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There are many techniques of IBA, each using characteristic properties of each element (e.g. mass, charge of nucleus or electromagnetic radiation emitted or absorbed) to determine the composition, concentration and distribution of various elements in materials. The following analysis techniques are routinely used at the Materials Research Group:
Rutherford Backscattering Spectrometry (RBS)
RBS is based on Rutherford’s experiment which lead to the discovery of the nucleus of the atom. Today RBS, is a powerful tool for determining elemental information, for example in the characterization of thin films. Helium ions (alpha particles) are accelerated to energies between 1 and 4 MeV. These alpha particles are then are focused on the sample to be analyzed.
Measurements are done in a vacuum chamber where an area of a few square millimeters is analyzed. Up to ten samples can be loaded into the chamber for standard RBS measurements. A silicon detector tilted at 165° detects the backscattered alphas from the sample. The chamber can be fitted with a cold-trap for liquid nitrogen that is used for in-situ heating measurements to obtain temperature dependent information on changes in the sample.
RBS analysis is used mostly for determining the composition and the depth distribution of elements but by aligning the crystallographic axes of the sample to the incoming alpha particles, RBS channeling analysis provides information about the crystal structure of the sample. RBS is a nondestructive and multielemental analysis technique.
Proton Induced X-Ray Emission (PIXE)
Charged particles (protons, alpha particles or heavy ions) are used to create inner-shell vacancies in the atoms of the specimen. Filling the vacancies by electrons from the outer shells leads to the emission of characteristic X-rays (and/or Auger electrons) and this forms the basis for a highly sensitive elemental analysis. Protons of 1-4 MeV energy are most often used. Their slowing down in matter is smooth and well characterized, with little scattering and deflection. The process is therefore easy to quantify. X-ray production yields are high and continuum background in PIXE is low. Therefore the detection limits are about two orders of magnitude better than with electron beams. PIXE spectra are usually collected in energy-dispersive mode and all elements with atomic numbers above 10 (Na and above) can in principle be detected at once. The characteristic X-rays of lighter elements are absorbed in the windows of routinely used Si(Li) or HPGe detectors. Typically reported sensitivities are 10-20 ppm for Na to Cl and 1-10 ppm for Ca and heavier elements. No information related to chemical identity, coordination chemistry or oxidation state of a particular element could be directly obtained.
The GeoPIXE software package is used for PIXE analysis and quantitative imaging. For point PIXE analysis GUPIX software can also be used.
Further reading:
S.A.E. Johansson, J.L. Campbell, and K.G. Malmqvist, Particle Induced X-ray Emission Spectrometry, Wiley, New York (1995).
Links:
GeoPIXE software package:
www.nmp.csiro.au
GUPIX:
www.physics.uoguelph.ca
Scanning Transmission Ion Microscopy (STIM)
The loss of energy by particles (ions) passing through a thin specimen depends on the elemental composition and thickness (areal density). The energies of transmitted ions and their number are measured using a semiconductor detector positioned behind the specimen.
Further reading:
M.B.H. Breese, D.N. Jamieson, and P.J.C. King. Materials Analysis Using a Nuclear Microprobe. Wiley, New York (1996).
(this book can be recommended for any ion beam technique)
Elastic Recoil Detection Analysis (ERDA) and Heavy Ion Elastic Recoil Detection Analysis (HI-ERDA)
The quantitative and sensitive analysis of light elements in thin films is in general a non-trivial task in materials science, since there are only a few techniques available to get reliable and accurate profiles. Elastic recoil detection (ERD) using energetic heavy ions is such a technique. Recoild ions scattered off a thin film by an energetic heavy ion beam impinging the surface at glancing angle are dtected under forward directions and anlysed for their nuclear charge or mass and energy. A sensitivity in the ppm reggion with a depth resolution of some 10 nm and a depth range of 1 micron is obtained in standard ERD set-ups.
Conditions for ERD analyses are currently being investigated with respect to the technical and physical limits. As a second phase experiments are planned to characterize various thin films within different fields of physical and technological contexts. A time of flight (TOF) detection system in coincidence with energy analysis will be used and is currently under development.
Contact Dr. Mlungisi Nkosi for further details.
