Rutherford Back Scattering
Rutherford Back-scattering Spectrometry (RBS) is a powerful tool for determining the structure and elemental composition of thin film samples: a high-energy beam of helium or hydrogen ions is directed at a sample. Ions that scatter elastically from the nuclei of atoms in the sample are detected using a silicon particle detector. The energy of the scattered ion depends on the mass and depth of the target atom, so that the composition and depth of the sample atoms can be determined. The Rutgers General Ionex tandem accelerator is capable of producing He++ ions up to 5.1 MeV via a multi-stage acceleration process.
A 2 MeV He++ RBS spectrum from an Al2O3 sample with Au and Ni thin films deposited on the surface. The data has been fitted using a SIMNRA simulation in order to determine the thickness of the different films and the order in which they were deposited on the surface. The He++ ions that scatter from Au retain most of their kinetic energy due to the relatively high mass of the Au nucleus while those ions that scatter from the Ni, Al, and O nuclei lose more energy and hence reach the detector with a smaller kinetic energy.
​
Variations of RBS include Elastic Recoil Detection Analysis (ERDA) for quantitative determination of hydrogen and nuclear reaction analysis (NRA) for sensitive determination of light elements from Li-Ne.
Summary
Ideal uses:
-
Measure thin Film composition/thickness/stoichiometric ratio
-
Determine areal concentrations (atoms/cm²)
-
Quantify film density (when thickness is known)
-
Analyze hydrogen content (ERDA)
-
Determine near surface crystallinity
Technical specifications:
-
Signal Detected: Backscattered H or He atoms
-
Elements Detected: H-U
-
Detection limits: 0.001-10 at%
-
Depth resolution: 100 – 200Å
-
Lateral resolution/probe size: 2mm
Strengths and limitations:
-
Non-destructive compositional analysis
-
Quantitative without standards
-
Analysis of large samples
-
Conductor and insulator analysis
-
Useful information limited to top ~ 1µm of samples