Secondary Ion Mass Spectroscopy (SIMS) is an analytical technique that uses ion beams. There are two basic types, dynamic and static; these differ only in the intensity of the incoming ion beam. The beam itself consists ions of argon, nitrogen, oxygen or cesium. After being generated, the beam is focused to a point varying in size from a few µm's to about a mm. The beam can be rastered (swept) across the sample to analyze larger areas. When the beam hits the sample, fragments of the sample are knocked loose; these fragments may be molecules, neutral atoms or ions. All but the ions are rejected; the positive or negative ones are selected and then sorted according to their mass/charge ratio. A spectrum is then built up showing the frequency of ions at each mass/charge ratio.
Unfortunately the mass/charge
ratio is not a unique characteristic. For example one ionized
atom of silicon has the same ratio as a singly-ionized molecule
of nitrogen or C2H4. A
singly ionized molecule of SiCl has the same ratio as copper.
Fortunately there is more than one ion produced for each material. Each isotope has its own peak in the chart. For example silicon has a peak at AMU 28, 29 and 30. Since each element has a specific pattern of isotopes, the ratio of peak heights can be used to identify a specific element.
As mentioned above, there are 2 types of SIMS. Dynamic SIMS (D-SIMS) is used mainly for inorganic materials with layers to be analyzed that are at least a few nm's thick. The ion beam current is typically a few nA. Static SIMS (S-SIMS) by contrast uses a beam of a few pA. It is used for very thin layers or very delicate materials, especially organic compounds. Because of the relatively delicate nature of the beam, organic compounds come off in molecular fragments rather than just the atoms. Thus organic compounds, or at least organic families, even in very thin layers can be identified. Time - of - flight SIMS (TOF-SIMS) is a cross between D - and S-SIMS using a field emission metal ion beam (usually Cs) for good areal resolution, TOF sorting for very high mass resolution and moderately weak ion currents for near S-SIMS sputter rates and ability to analyze organic materials. Its weakness is its strength - it generates too much data for many applications and so must be used judiciously.
SIMS is a very sensitive technique, with detection limits often in the ppm range, sometimes in ppb. It is not a particularly quantitative technique except in very special circumstances. It does provide useful relative information, that is, area A has 2.5 times as much of compound X than does area B, for example.
There are 2 types of SIMS analysis as with most surface analysis methods, the survey and the profile. In a survey, a whole range of masses is scanned to see what materials are present in a given region of a surface. This determines whatever elements and molecules are present. A profile is done with more rapid sputtering with certain specific ions selected. These will be ions determined by the surveys. The profile then shows how the composition of these specific ions varies with depth. By including one or more of the major elements of the structure one can determine where one layer stops and the next one starts. This makes it possible to determine where concentrations of the elements of interest are located vertically in the structure. The picture at the top of the page is a depth profile through a complex repeating structure.