in a lab.

Rutherford backscattering spectrometer that is one of the Stand-alone Measurements and Characterization tools in the Process Development and Integration Laboratory.

This page provides details on the Rutherford backscattering spectrometry (RBS) tool in the Stand-Alone Measurements and Characterization bays of the Process Development and Integration Laboratory (PDIL).

RBS is a well-established technique in thin-film characterization where a beam of high-energy (1–3-MeV) helium ions is directed at a sample. The helium ions are detected that are scattered elastically by nuclei in the sample. The higher the mass of an atom hit by a helium (He) ion, the higher the energy will be of the backscattered ion. This mass discrimination thus provides information on the sample's composition: the number of helium ions are counted as a function of energy, which allows one to determine the number of atoms of each element present.

RBS provides depth, as well as mass, information. Ions lose energy as they travel through the sample. This energy loss introduces a depth scale in the RBS spectrum. An ion scattered at a certain depth experiences an additional energy loss on the way in and out of the target. Thus, its kinetic energy is lower than that of an ion that experiences similar backscattering at the sample surface.

Applications:

  • Performing depth analysis of thin films. RBS can obtain information from the surface down to a depth of about 1–2 µm without sputtering. RBS is thus well suited for thin-film research including:
    • Surface and bulk contamination
    • Interface mixing and reaction
    • Diffusion profiles.
  • Analyzing composition quantitatively for films, multilayers, and bulk material. RBS yields the amount of atoms present quantitatively without needing any calibration standard.

  • Determining hydrogen content. Hydrogen (H) content can be quantified through Elastic Recoil Detection (ERD), which measures the energy of the H ions that are scattered forward after being hit by a He ion. Deuterium content and ratios can also be determined by this same method.

  • Studying crystallinity. When the incoming beam is aligned with a major crystallographic direction of a single-crystal target, the backscattered yield decreases drastically because the incoming ions are guided into the plane (channel), which decreases the probability of direct collisions. This phenomenon is called channeling, and measurements from RBS channeling can help to characterize:
    • Epitaxial growth
    • Lattice location of dopants and contaminants (defects) in single crystals
    • Improve resolution of low-energy elements.

Special features:

  • Sample type: thin films and bulk solids
  • Sample size: maximum is 50 mm x 50 mm, minimum is 5 mm x 5 mm. Capability of measuring full PDIL spec samples (157 mm x 157 mm) will be available.
  • Depth information: 2 µm (RBS, channeling), 0.2 µm (ERD)
  • Lateral resolution: 1 mm x 1 mm
  • Depth resolution: 1–10 nm (RBS, channeling), 5–20 nm (ERD)
  • Mass resolution: 1 atomic mass unit for Z < 40
  • Detection limit:
    • H and elements heavier than B
    • Dependent on substrate and element
    • Surface sensitivity for
      • N on top of Si: 1015 atoms/cm2
      • Ar on top of Si: 1011 atoms/cm2
      • Au on top of Si: 1010 atoms/cm2
      • H in bulk: 0.1 atom%.

Contact John Perkins for more details.