Primary Ion Gun

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. 

Figure 13.2. TOF-SIMS images of blue (m = 413 u) and green (m = 641 u) pigments of color filter array. Above each image the primary ion gun and the measurement time is displayed. Corresponding signal intensity of emitted secondary ions from an analyzed surface is given under suitable image. (Reprinted from Kollmer, F. 2004. Appl. Surf. Sci., 231-232 153-158. With permission from Elsevier.) (See color insert.)...

PHI TRIFT IV ToF-SIMS (Physical Electronics, USA) employs three electrostatic analyzers in the ion path to filter the background and metastable secondary ions. Using liquid metal cluster ion guns (such as Aut ion beam for sputtering of sample surface) increased sensitivity compared to a Ga+ primary ion beam are obtained (www.phi.com). The application of dual primary ion guns is useful for an effective dual beam depth profiling on multi-layered samples. 

Cation emitters The alkali metal zeolites, and other alkali metal aluminosilicates, are efficient emitters of alkali metal cations. The cation emitters have been known for a much longer time than the anion emitters, but the anion emitters are better understood from a chemical perspective hence they are discussed here. Both types of emitters, however, can be scaled up in intensity readily to be used for the primary ion guns in static SIMS instruments. Ion beams of 50 pA to 1 nA focused to a 1-mm spot size are routinely produced by using these emitters. These emitters are primarily used in SIMS guns, as opposed to being used for isotope ratio analyses. 

A pioneering work on the simultaneous measurement of the angular and velocity distributions was carried out by Champion et al. [98—101] following the velocity work of Vance and Bailey [94] described above. Their apparatus, a tandem mass spectrometer system, consists essentially of three sections a primary ion gun, a collision chamber, and a product-ion analyser and detector. A mass-analysed, velocity-selected ion beam is directed into the collision chamber containing target gas at low pressure. The product ions are velocity-analysed with a 127° electrostatic velocity selector and mass-analysed in a quadrupole mass filter. The angular distributions of the product ions are obtained by rotating the analyser-detector system about the centre of the collision chamber. 

The IT-SIMS instruments were custom designed and fabricated in-house at the Idaho National Laboratory. All instruments were based on modifications of commercial gas chromatography/mass spectrometry (GC/ MS) instruments that were altered by removing the gas chromatography (GC) and inlet, and replacing it with a direct insertion probe, a modified conversion dynode and a Venetian blind electron multiplier, and a perrhenate primary ion gun. The primary ion gun and the direct insertion probe are located coaxial with the IT,... 

The primary ion gun was operated at 5 kV at a current of 5 x 10 A, as measured using picoam-meter attached to a probe with an electron collector instead of a sample. Samples were irradiated for 0.9 s/scan, and 10 scans were collected and averaged. Depending on the persistence of the secondary ion signal, multiple experiments might be performed, but in all instances, the total primary ion dose to which the sample was exposed was 1 X 10 ions/cm, which is less than the commonly accepted static SIMS limit [11]. 

FIGURE 5.1 Diagram of the primary-ion gun described by Lehrle et al. mounted on a Bendix time-of-flight mass spectrometer. (Reprinted with permission from reference I). 

As mentioned in Section 5.1, SIMS is a highly diverse micro-analytical technique, possibly the most diverse. This diversity, even noted when restricting the discussion to commercially available instruments, covers the capabilities provided and the freedom in instrumental parameters available. As an example, almost all commercially available SIMS instruments contain two or more primary ion guns and are aU able to collect secondary ion spectra in either the positive or the negative polarities. In addition, there are almost always multiple signals from which to choose from for following a specific element or molecule. In some cases, these may be summed. In short, one or more of the following may be collected ... 

The q/m ratio can be calculated from this flight time. Because a very well defined start time is required for the flight time measurement, the primary-ion gun has to be operated in a pulsed mode in order to be able to deliver discrete primary-ion packages (20. 25. 26J. Electric fields (e.g., ion mirrors 26, 27 or... 

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