The Primary Ion Source

An ideal ion source must possess high brightness, and must produce an ion beam of homogeneous composition with a small energy spread. The ion current density must be easily monitored and remain constant across the beam cross-section. 

All ion guns feature a source of ions and some form of lens to extract the ions from the source chamber. More than one type of ion gun may be fitted to the instrument, as each will have strengths in certain areas of SIMS analysis. 

---

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 I shows a block diagram of a generalized SIMS Instrument. The essential Instrument subunits are the primary Ion source the sample the mass analyzer and the secondary Ion detector a11 of which are typically contained In or connected to an ultra high vacuum chamber. Many variations are possible In each of the SIMS Instrument subunits. Different primary Ion sources may be used to provide different species of primary Ions for example Ar. ...

A detailed description of the fast-beam apparatus and of the experimental procedure employed in the determination of absolute partial ionization cross sections has been given in previous publications (Wetzel et al, 1987 Freund et al., 1990 Tamovsky and Becker, 1992 Tamovsky and Becker, 1993). For the measurements of the cross sections of silane radicals, a dc discharge biased at typically 2 to 3 kV through SiD4 served as the primary ion source. Deuterated rather than protonated target species were used in these studies to facilitate a... 

The selection of the primary ion source is guided by the sample composition and by the required lateral resolution of the analysis. Oxygen ions preferentially form positive secondary ions from elements with a low ionization energy, while cesium ions are chosen for the preferential production of negative secondary ions from species with a high electron affinity. The Ga+ ion source provides a high brilliant primary beam and an excellent lateral resolution. In contrast, El ion sources provide poor lateral resolution but easily operate in the pulsed mode when time-of-flight (TOF) mass analyzers are used. 

The TOF is the most widely used analyzer for SIMS experiments. The TOF is based on the measurement of the time elapsed between the impact of the pulsed primary beam on the sample surface and the detection of the emitted secondary ions by the ion detector. The flight time of ions with different m/z (typically larger than 1 ps) is proportional to the ratio itself and is used to analyze the different ions. TOF analyzers are often used in conjunction with pulsed primary ion sources because the latter offer the possibility to synchronize the ion detection with the primary ion source pulse frequency. Reflectron TOF analyzers compensate for the secondary ion kinetic energy dispersion by using an electrostatic mirror that gradually reflects ions with the same m/z but with different kinetic energy. 

Fig. 14.9 The PDMS mass spectrometer at Argonne National Laboratory. The fission fragment source is located in front of the sample film. An accelerating potential of 10 kV is maintained between the sample plate and grid located 3 mm from the sample plate. Region 1 houses the primary ion source, acceleration region, and start detector. Region 2 houses a time-of-flight guide wire, drift tube, and stop secondary-ion) detector. Both regions are maintained at a pressure of less than 1 x 10 mmHg.

Negative Ion Chemical Ionization Negative ions are produced under ci conditions by electron capture. Under the higher pressure conditions of the ci ion source, electrons, both primary (those produced by the filament) and secondary (produced during an ionization event), undergo collisions until they reach near-thermal energies. Under these conditions, molecules... 

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. 

Since APCI is a chemical ionization, it needs a gas. Indeed, a nebulising gas (generally nitrogen) is introduced into the source. The gas molecules are ionized by a corona discharge (analogous to the fdament used in Cl) thus forming the primary ions, mainly composed of N2+" and N4+". In turn, the latter ionize the vaporized solvent molecules by... 

TOF analyzer it is critical for the mass resolution that the secondary ions are ejected at a precisely defined time. This means that the primary ion pulse should be as narrow in time as possible, preferably < 1 ns. At the same time maximum lateral resolution is desired. Unfortunately, there is a trade-off between these two parameters if the primary ion intensity is not to be sacrificed [122], Therefore, TOF-SIMS instruments have two modes of operation, high mass resolution and high lateral resolution. An advantage with the pulsed source is that an electron flood gun can be allowed to operate when the primary ion gun is inoperative. Thus, charge-compensation is effectively applied when analyzing insulating materials. 

Electron ionization (El) was the primary ionization source for mass analysis until the 1980s, limiting the chemist to the analysis of small molecules well below the mass range of common bioorganic compounds. This limitation motivated the development of the techniques commonly known as ESI, 1 MALDI, 2 and fast atom bombardment (FAB) 3,4 (Table 1). These ion sources allow for rapid and easy peptide analyses that previously required laborious sample preparation or were not possible with electron ionization. The mechanism of ionization these ion sources employ, which is somewhat responsible for their ability to generate stable molecular ions, is protonation and/or deprotonation. 

---