Medium‐Energy Backscattering and Forward‐Recoil Spectrometry

Medium-energy backscattering spectrometry is a variation of Rutherford backscattering spectrometry (RBS) using ions with energies ranging from a few tens to a few hundreds of kilo-electron-volts. The ion beam, frequently He+, is directed onto a specimen and the number of backscattered ions per incident ion is measured as a function of scattered-ion energy at one or more angles relative to the beam direction. The fundamental physical process being used is that of a binary collision between the nuclei of an ion in the beam and a near-stationary atom in the target. In such a collision, the energy of the backscattered ion is determined uniquely by its original energy, the scattering angle (which is the angle between the ion's trajectories before and after the collision), and the ratio of the masses. Consequently, a measurement of the energy of a backscattered ion at a specific angle may be used to infer the mass of the target atom with which it collided. The number of backscattered ions is proportional to the collision cross-section and the total number of available scattering centers. Since collision cross-sections are known a priori in this energy range, the number of backscattered ions with a given energy is a direct measure of the number of atoms of the corresponding mass in the target. The method achieves additional richness as the result of a second physical phenomenon, the gradual loss of kinetic energy through collisions as an ion penetrates through matter. This is measured by stopping power (also called linear energy transfer), which is the average energy lost, typically in electron volts per unit of length (measured in atoms per square centimeter) of trajectory in the target. A second factor controlling the energy of backscattered ions is the depth within the target at which the collision occurs. As a result, it is usually possible to extract quantitative information about the distribution of specific species within the near-surface region of the specimen. For the purposes of this article, it is sufficient to note that for medium-energy ions the stopping power is dominated by collisions between the nucleus of the projectile and the electrons of the target, so-called electronic stopping, which results in relatively uniform loss of energy with minimal angular divergence of the beam. Widely accepted semiempirical procedures are available for computing electronic stopping power. Particle detection technology is an extremely important issue in medium-energy ion beam analysis. This article also is focus on time-of-flight spectrometry optimized for high-resolution measurements. Other methods, including magnetic and electrostatic spectrometers, pulsed-beam accelerator systems, and time-of-flight spectrometry optimized for other measurements, are discussed briefly along with several other methods for depth profiling, trace-element identification, and hydrogen measurement. Throughout, special attention is paid to the relationship between medium-energy backscattering and conventional high-energy RBS. Keywords: medium-energy backscattering spectrometry; rutherford backscattering spectrometry; complementary techniques; alternative techniques; principles; practical aspects; time-of-flight spectrometry; spectrometer efficiency; sensitivity; resolution; instrumentation; forward-recoil data; problems