Primary ion sources

The three most common types of mass analyzers in SIMS systems are (1) double-focusing magnetic sector instruments, (2) TOF mass spectrometers, and (3) quadrupole mass spectrometers. The choice of mass analyzer depends on whether dynamic or static SIMS is needed, on the requirements of mass range and resolution, and on transmission efficiency, among other factors. The mass analyzers have been discussed in Chapter 9 in detail and this chapter should be reviewed as necessary. 

Double-focusing magnetic sector instruments use a magnetic field to separate the ions by mass-to-charge ratio. The mass range covered by this type of analyzer is up to 500 Da with a 

The detectors used for SIMS are electron multipliers (discussed in Section 14.2.1.1 and in Chapter 9) array detectors are used for imaging. 

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The instrumentation for SSIMS can be divided into two parts (a) the primary ion source in which the primary ions are generated, transported, and focused towards the sample and (b) the mass analyzer in which sputtered secondary ions are extracted, mass separated, and detected. 

The Intensity for LEISS using a noble gas as the primary Ion source Is quite low since, as previously discussed, most Incoming Ions are neutralized upon scattering and thus go undetected. Further, the relative Intensity of the various components Is a strong function of the Incident Ion energy as shown In Figure 2. These changes In Intensity can be attributed to several factors as described below ... 

Figure 4.2 is a block diagram that illustrates the principle of the SIMS technique. The apparatus includes a primary ion source, a vacuum chamber where the objects under study are placed, a mass analyser and a secondary ion detector. 

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... 

ToF-SIMS analytical techniques for characterizing human hair surfaces using Ga+ and primary ion sources have been compared.84 Similar hair fingerprints were obtained with both ion sources. With the Cg0 ion source, on average, secondary ion intensities between two and three orders of magnitude higher than the Ga+ primary ion source were observed.84... 

Figure 2. Schematic view of the spectrometer. The components illustrated include M, crystal manipulator Q.M.S., quadrupole mass spectrometer I.G., primary ion source E.S., energy spectrometer G, Bayard-Alpert gauge T, crystal target and G.I., gas inlet. Auxiliary components are omitted for graphical clarity. The SIMS experimental geometry and coordinate systems are defined in the inset. Reproduced with permission from Ref. 4. Copyright 1981, American Institute of Physics.

Mahoney, J.F. Perel, J. S. Taylor, "Primary Ion Sources for Fast Atom Bombardment Mass Spectrometry", Am. Laboratory, p. 92, March 1984. 

LMIG filled with bismuth (or gold) has been developed for SIMS (especially employed in ToF-SIMS) measurements. This primary ion source provides cluster beams of Bi with n = 1-7 which allow higher secondary ion yields compared to gallium or indium LMIGs ° and is well suited for special applications (biological imaging). Bismuth cluster ion beams have much better... 

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