Secondary ion mass spectrometry; ionization mechanisms and the construction of a new instrument

Abstract

Secondary Ion Mass Spectrometry (SIMS) is a technique that is still in the stages of evolution, due to numerous instrument design configurations and applications that remain unexploited. A lack of understanding surrounding the fundamental mechanisms responsible for secondary ion production has also constrained development. In this work, one instrument avenue, Specimen Isolation (SI), was examined with the aim of providing more in depth information on secondary ion production, and allowing for the optimization of this technique in the design of a new SIMS instrument. SI controls sample charging and molecular interference problems by inducing steady state charging conditions on electrically isolated samples under large secondary ion extraction fields. In parallel, the fundamental processes responsible for the production of singly charged secondary ions were examined by analysing the isotope fractionation exhibited by Si, Cu, Ag, and Sn samples under both Cs+ and O- bombardment, over the 0-500eV emission energy range. The fact that isotope ratios, when plotted against emission energy, exhibit oscillations suggests that a near or quasi resonance charge transfer process is occurring during secondary ion production, and is responsible for isotope fractionation under SIMS conditions. Multiply charged secondary ion production arising from O- bombardment of Al, Au, Ag, Cd, Co, Cu, Hg, In, Mo, Sc, Te, Ta, and V, was studied by noting isotope fractionation trends and intensity variations with primary ion current density. These, in conjunction with a kinematic study, indicate that secondary electron collisions with the secondary neutral population some distance above the surface, are responsible for the production of multiply charged ions from the elements heavier than P. Finally, the large negatively charged post-ionized populations arising from Cs+ bombardment of Mn, Fe, and Co, under both conventional and SI conditions, were noted and examined. These were found to be due to a charge transfer process occurring between the secondary ion and secondary neutral populations. The intensity enhancements noted under SI conditions, was attributed to an increased collisional probability resulting from the increased secondary ion dispersion, above the sample. The design and construction of a high sensitivity SIMS instrument, to be used for elemental analysis of largely insulating samples, was then undertaken. To combat the problems of sample charging and molecular interferences, the SI technique was optimized and incorporated into the extraction optics. This allows analysis of secondary ions with emission energies ranging from 0 to 500eV (within a 1-20eV energy window). To retain sensitivity over this extreme energy range, specifically designed optics were incorporated to ensure efficient transmission into and through a conventional double focusing mass spectrometer. These include a unique 4kV extraction geometry, and a combination of quadrupole and einzel lenses. The design was optimized, with the SI technique in mind, by ensuring efficient emittance-acceptance phase space matching. Ray tracing methods were used to validate this information. Initial transmission results indicate that detection limits, in favourable cases, should extend into the ppb range even when analysing secondary ions of higher emission energies. The mass resolution and mass range extend from 100 to 2000, and 1 to 500amu, respectively.