Static SIMS: towards unfragmented mass spectra — the G-SIMS procedure

A study is presented of the effects of the different positive ion beam species: Arq, Gaq, Xeq, Csq and SF5q and of their energies from 4 to 25 keV, on the fragmentation behaviour in static Secondary Ion Mass Spectrometry SIMS.spectra for samples of the polymers: polytetrafluoroethylene PTFE., polystyrene PS.and polycarbonate PC.. The overall effect of energy is found to be weak over the entire mass spectrum. However, large differences are observed in restricted mass ranges amongst fragmentation groups. The fragmentation is quantified in terms of the partition functions of the fragments from a plasma with effective temperature, Tp. It is found that fragmentation is least for high mass projectiles at low energies, but that the trend is different for polyatomic ions. A methodology is developed, which unifies all of the fragmentation behaviour to a single plot — the Unified Cascade Gradient plot. An equivalence of mass and energy is shown and that the chemistry of the bombarding ion is unimportant. By extrapolation of the data to low Tp, a new spectroscopy, known as gentle-SIMS or G-SIMS is formed. The G-SIMS spectrum is in the static regime. Significant peaks in the G-SIMS spectra are those peaks, which would be emitted from a surface plasma of very low temperature and thus have little post-emission rearrangement or fragmentation. Those peaks are, thus, directly characteristic of the material without rearrangement and provide a direct interpretation and identification. In the tests of the method described, this is supported and indicates that the G-SIMS analysis will be significantly less ambiguous than static SIMS so that interpretation will be possible in the absence of a relevant reference spectrum. Crown Copyright q2000 Published by Elsevier Science B.V. All rights reserved.