Features
RBS is a method in which beams of high-energy ions (\(\text{H}^{+}\), \(\text{He}^{+ +}\))impinge on solid samples
and are subsequently scattered by RBS. By measuring the kinetic energy
and intensity of the beam of back-scattered ions and use the known
atomic masses, it is possible evaluate the constituents and layer
structures of the samples.
Figure 1. Conceptual figure of RBS
It is also possible to determine the hydrogen concentration in the
sample by measuring the H ions that are forward-scattered by the
impinging He ions. This measurement is called hydrogen
forward-scattering spectroscopy (HFS) or elastic recoil detection
analysis (ERDA).
Figure 2. Outline of the HFS measurement. The incident, scattering,
and recoil angle \(\beta,\ \ \theta\),
and \(\alpha\), is \(75{^\circ}\), \(160{^\circ}\), and \(30{^\circ}\), respectively.
The features of RBS include:
Elements with atomic numbers 5 (B) to 92 (U) can be analysed (H is
also possible by HFS).
Quantitative analysis possible without using standard samples.
Compositional distribution and density in the depth direction can be
obtained.
High sensitivity and accuracy for particularly heavy elements.
Application Examples
Analysis of single thin-film, non-patterned samples.
Elemental analysis including resolving the isotopes \(_{}^{10}\text{B}\) and \(_{}^{11}\text{B}\).
Ion implantation verification (dose, atoms \(\text{c}\text{m}^{\text{-2}}\)).
Elemental density profile analysis.
Principle
When ions, such as \(\text{He}^{+}\), are accelerated and
impinge on the surface of a solid sample, some of them are elastically
scattered by atoms in the sample. Since the energy of the scattered ions
depends on the mass number of the atoms and the position (depth) from
the surface, it is possible to identify the elements in the sample at
various depths. Since scattering cross-sections, and therefore energy
loss (blocking power), are known, it is possible to analyse the
composition in the depth direction without calibration by standard
samples. The mechanism is illustrated in Figure
3.
Figure 3. RBS ion
energies and the associated information.
When an incident ion propagates through a solid, its energy is
reduced due to its interaction with electrons. This is expressed in
terms of blocking power, which is a parameter of the solid. The total
energy lost depends on the propagation distance.
It also depends on the type of atom and generally increases with
increasing atomic number. Provided that the analysis target element is
known, RBS analysis provides depth information from the amount of energy
lost, rather than from the known blocking power of each element.
The energy difference \(E_{\text{SCATT}}\) (see Figure 3)determines the type of atom in a
solid sample from the proton mass of the target atom
The intensity of the beam of scattered ions is proportional to the
concentration of the target atom, and the probability that the incident
ions are scattered is proportional to the square of the atomic number of
the target atom. Therefore, the intensity of the beam of scattered ions
yields the concentration of the target atom in the sample. Because the
intensity of the detected signal of the RBS energy spectrum is
proportional to the probability of scattering, the heavier the target
element, the stronger the signal, and hence the higher detection
sensitivity. Therefore, the heavier the target element, the smaller
measurable concentrations.
Data examples
Figure 4. Example of an RBS analysis of WSi/SiO films on a Si
substrate. The horizontal axis (channel number) corresponds to the
energy of the scattered He ions and hence the analysis depth. The energy
of the incident He ions was 2.275 MeV, and their total charge was 40.0
\(\mu\text{C}\). The scattering angle
was \(160{^\circ}\).
Figure 5. Example of RBS (left) and HFS (right) analysis of SiOC on a
Si substrate. The RBS scattering and HFS recoil angle was \(160{^\circ}\) and \(30{^\circ}\), respectively.
Annotated energy/depth profile: PDF file.
Spectral data: Microsoft® Excel® file
Measurement specifications
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H is also possible by HFS
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Typical maximum detection depth
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Depends on structure and/or material
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Items for enquiries
Purpose and scope of the analysis
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Quantity, availability of preliminary samples
Shape, dimensions, film structure and thicknesses, estimated
elemental concentration (at %), ion implantation process conditions
Desired delivery dates of preliminary and final results
Other relevant information
Caution
This analysis is subcontracted
The following cases may negatively affect the accuracy of the
analysis:
Uneven sample surface or interfaces
Sample with several types of atoms of large masses (atoms may not be
possible to distinguish)
Samples with atomic numbers smaller than 5 (B) and larger than that
of B and larger than 92 (U) cannot be measured