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

A study is presented of the effects of the different positive ion beam spec ies: Ar+, Ga+, Xe+, Cs+ and SF5+ and of their energies from 4 to 25 keV, on the fragmentation behaviour in static Secondary Ion Mass Spectrometry (SIM S) spectra for samples of the polymers: polytetrafluoroethylene (PTFE), pol ystyrene (PS) and polycarbonate (PC). The overall effect of energy is found to be weak over the entire mass spectrum. However, large differences are o bserved in restricted mass ranges amongst fragmentation groups. The fragmen tation is quantified in terms of the partition functions of the fragments f rom a plasma with effective temperature, T-p. It is found that fragmentatio n is least for high mass projectiles at low energies, but that the trend is different for polyatomic ions. A methodology is developed, which unifies a ll of the fragmentation behaviour to a single plot - the Unified Cascade Gr adient plot. An equivalence of mass and energy is shown and that the chemis try of the bombarding ion is unimportant. By extrapolation of the data to l ow T-p, 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 spe ctra are those peaks, which would be emitted from a surface plasma of very low temperature and thus have Little post-emission rearrangement or fragmen tation. Those peaks are, thus, directly characteristic of the material with out rearrangement and provide a direct interpretation and identification. I n the tests of the method described, this is supported and indicates that t he G-SIMS analysis will be significantly less ambiguous than static SIMS so that interpretation will be possible in the absence of a relevant referenc e spectrum. Crown Copyright (C) 2000 Published by Elsevier Science B.V. All rights reserved.