Sputtered neutral atom mass spectrometry

Sputtered Neutral Mass Spectrometry (SNMS) is the mass spectrometric analysis of sputtered atoms ejected from a solid surface by energetic ion bombardment. The sputtered atoms are ionized for mass spectrometric analysis by a mechanism separate from the sputtering atomization. As such, SNMS is complementary to Secondary Ion Mass Spectrometry (SIMS), which is the mass spectrometric analysis of sputtered ions, as distinct from sputtered atoms. The forte of SNMS analysis, compared to SIMS, is the accurate measurement of concentration depth profiles through chemically complex thin-film structures, including interfaces, with excellent depth resolution and to trace concentration levels. Genetically both SALI and GDMS are specific examples of SNMS. In this article we concentrate on post ionization only by electron impact. 

The atom flux sputtered from a solid surface under energetic ion bombardment provides a representative sampling of the solid. Sputtered neutral mass spectrometry has been developed as method to quantitatively measure the composition of this atom flux and thus the composition of the sputtered material. The measurement of ionized sputtered neutrals has been a significant improvement over the use of sputtered ions as a measure of flux composition (the process called SIMS), since sputtered ion yields are seriously affected by matrix composition. Neutral panicles are ionized by a separate process after sputter atomization, and SNMS quantitation is thus independent of the matrix. Also, since the sputtering and ionization processes are separate, an ionization process can be selected that provides relatively uniform yields for essentially all elements. 

Another important characteristic is that ion beams can produce a variety of the secondary particles/photons such as secondary ions/atoms, electrons, positrons. X-rays, gamma rays, and so on, which enable us to use ion beams as analytical probes. Ion beam analyses are characterized by the respectively detected secondary species, such as secondary ion mass spectrometry (SIMS), sputtered neutral mass spectrometry (SNMS), electron spectroscopy, particle-induced X-ray emission (PIXE), nuclear reaction analyses (NRA), positron emission tomography (PET), and so on. 

Resonance ionization mass spectrometry as a combination of resonance laser ionization with mass spectrometry can be performed on gas atoms only. Therefore, in RIMS of solid samples, before resonance ionization, a neutral gas has to be produced using several methods known from solid state mass spectrometry. During the evaporation of solid material, e.g., by laser evaporation, thermal evaporation or by sputtering with a primary ion beam, the formation of ions should to be avoided. In RIMS, mostly the thermal evaporation of sample from a heated W or Re filament is applied. 

Resonant and non-resonant laser post-ionization of sputtered uranium atoms using SIRIS (sputtered initited resonance ionization spectroscopy) and SNMS (secondary neutral mass spectrometry) in one instrument for the characterization of sub-pm sized single microparticles was suggested by Erdmann et al.94 Resonant ionization mass spectrometry allows a selective and sensitive isotope analysis without isobaric interferences as demonstrated for the ultratrace analysis of plutonium from bulk samples.94 Unfortunately, no instrumental equipment combining both techniques is commercially available. 

The closely allied topics of secondary neutral mass spectrometry (SNMS), fast atom bombardment (FAB), and laser ablation SIMS are important, but are beyond the scope of this chapter. SNMS is a technique in which neutral atoms or molecules, sputtered by an ion beam, are ionized in an effort to improve sensitivity and to decouple ion formation from matrix chemical properties, making quantification easier. This ionization is commonly effected by electron beams or lasers. FAB uses a neutral atom beam to create ions on the surface. It is often useful for insulator analysis. Laser ablation creates ions in either resonant or nonresonant modes and can be quite sensitive and complex. 

In fast-atom bombardment mass spectrometry (FAB) primary neutral atoms of energies 3—10 keV are used to sputter clusters and molecular ions from the sample. 

There are numerous other ionization methods, but they have limited applications. Fast atom bombardment (FAB), also known as liquid secondary ion mass spectrometry (LSIMS), was one of the early methods developed for the ionization of polar molecules. FAB is based on bombarding analytes in a matrix of low volatility, such as glycerol, with accelerated energetic neutral atoms (argon or xenon) or ions (cesium) that wiU sputter [M + H]+ ions from the surface. Although of major importance during its heyday, FAB has been superseded by ESI. 

Figure 5.13 Sketch of a fast atom bombardment (FAB) ion source. The potential difference between the probe tip and extraction grid (not shown) accelerates the ions (from both the Uquid matrix and analyte) into the mass spectrometer. Since a large quantity of neutral matrix is also sputtered by the fast primary beam, the source region must be provided with adequate pumping and the m/z analyzer region differentially pumped. The fast atom beam can be replaced by a beam of fast primary ions (often Cs ) and the technique is then sometimes referred to as Uquid assisted secondary ion mass spectrometry (LSIMS).

FIGURE 40.18 Depth profiles by laser secondary neutral mass spectrometry (laser SNMS), secondary ion mass spectrometry (SIMS) with Ar and 02 primary ions, and Auger electron spectroscopy (AES) of implanted boron. Reprinted from Higashi, Y., Quantitative depth profiling by laser-ionization sputtered neutral mass spectrometry (1999) Spectrochimica Acta Part B Atomic Spectroscopy, 54(1), 109-122. Copyright (1999), with permission from Elsevier Science. 

Higashi, Y. (1999) Quantitative depth profiling by laser-ionization sputtered neutral mass spectrometry. Spectrochimica Acta Part B Atomic Spectroscopy, 54, 109-122. 

Secondary Neutral Mass Spectrometry and Glow Discharge Mass Spectrometry. SNMS and GDMS are group>ed together because of the close similarities in principle between them. In both techniques the surface to be analyzed is sputtered and the contribution to the sputtered flux of secondary neutrals is measured. They differ in the methods used to measure the secondary neutral flux. In GDMS, as in GDOS, atoms are sputtered... 

Bombardment of a surface of interest with a beam of primary projectiles (e.g. ions, electrons, atoms, etc.) results in the sputtering of material characteristic of that surface (see Figure 11.7). A small fraction of this sputtered material will be positively or negatively ionised species, termed secondary ions, the majority being neutral species and electrons. With the aid of purpose-designed ion-extraction optics, these secondary ions can be extracted and mass-analysed to yield a mass spectrum of the surface. This characterisation technique is known as secondary ion mass spectrometry, SIMS [4-6]. The detection of sputtered material is not limited to ionised species. Recent instrumentation developments [7-9] have shown that sputtered neutral species can also be... 

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