Rutherford Backscattering Spectrometry (RBS)
An alpha particle, typically 2-3 MeV, impinges on a sample and is backscattered into a nearby solid state detector as illustrated in the figure above. In this example we have a thin layer of GaAs on a Si substrate. The energy of a backscattered particle determines the mass of the target atom as seen in the spectra below. Particles scattered from below the surface lose energy at a measurable rate; hence the energy scale yields the depth of the scattering of the particle. That is to say that the peak width of the Ga or As signal is proportional to the areal density or thickness of the GaAs film.
Ion Channeling steers energetic ions by a series of gentle, small angle collisions through rows or planes of atoms in a single crystal. An aligned single crystal will experience a dramatic reduction of surface scattering events and display a reduction of spectral yield as seen in the figure above.This will provide the following information:
(1) Amount and depth distribution of lattice disorder.
(2) Location of impurity atoms in lattice site.
(3) Thickness of amorphous or defective epitaxial surface layers.
The Advantages of RBS:
RBS provides composition and depth information. Routine depth resolution is on the order of 150 Å but with very careful sample and detector alignment, 50 Å at the sample surface may be resolved.
The technique is very useful in the analysis of thin films Measurements are performed routinely on thicknesses ranging from microns down to a fraction of a monolayer.
Typical analysis times are 10 minutes or less.
RBS is very sensitive to heavy elements so accurate determination of coverages of a fraction of a monolayer are possible. It is less sensitive to light elements, however this makes it an ideal compliment to PIXE where light element x-rays are absorbed in the detector window.
The RBS spectrum is easy to interpret. Unlike x-ray based spectroscopies, RBS offers a simple and direct conversion of a spectrum into useful data. Simulation and analysis software is available to optimize measurement parameters or to analyze spectra.