The aim of this chapter is to impart an understanding of the physical principles behind unusual SIMS applications and to do so in terms of conveying mental pictures of processes rather than dwelling upon mathematics. Several texts are recommended for further reading: McPhail (2006), Stephan (2001), Vickerman & Briggs (2001) and Benninghoven et al. (1987).

Secondary Ion Mass Spectrometry relies upon the impact of an energetic ion into a surface, transferring enough energy to atoms in the surface to allow them escape and be ionized (creating the secondary ion). The process is complex to model but some simple pictures can give insight into the process.

As we are interested here in nanoscale analytical techniques, we concentrate on secondary ionization with high spatial resolution (here nanoscale refers to sub-micrometer). To obtain this spatial resolution, primary ions are focused to a small spot size onto the sample. How this can be achieved will be discussed in section 1.1.3 below. The ions have high energy relative to the surface, typically >10 keV, and collide with atoms and molecules on the surface. Quantum mechanical treatment of this process is not appropriate for this chapter; a quick qualitative picture is all that is desired. The energetic ion colliding with species on the surface is envisaged. Most collisions will transfer 100s of eV to the surface species and these in turn will collide with their neighbours (Fig. 1). The volume will become rapidly thermalized and indeed the spectrum of ions leaving the surface matches a high-temperature plasma quite closely. a con sequence is that ions are ejected with a large range of energies (Fig. 2).