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Secondary Ion Mass Spectrometry (SIMS)

    SIMS (Secondary Ion Mass Spectrometry) is based on the surface phenomena called sputtering.
 
     
   
     
     

When a fast ion hits a solid surface it transfers all or part of its kinetic energy to the target atoms. This primary ion ends up either as reflected (scattered back into vacuum) or stopped (implanted) within the target.The transferred energy is distributed in a series of ion-atom and atom-atom collisions (called atomic collision cascade). If some atomic or molecular species near the surface acquire suitable momentum and enough energy to overcome the surface binding energy, it will be ejected (sputtered) from the target. Most of the emitted particles are neutral in charge, but a small proportion are also positively or negatively charged ions or electrons. The subsequent mass analysis of the emitted (secondary) ions provides detailed information on the elemental and molecular composition of the surface.

The mass separation and measurement of the secondary ions applies electric or magnetic field analyzers. Most common types are quadrupole, magnetic-sector or time-of-flight (TOF) analyzers.

 
   
   

Since the emitted particles originate predominantly from the uppermost one or two monolayers the information obtained from SIMS data refers to the surface. The dimensions of the collision cascade are rather small (a few nm in diameter), therefore the lateral resolution is determined by the provision of finely focussed primary ion beam.

SIMS is destructive in nature because particles are removed from the surface. This can be used to erode the target in a controlled manner and obtain information on the in-depth distribution of elements. This dynamic SIMS mode is widely applied to analyze thin films, layer structures and dopant profiles. In order to receive chemical information on the original undamaged surface, the primary ion dose density must be kept low enough (< 1013 cm-2) to prevent a surface area from being hit more than once. This so-called static SIMS mode is used for the characterisation of molecular surfaces.

From SIMS stems a technique using analysis of the emitted neutral particles. Post-Ionisation of these particles by electrons, plasma or photons allows to use the same types of analyzers for mass analysis of species originally emitted as neutrals. This technique is called Secondary Neutral Mass Spectrometry, SNMS. One of the most efficient ways to ionise the emitted neutrals is Laser Post-Ionisation (Laser-SNMS). The fraction of the neutrals detected in this way can be as high as 10 %, and the technique is attractive for the analysis of extremely small volumes.