Advanced Materials Characterization Chapter 12 Rutherford Backscattering Spectrometry

Advanced Materials Characterization Chapter 12 - by Igor Bello

Rutherford backscattering spectrometry (RBS) is the analytical technique named after Lord Ernst Rutherford. RBS belongs to the group of analytical techniques known as ion scattering spectrometry (ISS) that use phenomena of ion backscattering at the interaction of an ion beam probe with material surfaces. RBS was introduced as an analytical method of surfaces by Rubin et al. in 1957. It became a routine analytical technique in 1970’ due to the development of a detection device (solid surface-barrier silicon detector) and recording method of backscattered alpha particles. However, the key idea using backscattered ions in materials analysis is based on the so-called gold foil experiment also known as Geiger-Marsden experiment conducted by Ernest Marsden, while being undergraduate student, together with Hans Geiger under the supervision of Ernest Rutherford.

In RBS, alpha particles are induced by the electron impact ionization in helium gas in ion sources or conversion of positive-negative-positive ion conversion processes. The helium ions (He2+ ions that are in fact alpha particles or single charge He+ ions) are then extracted from the ion sources, focused, processed to ion beams, and accelerated to gain constant and known energy. At their impact on the analysis sample, some impact ions are backscattered and measured by a detector at a typical backscattering geometry that is determined by placing the detector at the backscattered angle ranging from 165° to 170°. Since the He2+ ions are scattered at different angles only a portion of backscattered ions is measured at a selected angle theta using a solid surface-barrier silicon detector. If the target of analysis is a layer sample, backscattered alpha particles may yield an RBS signal (RBS spectrum) because they do not only backscatter from the surface atoms, but they also penetrate below the surface in major quantities and backscatter from atoms in different material depth. Then, from the collected RBS spectrum, we can deduce the depth profile of a layered structure.