Accelerator-based Techniques for Analysis
High Voltage Engineering (HVE) Van de Graaff Accelerator
- Rutherford Backscattering Spectrometry (RBS)
- Elastic Recoil Detection Analysis (ERDA)
Particle accelerators create confined beams of charged particles that are propelled with a desired trajectory using electromagnetic fields. A Van de Graaff accelerator is an electrostatic accelerator that employs a Van de Graaff generator, using static electric fields to accelerate the particles. The Van de Graaff accelerator at CMM is a linear particle accelerator that propels positively-charged ions down one of two beam lines. Rutherford Backscattering Spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA) are two nondestructive analysis techniques that utilize the generated ion beam to characterize the material to a maximum depth of about one micron. Ion implantation can be performed for the purposes of analyzing the interaction of the ions with the target material during the implantation/irradiation as well as analyzing the behavior of the particles within the irradiated material post-implantation.
HVE VAN DE GRAAFF ACCELERATOR
B70 Materials Research Laboratory
The Van de Graaff accelerator operates at medium to high energies, within the range of 0.8-2.3 MeV, and the following gases can be used: H, He, Ne, Ar, Kr, and Xe. There is one beam line for RBS and ERDA, while a second beam line can be utilized for implantation/irradiation.
Rutherford backscattering spectrometry can provide information about a material’s composition as a function of depth, as well as its crystallinity and degree of disorder. Different information can be obtained depending on the gas species used to form the ion beam as well as the incident energy of those ions.
The most common usage of RBS at CMM employs a He+ beam to determine the composition of a material. A majority of the incident He+ ions implant into the target sample, but a small fraction undergo a direct collision with a nucleus of an atom inside of the sample. These ions backscatter with a given energy, based on the element of the atom that was collided with as well as its depth in the sample. The He+ ions also lose energy traveling through the solid target, and therefore useful information can only be gathered up to a depth of about one micron below the surface of the sample. For this reason, this technique is most useful for thin films and membrane specimens. Software is available that allows the user to fit the collected data so that thin film/membrane thicknesses as well as compositions can be determined.
The RBS chamber at CMM is equipped with apertures 1, 2, and 3 mm in diameter. There is a goniometer for channeling studies as well as sample rotation to prevent channeling. Further information about RBS theory and analysis can be found at the following website: http://www.eag.com/mc/rbs-theory.html.
Elastic recoil detection analysis is a second technique that can be performed on the same beam line as RBS. This technique is similar to RBS, but instead of analyzing the backscattering of particles from the ion beam, ionized atoms from the sample itself are recoiled toward a detector. Only elements with a lower Z than that of the incident ion beam can be analyzed, so with a He+ beam, only H can be detected. As with RBS, the accessible depth range is about one micron.
The second beam line connected to the Van de Graaff accelerator is used for the implantation/irradiation of samples. There are currently six apertures available: 3 circles and 3 squares with diameters/edges of 3, 4.5, and 6 mm. Implantations can be done at room temperature as well as elevated and low temperatures. The achievable implantation depth is dependent on the gas species, the incident ion energy, as well as the composition of the target sample.