SIMS The Techniques and Outputs

The impact of a primary particle on a surface causes energy and momentum transfer to a limited area around the point of particle impact, resulting in (i) a change in the lattice structure and (ii) a loss of surface material by sputtering. 

The ion-bombardment-induced emission processes include electrons and photons, besides the emission of surface particles (atoms or molecules) in a charged or uncharged, and possibly excited, state. All of these emission products are emitted with a certain angular distribution. 

The secondary ion emission includes all emitted ionized surface particles in the ground state and also in the excited state [112,113]. As virtually all secondary ions originate from the uppermost atomic layers of the bombarded surface, this represents one of the most important features of secondary ion mass spectrometry, namely its surface sensitivity. 

One very important question in SIMS concerns the charge state of the emitted atomic or molecular particles, which is heavily dependent on the chemical environment of the sputtered species. In fact, by changing this chemical environment (e.g., from a pure metal to an oxide) the ionization probability of the same species (e.g., a metal atom) may be changed by several orders of magnitude (via matrix effects) [112]. 

The most commonly used primary ions are Cs, 02, Ar+, Xe+ and Ga+, as well as the more recently reported Bi and Au cluster ions (Au +, Bi +) and even [114]. In comparison with the use of noble gas primary ions (Ar+ and Xe+), the use of 2 and Cs increases the ionization probability for species that tend to form cations and anions, respectively. Moreover, Ga is employed to obtain high lateral resolutions owing to the finely delivered focused beam [115]. Likewise, Ceo, Bi and Au clusters have been recently used in order to increase the yield of molecular mass fragments for the analysis of polymers and biomolecules [116]. 

---

Conceptually, SIMS can be considered a straightforward and direct technique. In practice, there are many complexities introduced as a result of the various methodologies that can be applied, whether in the static or in the dynamic mode of SIMS. This exists because there are numerous conditions under which SIMS can be carried out. Each condition is optimized to deal with the analysis of a different elemental or molecular species, from different solid matrices. In addition, relating the output to the compositional variations that may occur on or within the sohd being examined can be problematic. This stems, in part, from the complexities surrounding secondary ion generation, or more precisely, the matrix effect. As the term suggests, the matrix effect describes the effect of the matrix on the population of ions emitted. Matrix effects and their associated transient effects are discussed in Section 3.3.3.1.2. 

---