Tofsims Spectra and Secondary Ion Images Chemical Identification

TOF-SIMS produces mass spectra of positive and negative secondary ions from the outer few nanometers of material surfaces under static conditions. Secondary ions, including molecular ions, suggest the chemical structures and components of sample surfaces with a high mass resolution of over several 1000 atomic mass units (amu). In the case of an analysis of organic materials such as proteins, however, the organic analyte molecule cation yield is low. In order to achieve high yields oflarge molecular ions in SIMS with proteins, cationization of sputtered particles by means of complex-ation with substrate atoms is often used (Benninghoven et al. 1993; Wojciechowski et al. 2001). In these analyses, an organic analyte is deposited onto a clean metal surface, such as silver.

Chemical Imaging

The chemical components, elements, and molecules of a sample can be mapped by focusing the primary gallium ion beam to a diameter of 50 nm (the gold cluster ions and bismuth cluster ions may have the same spatial resolution; Winograd 1993) and rastering the sample surface as shown in Fig. 3. In order to obtain meaningful chemical images it is important to

Fig. 3. Schematic of TOF-SIMS imaging

Fig. 4. TOF-SIMS images of chromium positive ion (left) and silicon positive ion (right) on a patterned glass plate

Fig. 4. TOF-SIMS images of chromium positive ion (left) and silicon positive ion (right) on a patterned glass plate ensure a static condition by estimating the primary ion dose (Marletta et al. 1990); this will allow observation of the outer sample surfaces. Figure 4 shows some examples of secondary ion images obtained using chromium and silicon ions on a microscale patterned glass plate, on which the letters are patterned with chrome. These TOF-SIMS images were obtained with a gallium ion source.

Quantification

Quantitative analysis with TOF-SIMS requires very careful handling, as is the case with other SIMS applications, because the secondary ion yield depends on several sensitive factors, such as the sample matrix and elements involved, which are not directly proportional to the concentration of the components in the samples (Belu et al. 2003; Benninghoven et al. 1987).

The matrix effect is one of the most important factors in quantitative analysis with TOF-SIMS. The variability of the ion yield is constrained by changes in the surface composition. An identical substance does not have the same secondary ion yield in a different chemical environment, so simple comparisons among TOF-SIMS spectra are very difficult to carry out.

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