Static Secondary-ion Mass Spectrometry - Mechanism


Most energy of the primary ions is dissipated into the near surface region of the solid by a series of binary collisions. This results in ejection (sputtering) of so-called ‘secondary’ particles such as electrons; neutral species, atoms, and molecules; atomic and cluster ions from the surface. In SIMS it is these secondary ions which are detected and analyzed by a mass spectrometer to produce a mass spectrum of a surface for a detailed chemical analysis of the surface or the solid . Secondary ion current: Ii± = Ipfi± CiSiηi (± refers to a positive or negative particle) Ip= Incident ion current (ions/s); fi±= Fraction of particles sputtered as ions Si= Sputtering yield of both ions and neutrals (particles/incident ion) fi±= Fraction of particles sputtered as ions; Ci= Concentration of the ith element (corrected for isotopic abundance) in the sputtered volume; ηi= collection efficiency of the SIMS instrument Ip (ions/s) = 0.25 d²j; d= diameter of a Gaussian shaped beam j= current density (ions/cm²s)

All the secondary ions generated in SIMS analysis originate from the topmost monolayers of the bombarded solid. This means that all different modes of SIMS analysis are basically surface analyses secondary ion emission—atomic as well as molecular—reflect the chemical composition of the surface-near region of the bombarded solid. However, the intention of different SIMS analyses may be quite different. This depends on the erosion rate of the surface which is controlled by the dose of the primary ions. It may be bulk analysis (Dynamic SIMS) or a true analysis of originally uppermost monolayer of a condensed phase (Static SIMS).

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