Duoplasmatron

The Duoplasmatron (Eig. 3.18). In the Duoplasmatron, gas-discharge ion sources are used for bombardment with oxygen or argon. In dynamic SIMS, especially, the use of O2 ions is common because of the chemical enhancement effect. With a duoplasmatron ion beam currents of several microamps can be generated. The diameter of the beam can be focused down to 0.5 pm. 

The oxygen ion beam diameter is limited to 0.5 pm by the duoplasmatron source used. For mapping electropositive elements this drawback must be tolerated because of the chemical enhancement effect. 

Typical ion sources employ a noble gas (usually Ar). The ionization process works either by electron impact or within a plasma created by a discharge the ions are then extracted from the region in which they are created. The ions are then accelerated and focused with two or more electrostatic lenses. These ion guns are normally operated to produce ions of 0.5-10 keV energy at currents between 1 and 10 pA (or, for a duoplasmatron, up to 20 pA). The chosen spot size varies between 100 pm and 5 mm in diameter. 

The Mg isotopic measurements were performed with a modified AEI IM-20 ion microprobe [13,14]. Secondary ions were generated by bombarding the sample with a focussed ion beam to excavate a small volume of the sample. A fraction of the sputtered material is ionized during the sputtering process and is drawn off into the mass spectrometer. A duoplasmatron ion source produces a... 

The 6 x 1022 particles leaving the 6 GW(t) reactor per second carry 150 MW of power. With approximately 5 x 101 m2 of trapping surface (some 10% of the reactor vacuum wall area), the power loading becomes 3 x 106 Wm 2. The maximum ion current density which has to be considered is — 1 x 1021 ions m 2s 1 or 1.9 x 10s mA m-2. Such ion current densities incidentally are experimentally accessible with ion sources of the duoplasmatron type. 

In 1967 Liebl reported the development of the first imaging SIMS instrument based on the principle of focused ion beam scanning [24]. This instrument, the ion microprobe mass analyzer, was produced by Applied Research Laboratories (Fig. 4.5). It used an improved hollow cathode duoplasmatron [25] ion source that eliminated filaments used in earlier sources and allowed stable operation with reactive gases. The primary ion beam was mass analyzed for beam purity and focused in a two-lens column to a spot as small as 2 pm. The secondary ions were accelerated from the sample surface into a double focusing mass spectrometer of Mattauch-Herzog geometry. Both positive and negative secondary ions were de-... 

Current density and the type of primary ion have a critical effect on SIMS analyses. High current densities are desirable for rapid profiling and high-sensitivity analysis, whereas low current densities are chosen when the analyte layer is thin or when using static SIMS. Exotic polyatomic ion sources are an area of active research today Re04 [104], SF5+ [105], SF6 , NO, CF3+, C, and many others [106,107], have been reported to provide exceptional enhancement of secondary ion yields and ultrashallow depth profiling. However, the most heavily used ion sources today are the hollow cathode duoplasmatron (Fig. 4.31) [108], the thermal... 

The most common species used with SIMS sources are Ar+, 02+, 0 , and N2+. These ions and other permanent gas ions are formed easily with high brightness and stability with the hollow cathode duoplasmatron. Ar+ does not enhance the formation of secondary ions but is popular in static SIMS, in which analysis of the undisturbed surface is the goal and no enhancement is necessary. 02+ and 0 both enhance positive secondary ion count rates by formation of surface oxides that serve to increase and control the work function of the surface. 02+ forms a more intense beam than 0 and thus is used preferentially, except in the case of analyzing insulators (see Chapter 11). In some cases the sample surface is flooded with 02 gas for surface control and secondary ion enhancement. An N2+ beam enhances secondary ion formation, but not as well as 02+. It is very useful for profiling and analysis of oxide films on metals, however. It also is less damaging to duoplasmatron hollow cathodes and extends their life by a factor of 5 or more compared to oxygen. 

Oxygen and cesium are commonly used as primary ions. In the oxygen ion source (Duoplasmatron) ions (mostly Oj, O-) are produced in an arc discharge. Usually O is used as it reduces sample charging. Since O has the second highest... 

The most commonly used ion sources include duoplasmatrons, which are used to obtain an intense beam of micron size, and liquid metal sources, with a lower output level but providing a probe and thus a lateral resolution of a few nanometres. 

Schnitzer and Anbar s experiments are rather similar to those of Baumann, Heinicke, Kaiser and Bethge Both employed plasma ion sources that employed by Schnitzer and Anbar was a hollow cathode duoplasmatron. Also, both used einzel lenses and momentum/charge analysis. But the different lifetimes of the doubly-charged negative ions studied dictated somewhat different analyses. Baumann, et al. used an electric deflection analysis after the magnetic sector, as already seen, whereas Schnitzer and Anbar employed a Wien velocity filter and einzel lens voltage variation prior to the magnetic sector. 

Extraction and acceleration of the negative ions from the duoplasmatron was accomplished with voltages of 5 to 15 kV. After passing through the einzel lens, the ions encounter a 30 cm Wien velocity filter. The ExB field of this filter allows analysis based only on the ion velocity according to... 

Figure 8.7 Duoplasmatron ion source. The magnetic field marked B intensifies the plasma by confining electrons close to the axis. (Reproduced with permission from J.C. Vickerman and D. Briggs, ToF-SIMS Surface Analysis by Mass Spectrometry, IM Publications and SurfaceSpectra, Chichester and Manchester. 2001 IM Publications.)...

The procedure involves bombarding the sample surface with a fast-moving (5-20 keV) ion beam. This beam is produced in a highly efficient discharge source called a duoplasmatron (see Fig. 16.18). The primary ions are focused by a series... 

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