Gas-phase ionisation of sputtered rare gas atoms

Abstract

During bombardment of solid samples with rare gas ions, charge-transfer events can convert reemitted rare gas atoms to positively charged ions. In analytical applications of secondary ion mass spectrometry (SIMS) this mechanism of ion formation is of considerable interest because, owing to their high ionisation potential, the ion fraction of sputtered rare gas atoms is very low. A quadrupole-based SIMS instrument was used to study details of the gas-phase ionisation process, notably the variation of the ion production rate as a function of the distance from the surface. The relevant information was derived from the apparent energy spectra of gas-phase generated (GPG) ions, observed during bombardment of a variety of elemental targets with Ne +, Ar + and Kr + beams at energies between 3 and 12 keV. Owing to the use of a secondary ion extraction field of low strength, gas-phase ionisation events could be separated by distance δ from the surface, with δ up to about 6 mm. The results were compared with a simple model that describes the ion production rate as the product of the gas-atom flow rate and the ionisation probability. The first factor is proportional to the primary ion current and the second proportional to the current density j 0. Therefore, the intensity of GPG ions is not proportional to j 0 2 , as assumed previously. The mean ionisation probabilities of GPG ions (∼10 −5 at a moderate mean current density of ∼2 mA/cm 2) were found to be higher by more than four orders of magnitude compared to ‘ordinary’ SIMS. In part, this favourable result can be attributed to the low energy of rare gas atom ejection (∼0.1 eV). The experimental data suggest that the angular distributions of ejected rare gas atoms are strongly forward peaked. Presumably for this reason the yields of GPG ions observed with an amorphised target like silicon were distinctly higher than with polycrystalline metals. In the latter case, emission from unfavourably oriented microcrystals causes a large fraction of ejected gas atoms to escape from the interaction volume before ionisation can occur. Further enhancement in yield can be expected by the use of focussed primary ion beams with fairly uniform rather than Gaussian-like current density distributions. If the atomic number of the projectile is slightly lower than that of the target atom, as for Ne + impact on Mg, Al and Si or Ar + impact on Ti +, sizable or even high signals, independent of current density, were observed due to energetic multiply scattered ions.