Sputtering, SIMS characteristics

The ion microprobe was early recognized [2-4] to be a potential chemical microscope for many elements, and qualitative analyses rapidly became routine, especially in the study of metals and doped semiconductors. The ion microprobe uses a focused primary ion beam (O , 02, Cs ) to sputter a small volume of material from a target (usually a solid sample). A fraction of the sputtered atoms, characteristic of the surface composition of the sample, are ionized these secondary ions are the source of information in secondary ion mass spectrometry (SIMS) (Figure 43.1). 

As evident from Scheme 7.13, most modern ionisation techniques have been used for TLC-MS, and no single ionisation method is used exclusively with TLC-MS. Various ionisation methods may be applied that avoid the need to evaporate the sample into an El or Cl source these are based in particular on sputtering (FAB, SIMS) or laser desorption. Several sputtering methods of ionisation do not require the use of a liquid matrix, e.g. TLC-SIMS [797], Recent developments include the use of matrix-assisted laser desorption ionisation (MALDI) and surface-assisted laser desorption ionisation (SALDI). It is obvious that TLC-MS is complemented with TLC-MS11 [800] and TLC-HRMS techniques. Table 7.82 lists the general characteristics of TLC-MS. 

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. 

The single most unique characteristic of the SIMS technique is its sensitivity. It can be as good as one part per billion (ppb). For example, if silicon is sputtered at a rate of 10 A/sec over an area of 100-pm x 100-/zm, then 10"u cm3/sec of material is removed. Given the density of silicon, this reduces to approximately 5 x 1011 atoms/sec. If 1% of these atoms are ionized (by charge transfer with the surface) and 10% of those ionized are collected in the mass spectrometer, then the measured ion intensity will be 5 x 10s ions/sec. If we assume we can distinguish 5 ions/sec, then a detection sensitivity of 1 part in 108 is achievable. This sensitivity is many orders of magnitude better than other techniques. 

Unger et al. (36) were the first to describe direct TLC analysis by secondary ion mass spectrometry without the interdiction of an extraction solvent. Muscarine (a quaternary alkaloid from mushrooms with a high secondary ion yield) could be sputtered directly from a cellulose TLC matrix (Figure 5). The experiment is based on the relatively low secondary ion yield of the matrix on bombardment by the primary ion beam, and the characteristic signal for the intact cation of the muscarine at m/z 174. The total amount of muscarine present in the TLC spot was about 16 micrograms. More recent SIMS experiments use the liquid matrix typical of FAB experiments, and thus involve an extraction of the sample from the matrix. 

A characteristic of sputtering in SIMS is the formation of molecular ions. Several atoms can leave the surface almost simultaneously and can leave the surface as a bound diatomic cluster. The secondary ions can combine with residual vacuum species such as C, H, and O, with the primary beam ions, and with other ions from the sample. If clusters or molecular ions are detected, it does not mean that they were nearest neighbors in the analyzed surface [5]. 

Sputtering a sample yields secraidary irais which ccmtain analytical information characteristic of the sample. Several types of SIMS instmments are available to perform the analytical measurements. The basic components are a primary imi source, a sample chamber, a mass spectrometer, a secondary ion detectiOTi system, a vacuum system, and a data acquisition system as illustrated in Fig. 4.3. There are many variations and combinations of the basic components and it is beymid the scope of this chapter to comprehensively cover all instrumentation so only some general and more conmuMi types will be discussed. An in-depth discussion on SIMS instrumentation is provided in the very comprehensive SIMS reference by Benninghoven et al. [10] and in the instrumentation chapter of reference [11]. As SIMS has expanded into more areas of analysis, it has become more difficult to have one instrument to perform all types of analysis so instrument development has trended toward dedication to specific applications [12]. 

Principles and Characteristics The twin techniques of secondary neutral mass spectrometry (or sputtered neutral mass spectrometry, SNMS) and SIMS, which share bombardment of the sample surface with a focused primary ion beam (Ar+, Cs+, Ga+, O2) of sufficiently high ion energy (some keV), are among the most powerful surface analytical techniques for compositional characterisation of surfaces. As in SIMS, in SNMS the implanted primary ions penetrate into the solid surface to different depths (1-10 nm) and transfer their kinetic energy as a function of the sample material, primary ion energy and mass. Whereas SIMS detects the directly emitted secondary ions, in SNMS the secondary sputtered ions are suppressed by a... 

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... 

The SIMS technique, as originally developed, was limited to relatively low molecular weights (m/z < 500 Da) due to limitations of the quadrupole mass analyzer. Latter, the development of TOF-SIMS extended the mass range to 10,000 Da. The problem with SIMS is that organic compounds often yield both intense structurally characteristic secondary ions and abundant nonspecific secondary ions. Quantitative analysis with SIMS is difficult mainly because of ion optical and matrix effects. With inorganic samples, uses of internal standards are required. For organic systems, factors such as sample/matrix evaporation, sputtering, and irreversible chemical reaction need to be determined. SIMS is always a surface technique [14]. 

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