Secondary Ion Emission during the Proton Bombardment of Metal Surfaces

Abstract

Secondary ion mass spectrometry (SIMS) is successfully used for fundamental and applied studies of the solid surfaces. Thus, it is important to know regularities in the secondary ion emission (SIE) induced by the primary beams of the inert or chemically active elements. The SIE intensity is found to be dependent not only on the surface sputtering processes (the intensity increases with the atomic number of the bombarding ion), but on the ionization probability of the sputtered atoms (the probability is strongly dependent on the physical and chemical states of target surface and experimental conditions) too. Analytical capabilities of SIMS might be improved by the use of light ions producing low sputtering and causing minimum surface erosion. The goal of the present review paper is a systematic study of various SIE aspects for metals, alloys, and chemical compounds in the case when the surface is bombarded with lightest ions — protons. To reveal the possibilities of proton applications in SIMS, the target surface was also bombarded with argon ions. The SIE regularities for metal targets were studied on the ion microprobe analyser equipped with a spherical 180°C sector energy analyser and a system, which pumps gas into the chamber. This made it possible to investigate not only the SIE factors, but also the secondary ion energy distribution and the change of the emission character at the target interaction with active gases. The SIE coefficients are measured for metals of I–III large periods. The use of primary protons and argon ions leads to a similar dependence of the SIE differential coefficients on the target atomic number. The SIE coefficients for primary protons and argon ions for the same elements are similar. Sputtering coefficients for the Ar+ ions are known to exceed those for protons. The fact that the metal-ion emission intensity for bombardment with lighter ions commensurate with a signal of secondary ions sputtered from the surface by heavy species is of a high practical importance for use of protons with analytical purposes. This fact may be explained as a result of extraordinary high ionization probability due to H+ ions. However, along with the general regularities based on the nature of materials at hand, there are essential differences in mechanisms of the atom ionization for the sputtering by protons and inert gas ions. Therefore, we have investigated the energy distributions of metal secondary ions ejected from surface bombarded with H+ and Ar+ ions. It is shown that, in the case of proton bombardment of multicomponent targets, a partial suppression of the structural, phase, and other matrix effects is observed. These effects as well as the increasing ionization probability of metal atoms sputtered by protons are quantitatively described concerning the important role of the local surface bonds in the ionization mechanism.

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