Ion Beam Analysis Methods
A range of analytical techniques are possible with an ion microbeam system, enabling non-destructive elemental analysis with high sensitivity and spatial resolution.
Particle Induced X-ray Emission
When a fast moving charged particle collides with an atom there is a reasonable probability that an electron will be ejected from an inner atomic shell. Subsequently, the electrons in the outer shells re-arrange themselves with the emission of a quantum of energy (X-ray), the energy of which is characteristic of the parent atom. The measurement of these characteristic X-rays enables the chemical composition of a sample to be determined with high quantitative accuracy and sensitivity (10–20 ppm for Na to Cl, and 1–10 ppm for Ca and higher in the periodic table).
Rutherford Backscattering Spectroscopy
As the ion traverses the sample, there is a small probability that a direct elastic nuclear collision will also occur causing the ion to recoil out of the sample. By measuring the energy of recoiling ions, information can be obtained on the concentration and depth distribution of major constituents of the sample, such as the light element matrix C, N and O. RBS is complementary to PIXE and allows the sample matrix to be characterised, thus making quantitative measurements of the trace elements more accurate.
Scanning Transmission Ion Microscopy
For most biological samples (tissue sections, isolated cells), the high energy particle beam passes through the sample, losing energy by collisions with electrons. By measuring the energy loss of individual particles in the transmitted beam, information on the density or thickness of the specimen can be obtained.
Nuclear Reaction Analysis
Measurement of reaction products such as gamma rays, alpha particles and protons following nuclear reactions between the incident ion and the target atoms. Used mainly for light ions up to Mg in the periodic table and their isotopes.
Elastic Recoil Detection Analysis
Measurement of recoiling atoms following elastic nuclear collisions at a glancing angle. Used mainly for profiles of very light ions such as hydrogen or deuterium.
Ion Beam Induced Current
A technique which images the active regions in micro-electronic devices, by measuring the induced current caused by a penetrating beam of focused ions. This technique enables active regions to be assessed on-line for resistance to radiation.
Channeling
If the direction of the ion beam is aligned with a plane or axis of a crystal, then channeling can occur and the ions travel through the crystal with much reduced energy loss. This phenomenon can be used to measure crystal quality and the mapping of the lattice plane directions (strain measurements) when used in conjunction with PIXE and RBS. When used with STIM, the technique can provide information on faults such as lattice distortion and dislocations at depth in the crystal.
Ion Microbeam Tomography
The use of a penetrating focused beam of high energy ions to produce information at depth on the elemental constituents and internal structure of a specimen. By scanning the beam over the specimen at many orientations, 3D information can be obtained and 3D images generated.
Ion Microbeam Microfabrication
The use of finely focused and penetrating high energy ion beams to form deep (~10 µm) and narrow (~100 nm) channels in resists used in lithography. Also known as Deep Ion Beam Lithography (DIBL).