Rutherford Backscattering Spectrometry

Rutherford Backscattering Spectrometry (RBS ), German Rutherford backscattering spectrometry is a method for the investigation of near-surface thin layers by means of ion beams. It is therefore closely related to other methods of ion scattering spectroscopy, such as the low-energy and medium -energy ion scattering spectroscopy ( LEIS and MEIS ).

The method is named after Ernest Rutherford, who was able to explain the observed first in the so-called gold foil experiment backscattering of alpha particles and developed his model of the atom.

Operation

For a measurement to shoot high-energy ions (0.1 to 4 MeV ) low-mass (hydrogen or helium) to a sample. A detector measures the energy of the backscattered ions. Their energy depends on the initial amount of energy, the mass of the atom samples taken in each case and the angle at which from is detected. The ratio of the energy of the backscattered ions ( ) to the incident ion beam ( ) is referred to as k- factor. The k- factor depends on the backscattering angle () and the ratio of the projectile mass ( ) for sample atomic mass ( ). General results from momentum and energy conservation for the k- factor:

Typically, the detector is positioned at an angle close to 180 °, since the loss of energy in the scattering becomes maximum and thus, the mass resolution is higher.

Furthermore, the ions lose their way through the sample continuously energy. The energy of the detected particle after scattering arises therefore to:

This corresponds to the term in the square brackets of the incident energy minus the energy loss until reaching the depth in the scattered, multiplied by the k- factor, the energy after scattering. From this then the energy loss is subtracted while exiting the sample. In this case, and the distances to the surface of which depend on the specific experiment. The so-called stopping power results from the Bethe formula and depends on the energy of the particle and the sample composition.

Because of this depth dependence of the energy of backscattered particles, the measurement of the composition by RBS is also deeply dissolved. Depending on the test parameters can be studied depths of a few hundred nanometers to a few micrometers. This deep penetration is possible because of the scattering cross section for high-energy ions ( several MeV) is very much smaller than in the low energy ion spectrometry (e.g., Leis, energies up to 10 keV, only suitable for surface analysis), and the stopping power typically at more than 100 eV / nm is located ( for bombardment with He ions). Accordingly, the analyzable depth is mainly dependent of. The higher the higher the analyzable depth, the lower but also the depth resolution. For greater depths, the depth resolution is lowered even further by the so-called straggling ( broadening of the energy distribution of the ion beam as it passes through the sample), which results from the random nature of the energy loss in passing through the sample.

For not too high energies, the scattering potential ( nearly ) corresponds to the Coulomb potential and thus the likelihood of a back scattering ( scattering cross-section ) is well known and is given by the Rutherford scattering cross section. For higher energies there may be deviations from the Rutherford scattering cross -section.

The composition of a binary alloy can be determined with the following formula:

With the measured backscattering yield of the respective element and as the differential cross section.