Elastic Ion Scattering for Composition Analysis

Composition analyses for virtually all of the elements on the periodic table can be performed through a combination of elastic ion-scattering techniques: Rutherford backscattering spectrometry (RBS), elastic recoil detection (ERD), and enhanced-cross-section backscattering spectrometry (EBS). This article reviews the basic concepts used in elastic ion scattering for composition analysis and give examples of their use. Elastic ion scattering is also used for structure determination utilizing techniques such as ion channeling. Composition determination using ion-beam analysis (IBA) falls into two categories: composition-depth profiling and determination of the total integrated composition of a thin region. Unknown species in a sample may be identified using elastic ion-scattering techniques, but in general, analyses involving characteristic x rays or γ rays are more accurate for determining the presence of unknown constituents. For elastic ion scattering, if the ion beam and ion energy are chosen to have sufficiently high energy loss in the sample, then a composition profile is determined as a function of depth by collecting one ion-scattering spectrum. To represent accurately the composition profile as a function of depth in the sample, an accurate knowledge of the sample's volume density is required because IBA techniques are insensitive to density. If the density of the sample is unknown, then the depth in the sample is given in terms of an areal density: depth times volume density. When the incident ion beam is very light or the ion energy very high, then the composition as a function of depth may not be resolved fully, in which case the average composition of the layer of interest is determined by integrating the signal. In this way, the number of atoms per unit area (areal density) is determined from the integrated signal. Keywords: elastic ion-scattering techniques; principles; practical aspects; data analysis; initial interpretation; composition analysis; non-Rutherford cross sections; radiation hazards; examples; enhanced cross-section backscattering