Ion Beam Analysis (IBA) uses a high-energy, light ion beam (typically He++, i.e., nuclei: alpha particles) to probe elemental composition as a function of depth to several microns with a depth resolution of 50-200 angstroms. It is a fast, nondestructive (i.e., nonsputtering) and standardless technique to quantify the absolute atomic ratios (stoichiometry) in compounds or mixtures, insensitive to their chemical environments. It can also determine the film thickness given knowledge of atomic density, or density (i.e., porosity) given knowledge of film thickness (e.g., from a SEM cross section image) as well as structural disorder in single crystals or epitaxial films, by examining ion channeling. The energy distribution of backscattering ions (He++) quantifies the depth distribution for a given element. Characteristic X-rays are also emitted from the different target elements because of core electrons ejected by the passing He++ nucleus. This X-ray emission spectrum can be used to ensure the accurate identification of similar mass elements (i.e., heavy impurities in a glass). Even higher-energy gamma rays emitted from the beam-induced nuclear reactions can provide excellent sensitivity for certain light elements such as Li and F. Finally, one can also tilt the sample and detect recoiling protons and deuterons from the sample, and thereby measure a hydrogen depth profile.
Thus Ion Beam Analysis is a broad term (worth wiki-ing or googling) that involves several specific techniques, mainly:
- Rutherford backscattering spectrometry (RBS)
- Forward recoil spectrometry (FReS) or elastic recoil detection analysis (ERDA)
- Nuclear reaction analysis (NRA)
- Particle induced X-ray emission (PIXE) analysis
- Ion channeling analysis
MAS 1700 pelletron tandem ion accelerator (5SDH) equipped with charge exchange RF plasma source by National Electrostatics Corporation (NEC). Analytical endstation (RBS 400) by Charles Evans & Associates:
- Fixed ion detector at 165° for Rutherford backscattering spectrometry (RBS).
- Movable ion detector (90° - 150°) for grazing-angle scattering (enhanced surface sensitivity).
- Fixed ion detector at 30° for forward-recoil spectrometry (FReS) of H and D.
- Scintillation NaI(Tl) gamma-ray detector for particle induced gamma-ray emission (PIGE), a form of Nuclear Reaction Analysis (NRA) of light elements.
- Retractable Si(Li) X-ray detector for particle induced X-ray emission (PIXE) analysis of trace elements.
- Sample goniometer controlled translational and rotational movement.
Graphic-interface computer control of data acquisition and ion beam characteristics:
- Computer control of ion beam parameters (mass, energy, charge, current and focusing).
- Automated collection of data on multiple samples.
- Sample positioning/tilting (automatic orientation for axial or planar channeling).
- Simultaneous collection of RBS, FReS, PIGE, and PIXE spectra.
- Cryogenic sample stage, liquid-nitrogen cooled (to reduce beam damage on organic samples).
- Different sizes of apertures available to control beam size on target from 0.2 mm to 10 mm.
- Auxiliary analysis workstation.
- Quark, SimNRA, HYPRA, RUMP, and GUPIX software available for data analysis.
- Batch data-file format conversion.
Rutherford backscattering spectrometry (RBS):
- Nondestructive and multielemental analysis technique
- Elemental composition (stoichiometry) without a standard (1 - 5% accuracy).
- Elemental depth profiles with a depth resolution of 5 - 50 nanometers and a maximum depth of 2 - 20 microns.
- Surface impurities and impurity distribution in depth (sensitivity up to sub-ppm range).
- Elemental areal density and thus thickness (or density) of thin films if the film density (or thickness) is known.
- Diffusion depth profiles between interfaces up to a few microns below the surface.
- Channeling-RBS is used to determine lattice location of impurities and defect distribution depth profile in single crystalline samples
Forward recoil spectrometry (FReS):
- Nondestructively and simultaneously determines hydrogen isotopes (H and D) and their depth profiles in polymers and other solids with a sensitivity of 0.01 at.%, a depth resolution of 30-80 nm, and a maximum depth of 1 micron.
- Measurements of other light elements (Z<9) are also possible if heavy ions like Cl or Au are used.
Nuclear reaction analysis (NRA):
- Nondestructively measures light elemental depth profiles (Z<9) with a superb sensitivity of a few ppm, a good depth resolution of a few nanometers, and a maximum depth of a few microns. Elements like H, D, Li, B, C, O, and F can be analyzed.
- Unlike ion scattering techniques, NRA is an isotopically sensitive technique with an excellent mass resolution and has no mass-depth ambiguity of RBS and FReS in data interpretation.
- Channeling-NRA can be used to determine lattice location of impurities and defect distribution depth profile in single crystalline samples.
Particle induced X-ray emission (PIXE) analysis:
- Nondestructive and multielemental analysis of trace elements with an excellent detection limit of up to 20 ppb.
- Used together with RBS for accurate mass identification of medium to heavy elements with similar masses.
- Elemental composition of magnetic films in which RBS does not have an enough mass resolution to resolve Mn-Fe-Co-Ni elements.
- Channeling-PIXE can be used to determine lattice location of impurities in single crystalline samples
Ion channeling analysis
- Assess crystallinity of MBE-grown thin films such as type of defect structures, impurity location, type of atomic site, lattice strain and alignment in epitaxial growth
- Enhance surface sensitivity of light elements on heavier single crystal substrate
- Channeling-RBS, Channeling-NRA, and Channeling-PIXE are available for different applications
- Accelerator terminal voltage tunable from 80 kV to 1.7 MV (source injection voltage is up to 30 kV).
- H+, He+ and He++ beams in standard configuration with maximum energies of 3.4, 3.4 and 5.1 MeV, respectively.
- Beam spot size from 0.2 mm to 1 mm.
- Beam current on target up to a few tens to hundreds nA depending on ion species and energies.
- 3He, 15N and 16O beams are also available for nuclear reaction analysis (NRA) or elastic recoil detection analysis (ERDA).
- Ortec Ultra ion detectors: energy resolution of 12 keV
- Kevex Retractable Si(Li) X-ray detector with 5 mm Be-window: energy resolution of 145 eV.
- Canberra 2" x 2" NaI(Tl) gamma detector: energy resolution of 6.5%.
- Sample lateral movements: ± 25mm with a minimum step size of 0.001mm.
- Tilting movements: ± 90° along vertical axis and ± 20° along horizontal axis with a minimum step of 0.01°.
- Typical sample size: 5 x 5 mm2 or 10 x 10 mm2for RBS/NRA/PIXE/Channeling and 5 x 15 mm2 for FReS.
- Minimum size: 0.5 x 0.5 mm2 and Maximum size: 50 x 50 mm2
- Sample thickness is typically no more than 5 mm, but thicker samples can be accommodated with special preparation.
- Sample has to be a vacuum-compatible solid with reasonably smooth surface.