Backscattered ions

The three main components of an RBS instrument are a source of helium ions, an accelerator and a detector to measure the energies of the backscattered ions. 

Figure 4.15 Schematic representation of RBS. Top the incident ions are directed such that they either scatter back from surface atoms or channel deeply into the crystal. Middle the ions scatter back from target atoms throughout the outer micrometers and suffer inelastic losses, causing the energy of the backscattered ions to tail to zero. Bottom scattering from the heavy outer layer gives a sharp peak separated from the spectrum of the substrate as in the middle diagram.

As discussed above, the Si(001) surface is reconstructed into dimers, a side view of which is shown in Fig. 7. In addition to the reconstruction of the surface atoms there is significant distortion of the subsurface region. This distortion blocks channels which enter into the bulk, and causes an excess of backscattered ions over what would be expected from atomic rows in the ideal crystal. This local distortion is the property monitored by backscattered ion intensities. The dimer pairs also tend to form in rows, which is the source of the (2 X 1) LEED pattern observed for this surface. 

Rutherford scattering is an elastic event, that is, no excitation of either the projectile or target nuclei occurs. However, due to conservation of energy and momentum in the interaction, the kinetic energy of the backscattered ion is less than that of the incident ion. The relation between these energies is the kinematic factor, K, which is given by the expression... 

Rutherford Back-scattering Spectroscopy Elemental analysis, profiling He+ 2meV Backscattered ion He-U 10-1-10-4... 

It seems relevant to remind that this technique is based on recording the backscattered light (usually helium) ions occurred as a result of their interaction with the matter of a solid layered specimen (for more detail, see Ref. 124). Experimental data are presented as plots of intensity, /, against energy, E, of the beam of backscattered ions (Fig. 2.15). 

For the analysis of the new surface after every removal one may use all the surface techniques already mentioned in Sect. 4.3.1 as long as their information depth does not exceed the thickness of the layer removed Auger and ESCA-spectroscopy, secondary-ion mass spectrometry (SIMS), backscattering, ion-induced X-ray and nuclear reaction analysis. In addition, one may investigate the content of the element of interest in the removed layer. Because of the low absolute concentration of implanted ions most of the standard methods of analysis fail. The best results come from implantations of radioactive elements followed by measuring the radioactivity of the dissolved removed layer. 

RBS Rutherford Back Scattering Solids, thin films Mono-energetic Ions (H or He ) 0.6-3 MeV Backscattered ions 10 nm-1 pm 1 mm Element Identification (LI to U) detection limit 0.01-1% 46... 

FIGURE 21-13 Rutherford backscattering apparatus. An ion beam from an accelerator is focused on the sample. Backscattered ions are delected with a solid-state particle detector. 

LEIS measurements result in an energy spectrum of the backscattered ions from the sample surface. The energy of scattered ions provides information on the mass of the surface atoms, which is directly related to their chemical identity. The cross-section, solid angle and transmission factor (the analyser acts as an energy filter, with transmission characteristics that depend on the mode of operation) are used for these calculations. 

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