Rutherford Back Scattering Spectrometry (RBS)
| Rutherford backscattering spectrometry (RBS) is the most widely used technique for elemental identification and composition analysis. RBS as a method for materials analysis was firstly described in 1957 by Rubin et al.4 In this technique, a low mass (generally He) and highly energetic ion is bombarded on a material and get scattered back from the target atoms. It relies on the fact that the energy of an elastically backscattered particle depends on the mass of the target atom (kinematic factor) and on the depth at which the scattering took place (energy loss on the way to and from the point of interaction). This allows profiling the elemental composition of the sample close to the surface. The schematic of RBS geometry is shown in Fig. m5.3. | ||
/m5.3.png) | ||
FIGURE m5.3 Schematic of RBS set up | ||
| The detection of backscattered particles can be done with simple silicon surface barrier detectors. In principle all elements from Be to U can be detected, though the sensitivity depends largely on the combination of elements and the sequence of layers in the target. RBS is suited best for the detection of heavy elements on light substrates. | ||
| Let us now look at the formulation used for RBS and the knowledge of parameters required in RBS: | ||
| (i) The kinematic factor: | ||
| The kinematics of RBS is a simple two body classical collision process as discussed in Lecture 2 of MODULE II [Please look at Fig. m2.4]. | ||
| The kinematic factor as defined in equation m2.9 is reproduced below: | ||
/equ5_7.png) | ||
| where E1 is the energy of backscattered atom and E0 is the energy of the incident atom/ion and the ration E1/E0 is known as kinematic factor, k. The backscattered angle is α is shown in Fig. m2.4. The k value plays an important role in designing a RBS experiment. In Fig. m5.4 kinematic factor for various projectiles and for a range of masses is shown. | ||
/m5.4.png) | ||
Figure m5.4 The value of k (at α = 1800) for various projectiles from various mass number is shown | ||
| (ii) Depth scale:5 | ||
| Depth scale in RBS basically gives an idea from which part (from surface to probing depth) of the sample the information comes to the detector. The backscattered species so recorded in the detector give the depth profile of a particular element present in any sample. The signal from an atom at the sample surface will appear in the energy spectrum at a position E1 = k•E0. The signal from atoms of the same mass below the sample surface will be shifted by the amount of energy lost while the projectiles pass through the sample, both before (ΔEin) and after a collision (ΔEout). Figure m5.5 and the associated equations are self explanatory to understand the energy detected from any detector from any depth scale. | ||
/m5.5.png) | ||
Figure m5.5 Energy loss from the surface and from the depth of a sample gives the energy detected at the detector. | ||
| Let us now see necessary equations associated with this figure and deduce the energy of the backscattered He from a depth Δx detected at the detector (Edet). | ||
| The energy loss (ΔEin) of the incoming He is | ||
/equ5_8.png){kind=small} | ||
| Energy of He at a depth Δx is | ||
/equ5_9.png){kind=small} | ||
| The energy of He after backscattering from an atom located at a depth Δx is kEs. | ||
| Finally the energy of the backscattered particles after traversing back to the outward direction and reaching to the detector i. e. Edet is . | ||
/equ5_10.png){kind=small} | ||