SSIMS

The study of surfaces of catalytic interest was one of the earliest applications of static SIMS, and Vickerman et al. (2000) have recently reviewed recent applications of the technique in this area. As an example of this 

These catalysts are able to oxidize simulataneously the unburnt hydrocarbons and carbon monoxide to H20 and C02, and to reduce N02 emissions to N2. They 

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Together with XPS and AES, SSIMS ranks as one of the principal surface analytical techniques. Because its sensitivity for elements greatly exceeds that of the other two techniques and much chemical information is available, its use is rapidly expanding in many fields of application. 

The instrumentation for SSIMS can be divided into two parts (a) the primary ion source in which the primary ions are generated, transported, and focused towards the sample and (b) the mass analyzer in which sputtered secondary ions are extracted, mass separated, and detected. 

Several ion sources are particularly suited for SSIMS. The first produces positive noble gas ions (usually argon) either by electron impact (El) or in a plasma created by a discharge (see Fig. 3.18 in Sect. 3.2.2.). The ions are then extracted from the source region, accelerated to the chosen energy, and focused in an electrostatic ion-optical column. More recently it has been shown that the use of primary polyatomic ions, e. g. SF5, created in FI sources, can enhance the molecular secondary ion yield by several magnitudes [3.4, 3.5]. 

Fig. 3.3. Experimental arrangement used by Krauss and Gruen for SSIMS [3.8] a qua-drupole mass spectrometer was used for mass analysis and a retarding-field analyzer for prior energy selection (a) ion gun (b)-(d) lenses 1-3 (e) quadrupole mass spec-...

Fig. 3.4. Schematic diagram ofthe imaging time-of-flight SSIMS system used at the University of Munster, Germany.

With such aTOF-imaging SSIMS instrument, the useful mass range is extended beyond 10000 amu the mass resolution, m/Am, is -10000 with simultaneous detection of all masses and within each image, all masses can be detected. The number of data generated in a short time is enormous, and very sophisticated data acquisition systems are required to handle and process the data. 

A SSIMS spectrum, like any other mass spectrum, consists of a series of peaks of dif ferent intensity (i. e. ion current) occurring at certain mass numbers. The masses can be allocated on the basis of atomic or molecular mass-to-charge ratio. Many of the more prominent secondary ions from metal and semiconductor surfaces are singly charged atomic ions, which makes allocation of mass numbers slightly easier. Masses can be identified as arising either from the substrate material itself from deliberately introduced molecular or other species on the surface, or from contaminations and impurities on the surface. Complications in allocation often arise from isotopic effects. Although some elements have only one principal isotope, for many others the natural isotopic abundance can make identification difficult. 

Figure 3.5 shows the positive SSIMS spectrum from a silicon wafer, illustrating both the allocation of peaks and potential isobaric problems. SSIMS reveals many impurities on the surface, particularly hydrocarbons, for which it is especially sensitive. The spectrum also demonstrates reduction of isobaric interference by high-mass resolution. For reasons discussed in Sect. 3.1.3, the peak heights cannot be taken to be directly proportional to the concentrations on the surface, and standards must be used to quantify trace elements. 

The relationship between what is recorded in a SSIMS spectrum and the chemical state of the surface is not as straightforward as in XPS and AES (Chap. 2). Because of the large number of molecular ions that occur in any SSIMS spectrum from a multi-component surface (e. g. during the study of a surface reaction), much chemical information is obviously available in SSIMS, potentially more than in XPS. The problem in using the information from a molecular ion lies in the uncertainty of knowing whether or not the molecule represents the surface composition. For some materials. 

Fig. 3.7. SSIMS spectra after interaction ofoxy- bed on silver, at 400 K [3.16] (A) positive SSIMS gen with Haifa monolayer of potassium adsor- spectrum (B) negative SSIMS spectrum.

In conclusion, SSIMS spectra provide not only evidence of all the elements present, but also detailed insight into molecular composition. Quasimolecular ions can be desorbed intact up to 15000 amu, depending on the particular molecule [3.17] and on whether an effective mechanism of ionization is present. Larger molecules can be identified from fragment peak patterns which are characteristic of the particular molecules. If the identity of the material being analyzed is completely unknown, spectral interpretation can be accomplished by comparing the major peaks in the spectrum with those in a library of standard spectra. 

Compared with XPS and AES, the higher surface specificity of SSIMS (1-2 mono-layers compared with 2-8 monolayers) can be useful for more precise determination of the chemistry of an outer surface. Although from details of the 01s spectrum, XPS could give the information that OH and oxide were present on a surface, and from the Cls spectrum that hydrocarbons and carbides were present, only SSIMS could be used to identify the particular hydroxide or hydrocarbons. In the growth of oxide films for different purposes (e.g. passivation or anodization), such information is valuable, because it provides a guide to the quality of the film and the nature of the growth process. 

Fig. 3.8. SSIMS spectra from oxide (A) positive SSIMS spectrum, 3 s ...

Later it was found that the polluting lubricant droplets originating from the transport belts used in the production they had fallen into the paint bath and prevented adhesion of the paint to the metal. It can be concluded that the high sensitivity of SSIMS in the detection of submonolayer coverage of organic species makes it an extremely powerful tool for solving such interface problems. 

Treatment of polymer surfaces to improve their wetting, water repulsion, and adhesive properties is now a standard procedure. The treatment is designed to change the chemistry of the outermost groups in the polymer chain without affecting bulk polymer properties. Any study of the effects of treatment therefore requires a technique that is specific mostly to the outer atomic layers this is why SSIMS is extensively used in this area. 

Other SSIMS studies of polymer surfaces have included perfluorinated polyether [3.21], low-density polyethylene [3.22], poly(ethylene terephthalate) [3.23], and the oxidation of polyetheretherketone [3.24]. 

The data show that SSIMS can be used as a tool for characterizing the different steps in the production of biosensors, or even for sequencing. Similarly, SSIMS can be used to solve a variety of problems in bioanalytical chemistry, e. g. screening of combinatorial libraries, characterizing Langmuir-Blodgett layers, etc. 

Because of the inherently destructive nature of ion bombardment, the use of SSIMS alone in the study of the reactions of surfaces with gases and vapor must be viewed with caution, but in combination with other surface techniques it can provide valuable additional information. The parallel techniques are most often XPS,TDS, and LEED, and the complementary information required from SSIMS normally refers to the nature of molecules on surfaces and with which other atoms, if any, they are combined. 

A typical SSIMS spectrum of an organic molecule adsorbed on a surface is that of thiophene on ruthenium at 95 K, shown in Eig. 3.14 (from the study of Cocco and Tatarchuk [3.28]). Exposure was 0.5 Langmuir only (i.e. 5 x 10 torr s = 37 Pa s), and the principal positive ion peaks are those from ruthenium, consisting of a series of seven isotopic peaks around 102 amu. Ruthenium-thiophene complex fragments are, however, found at ca. 186 and 160 amu each has the same complicated isotopic pattern, indicating that interaction between the metal and the thiophene occurred even at 95 K. In addition, thiophene and protonated thiophene peaks are observed at 84 and 85 amu, respectively, with the implication that no dissociation of the thiophene had occurred. The smaller masses are those of hydrocarbon fragments of different chain length. 

SSIMS has also been used to study the adsorption of propene on ruthenium [3.29], the decomposition of ammonia on silicon [3.30], and the decomposition of methane thiol on nickel [3.31]. 

Fig. 3.14. Positive SSIMS spectrum from Ru (001) after exposure to 0.5 L thiophene at 95 K [3.28].

SSIMS has been used in the TOP SSIMS imaging mode to study very thin layers of organic materials [3.32-3.36], polymeric insulating materials [3.37], and carbon fiber and composite fracture surfaces [3.38]. In these studies a spatial resolution of ca. 80 nm in mass-resolved images was achieved. 

The basic instrumental set-up for dynamic SIMS is the same as for SSIMS (Sect. 3.1.2). Depending on the intensity, beam diameter, and ion species needed, dif ferent ion sources are used. Several mass analyzers with different characteristics enable a broad field of applications. 

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