Elastic Recoil Detection

Elastic Recoil Detection

Elastic Recoil Detection, also referred to as forward recoil scattering (or spectrometry), is an Ion Beam Analysis technique in materials science to obtain elemental concentration depth profiles in thin films. An energetic ion beam is directed at the sample to be depth profiled and (as in Rutherford backscattering) there is an elastic nuclear interaction with the atoms of the sample. The incident energetic ions typically have MeV of energy, enough to kick out (recoil) the atoms being struck. The technique depends on putting in an appropriate detector to detect these recoiled atoms.

The great advantage in ERD is that all the atoms of the sample can be recoiled if a heavy incident beam is used, so a complete analysis of the sample is immediately available. For example, a 200 MeV Au beam can be used with a gas ionisation detector (see for example Timmers et al., Nucl.Instrum.Methods A, 447, 2000, 536-543; doi:10.1016/S0168-9002(99)01307-8). With the right recoil angle the scattered incident beam is kinematically prohibited, and therefore does not enter the detector. Alternatively, a 35 MeV Cl (or 50 MeV I) beam is often used with a time-of-flight detector: this is good for light elements (or transition metals) in silicon. The earliest example of ERD used a 35 MeV Cl beam (L'Ecuyer et al., J.Appl.Phys. 47, 1976, 381; doi:10.1063/1.322288, 2 pages).

ERD is also often done using a relatively low energy (2 MeV) 4He beam specifically to depth profile hydrogen. In this technique multiple detectors are used, at backscattering angles to detect heavier elements by RBS and a forward (recoil) detector to simultaneously detect the recoiled hydrogen (the cross-section for which reaction is strongly non-Rutherford at these energies). The recoil detector has to have a "range foil": a thin film (typically 6 micrometres of PET film) to preferentially stop the incident He beam scattered into the forward direction. Round robin intercomparisons have shown that 6% absolute accuracy for the H content can be obtained routinely with current methods (Boudreault et al., Nucl.Instrum. Methods B, 222, 2004, 547-566; doi:10.1016/j.nimb.2004.02.022)