The Relation between Rutherford Backscattering and Low Energy Ion Scattering
Rutherford Backscattering Spectrometry (RBS analysis) is a well-known technique for the element analysis of solids. In the analysis, a beam of high energy light ions (typically 1-3 MeV He-ions) is aimed at the sample. These fly into the material and, at some point, collide with an atom in the sample, after which they bounce back and fly out of the material again. In the collision, these He-particles have lost a characteristic amount of energy, depending on the mass of the sample atom that they collided with. This energy loss is, just like in macroscopic collisions between e. g. billiard balls, determined by the laws of conservation of momentum and energy. In addition to the energy loss by the collision, they are slowed down on their paths into and out of the sample. After they leave the sample, the particles are analyzed for their energy and the number of detected particles at each energy is plotted as a function of their energy in an RBS spectrum.
A Comparison of LEIS and RBS
If the sample consists of a layer of heavy atoms on top of a light substrate, part of the spectrum will have the block shape of figure 1. The ions that scattered from atoms at the surface will not have traveled through the sample and they show up on the high energy side of the signal. The rest of the block, that extends to lower energy, is caused by ions that scattered from atoms in this layer below the surface. The low energy edge of the block represents the atoms that are in the bottom of the layer and, therefore, the width of the block is a measure for the thickness of the layer.
If the layer of heavy atoms would be covered by a second layer, this entire block shaped signal in the Rutherford Backscattering spectrum moves to lower energy, with the energy difference corresponding to the thickness of the layer above it.
Ion scattering that is not Rutherford Backscattering Spectrometry
Because of the use of high energy ions, Rutherford Backscattering Spectrometry is sometimes called HEIS: High Energy Ion Scattering. There are more ion scattering techniques than Rutherford Backscattering Spectrometry: Two of those are Medium Energy Ion Scattering (MEIS) and Low Energy Ion Scattering (LEIS). The LEIS technique is commercially available as the instrument fits in any normal laboratory.
There are two critical differences between LEIS and HEIS (RBS analysis). The first difference is the energy of the ions that are used. LEIS uses 1-8 keV ions, an energy that is 3 orders of magnitude lower energy than that used in HEIS. The spectrum encompasses a much smaller energy scale, which, in turn, translates to a smaller depth scale (< 10 nm), but also in a much better depth resolution (< 1 Å).
The second difference lies in the type of particles that are detected. In Rutherford Backscattering Spectrometry all He particles are detected, irrespective of their charge. This is important since the ions are neutralized when they enter the sample. If in RBS analysis only ions would be detected, there would hardly be any signal.
In LEIS analysis only ions are detected. This means that the primary signal in a LEIS measurement is due to particles that never entered the sample (otherwise they would be neutralized): they scattered from the outermost layer of atoms on the sample. But there also is a secondary signal. This stems from He particles that have traveled through the sample (as in Rutherford Backscattering Spectrometry) and are re-ionized as they leave the sample. The primary signal in LEIS shows up as a narrow peak, whereas the secondary “in depth” signal appears as a block, like in Rutherford Backscattering Spectrometry.
An example of a Low Energy Ion Scattering measurement
Figure 3 shows an actual LEIS spectrum of a sample consisting of alternating W/Si layers that was measured in our lab. Since the sample has been in contact with air in the laboratory, hydrocarbons from the atmosphere have adsorbed onto the surface.
The spectrum has peaks for C, O, and Si. There is no signal for W. This measurement shows that C, O, and Si occur in the outermost atomic layer of the sample and W does not. The majority of atoms at the surface, however, is H (from hydrocarbons). This element can not be detected in the measurement, since it is lighter than He and He ions scatter forward from H instead of backward. As a result, the other signals are weak. The ability to specifically analyze the outermost atomic layer is an advantage of Low Energy Ion Scattering over Rutherford Backscattering Spectrometry.
The similarity with Rutherford Backscattering Spectrometry can be seen in the two steps in the background: one at around 2200 eV and one at 1600 eV. They represent the top of the first and second W layers, respectively, in a way that is similar to Rutherford Backscattering Spectrometry.
When is LEIS the "better" RBS?
It is clear that Low Energy Ion Scattering and Rutherford Backscattering Spectrometry have a lot in common. Both analysis techniques determine the mass of sample atoms by colliding light particles with the sample atoms. The big difference lies in the depth scale and depth resolution. Whereas Rutherford Backscattering Spectrometry analyzes the sample up to a depth of a few hundred nm, Low Energy Ion Scattering analyzes at two separate depths in the same spectrum: The outermost one atomic layer, as well as the first few (<10) nm in the sample. The latter analysis can be performed with a remarkable depth resolution: Differences in average layer thickness of 1 Å can be detected. Since there is a drive for films to be thinner by the day, analysis with LEIS will grow in importance.
Are you interested in more details about LEIS or RBS analysis? Are you considering using one of the techniques in your laboratory? Then contact us and let one of our experts advise you.