RBS - Rutherford backscattering spectroscopy

The RBS method relies on interpretation of the energy spectrum of the projectiles backscattered from the sample. Due to the collision dynamics the energy loss of the scattered projectile depends on the type of the scattering center (the projectile ion scatters on the Coulomb potential of the atoms in the sample) and on the scattering angle. The projectiles are observed at large scattering angles. The energy of the ion scattered for 180° is E=E0.[(MT-MP)/(MT+MP)]2, where E0 is the initial ion energy, and MP, MT is atomic mass of the projectile and of the target atom, respectively.

The RBS spectrum contains information about the depth concentration of the scattering centers, that is about the depth profiles of the different elements in the sample. When the ion travels through the sample it continuously losses its energy by the collisions with electrons but only slightly changes the direction. At certain depth it suffers a close encounter by some atomic nucleus which can drastically change its direction. On the way back from the sample the projectile again transfers some energy to the target electrons. By fitting the model RBS spectrum with parametrized depth distributions of elements to the experimental spectrum, one can determine the depth profiles of these elements in the sample.

With RBS method one can measure depth profiles of elements heavier than the projectile ion. Better resolution is achieved when the mass difference is not large. The equipment at MIC allows to measure depth profiles up to several hundred nm (max. 1 µm) with 20 nm resolution. The energy of the projectiles is measured by the semiconductor detector. The detector energy resolution determines the depth profile resolution. With the recently developed method of Time-Of-Flight spectrometry the depth profile resolution was further improved.

The smallest relative concentration of the elemental component, which can be detected by RBS method depends on the sample composition. The limit concentration of the heavy element in the sample composed mainly of light elements can reach about 0.1%, while the typical sensitivity of the method is about 0.5%. In figure there is RBS spectrum of thin layer of TiWO3 recorded by using helium ions as the projectiles.

RBS

RBS method is used on our PIXE/RBS, ERDA/RBS and external beam beamline.



Last updated: 01/22/2014