Atomic SIMS, sputtering techniques

Mass-spectrometry principles and techniques have been employed in other kinds of surface studies in which sample atoms are sputtered by interaction with a laser beam or by RF glow discharges. These approaches are more highly specialized, but it should be clear that mass spectrometry is an important tool in surface chemistry. The student should compare SIMS and ISS with other surface analytical techniques such as ESCA, Auger spectroscopy, electron microprobe, and low-energy electron diffraction (see Chaps. 14 and 15). 

Compensation of Preferential Sputtering. The species with the lower sputter yield is enriched at the surface. This effect is called preferential sputtering and complicates, e. g.. Auger measurements. The enrichment compensates for the different sputter yields of the compound or alloy elements thus in dynamic SIMS (and other dynamic techniques in which the signal is derived from the sputtered particles, e.g. SNMS, GD-MS, and GD-OES), the flux of sputtered atoms has the same composition as the sample. 

Since ion beams (like electron beams) can be readily focussed and deflected on a sample so that chemical composition imaging is possible. The sputtered particles largely originate from the top one or two atom layers of a surface, so that SIMS is a surface specific technique and it provides information on a depth scale comparable with other surface spectroscopies. 

SIMS is by far the most sensitive surface technique, but also the most difficult one to quantify. SIMS is very popular in materials research for making concentration depth profiles and chemical maps of the surface. The principle of SIMS is conceptually simple A primary ion beam (Ar+, 0.5-5 keV) is used to sputter atoms, ions and molecular fragments from the surface which are consequently analyzed with a mass spectrometer. It is as if one scratches some material from the surface and puts it in a mass spectrometer to see what elements are present. However, the theory behind SIMS is far from simple. In particular the formation of ions upon sputtering in or near the surface is hardly understood. The interested reader will find a wealth of information on SIMS in the books by Benninghoven et al. [2J and Vickerman el al. [4], while many applications have been described by Briggs et al. [5]. 

In secondary-ion mass spectrometery (SIMS) and its sister technique fast atom bombardment mass spectrometry (FARMS), a surface is bombarded with energetic particles, and the kinetic energy of the particles converts substrate and chemisorbed atoms and molecules to gas-phase species. The ejected (or sputtered) material is subsequently interrogated using various analytical tools, such as lasers and mass spectrometers, to indirectly deduce information about the initial surface. The relationships between sputtered material and the surface, however, are not always clear, and erroneous conclusions are easily made. Computer simulations have demonstrated that a fundamental understanding of the sputtering process is required to interpret experimental data fully ... 

Quantitative depth profiling using polyatomic MCs+ and MCs2+ ions instead of atomic ions M= ions is well established in surface analysis using SIMS. The MCs+ technique, which reduces matrix effects significantly, was proposed by Gao in 1988.100 The formation of MCs+ has been explained by the recombination of sputtered neutral atoms (M) with... 

On the other hand, SIMS takes advantage of the destructive nature of the ion probe. Atoms can be knocked free (sputtered) from the surface by the bombarding ions and those that become ionized are analyzed by conventional mass spectrometry I70). A large number of different kinds of ions can be emitted from the surface. The resolution is also quite good. Thus, although SIMS is not as surface sensitive as ISS, it does provide more detailed information about the surface chemistry. ISS and SIMS, therefore, complement one another. Furthermore, since the ion probe sputters away the surface that is being analyzed, the change in the chemistry of the surface as a function of depth below the surface can be studied by these techniques. 

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. 

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