Secondary ion sources

Lasers are used to deliver a focused, high density of monochromatic radiation to a sample target, which is vaporized and ionized. The ions are detected in the usual way by any suitable mass spectrometer to produce a mass spectrum. The yield of ions is often increased by using a secondary ion source or a matrix. 

Glow discharge source (GDMS) Laser ion source (LIMS) Secondary ion source (SIMS) Sputtered neutral source (SNMS) Thermal ionization source (TIMS) Inductively coupled plasma ion source (ICP-MS)... 

The ion source is an essential component of all mass spectrometers where the ionization of a gaseous, liquid or solid sample takes place. In inorganic mass spectrometry, several ion sources, based on different evaporation and ionization processes, such as spark ion source, glow discharge ion source, laser ion source (non-resonant and resonant), secondary ion source, sputtered neutral ion source and inductively coupled plasma ion source, have been employed for a multitude of quite different application fields (see Chapter 9). 

Figure 2.29 Schematic diagram of the sputter process in a secondary ion source.

Figure 2.30 a) Principles of a secondary ion source b) Schematic diagram of an ion source of a secondary neutral mass spectrometer (SNMS). (J. S. Becker and Id. J. Dietze, Int. J. Mass Spectrom. Ion. Proc. 197, 1-35. (2000). Reproduced by permission of Elsevier.)... 

Secondary ion sources (using primary ion beams to sputter solid sample surfaces) are applied not only in SIMS and SNMS, but also for the formation of negative ions in accelerator mass spectrometry (AMS). 

In SIMSLAB from VG Scientific, both surface analytical techniques - SIMS and SNMS - have been applied (see Figure 5.34). In this mass spectrometer different types of primary ion sources are available. Ar+, Cs+, Ga+ or O) primary ions are accelerated in the secondary ion source on the solid sample surface. Similar to the CAMECA IMS-7f, with this experimental arrangement, besides depth profiling, a microlocal analysis can also be performed. The sputtered secondary ions (for SIMS) or the post-ionized sputtered neutrals (for SNMS) - the post-ionization is carried out by an electron beam in an ionizer box (right-hand schematic in Figure 5.34) - are separated... 

Trace impurities including B, N, F, Mg, P, Cl, Cr, Fe, Ni, Co, Cu, Zn, Ge, As, Se, Mo, Sn and Sb in semiconductor materials (Si, SiGe, CoSi2, GaAs and GaN) can also be measured by TEAMS (trace element accelerator mass spectrometry), which utilizes a secondary ion source in an accelerator mass spectrometer.51 TEAMS is an interesting complementary technique to SIMS and can eliminate interferences that may not be resolvable in SIMS. 

The main components of a secondary ion mass spectrometer are the primary particle source (charged bombarding particles), the secondary ion source (containing the bombarded target with the sample molecules M on its surface), the e/m analyser, and the ion detection unit, see Scheme 4.1. 

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