Static secondary-ion mass spectrometry
Static secondary-ion mass spectrometry, or static SIMS is a technique for chemical analysis including elemental composition and chemical structure of the uppermost atomic or molecular layer of a solid which may be a metal, semiconductor or plastic with insignificant disturbance to its composition and structure. It is one of the two principal modes of operation of SIMS, which is the mass spectrometry of ionized particles emitted by a solid (or sometimes liquid) surface upon bombardment by energetic primary particles.
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 [4]. 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).
Ion bombardment of a surface may result in a drastic change of its chemical composition and structure. These changes include sputtering, amorphization, implantation, diffusion, chemical reactions, and so on. All these changes are limited to a very small region surrounding the path of the primary ion into the solid. For Static SIMS each subsequent primary ion hits an undamaged area and total of only 0.1-1% of the atomic sites are bombarded during the measurement. To ensure this very low primary current densities are used generally in the range of 10−10 – 10−9 A/cm² (primary ion dose is below 1012 - 1013 ions/cm2). This leads to extremely small sputtering rates of fraction of a monolayer per hour and hence small secondary ion current density. Additionally, these emitted secondary ions are of low kinetic energy and emitted up to 20 nm from the impact site with surface annealing occurring in femto-seconds. These reasons make SSIMS a purely surface analysis technique causing negligible damage to the surface and with detection limit as low as 10−8 monolayer (ML).
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