Low-energy (5–15 keV) ion scattering spectroscopy (ISS) and direct recoil spectroscopy (DRS) are well-established analysis techniques that have been used for several years to characterize surfaces of materials. ISS utilizes a beam of ions in the 1–15 keV range which are scattered by a surface atom at an angle θ1 relative to the direction of incidence. In this energy range, the collisions are elastic and are described by classical two-body collision kinematics. The kinetic energy E1 of the scattered primary ion is given by
where E0 is the primary ion kinetic energy and α = M2/M1. Note that Equation 1 has real values only if M2 > M1, and ISS is not able to detect surface atoms which are lighter than the primary ion beam.
The kinetic energy of the backscattered primary particle may be measured either by using an electrostatic energy analyzer (ESA) or by pulsing the beam and using a time-of-flight (TOF) detection scheme. The ESA only detects the ion fraction (typically 10 2−10−4) of the scattered primary beam and transmits only a narrow portion of the energy distribution of these ions, whereas the TOF detection scheme simultaneously detects the full energy range of both ions and neutrals. Consequently, the required beam dose for TOF detection is 3–4 orders of magnitude smaller than that required by the ESA. A typical analysis beam dose for TOF detection is ∼1011−1012 ions/cm2, removing or displacing one part in 103−104 of the near-surface atoms and making the TOF scheme essentially nondestructive.