ToFSIMS

LIMA is a similar technique to ToFSIMS except that the ionising primary beam is a laser. One of the main benefits of LIMA is that it has a small analysis area which allows the study of small particles on or in sample surfaces. The data are obtained quickly and from a deeper depth ( 1 mm) than SIMS. However, because the laser/surface interaction can be damaging, only elemental and simple molecular information is available. 

LIMA is a good technique for rapid micro-point analysis of any material and can be used to examine buried features. It is especially sensitive to low-mass elements which makes LIMA a highly complementary technique to SEM/energy dispersive analysis using X-rays. 

---

Characterization of films of organofunctional silanes by TOFSIMS and XPS. Part I. Films of A/-(2-(vinylbenzylamino)-ethyl]-3-aminopropyltri-methoxysilane on zinc and y-aminopropyltriethoxysilane on steel substrates... 

In this series of papers we will report on the use of TOFSIMS for the characterization of films of a number of commonly used silanes deposited on various substrates, mainly metals. The background of this interest in metals is the possible application of silanes as corrosion-inhibiting metal pretreatments. 

In this paper we report on TOFSIMS and XPS analyses of thick and thin films of SAAPS on zinc in order to demonstrate the effect of film thickness. We also present some TOFSIMS results obtained with films of y-APS on mild steel in which the effects of the cleaning process of the substrate and the pH of the silane solution are demonstrated. 

TOFSIMS analyses were performed on a Kratos PRISM instrument. It was equipped with a reflectron-type time-of-flight mass analyzer and a pulsed 25 kV liquid metal ion source of monoisotopic 69Ga ions with a minimum beam size of 500 A. Positive and negative spectra were obtained at a primary energy of 25 keV, a pulse width of 10-50 ns, and a total integrated ion dose of about 10" ions/cm2. This is well below the generally accepted upper limit of 5 x 1012 ions/cm2 for static SIMS conditions in the analysis of organic materials [12], The mass resolution at mass 50 amu varied from M/AM= 1000 at 50 ns pulse width to about 2500 at 10 ns pulse width. 

XPS and TOFSIMS analyses of SAAPS films on zinc substrates... 

TOFSIMS analysis. The positive and negative TOFSEMS spectra of the same films are shown in Fig. 2. It should be noted that these were acquired in the low mass resolution mode, hence the peak identification is not completely unequivocal in some cases. 

Several highly characteristic peaks are observed in Fig. 2. The compositions of till major peaks are given in Table 2. These are based on their exact mass determination, which is accurate to within 0.01 amu. This is possible in a TOFSIMS instrument because of its higher mass resolution as compared with quadrupole instruments. Therefore, peak identities can be determined with higher certainty than in quadrupole instruments which have only unit mass resolution. 

The TOFSIMS results thus indicate that the surface of the silane film consists mainly of a random mixture of partly oxidized vinylbenzyl groups and the silane end of the molecules. The silanol groups have formed siloxane groups to some extent and some crosslinking of the molecules has occurred via the vinyl groups. 

Figure 2. Positive (a, b) and negative (c) TOFSIMS spectra of the S A APS film of Fig. 1. before heating.

The elemental and binding state information obtained from the XPS analysis is in good agreement with, and sometimes complementary to, the information from the TOFSIMS spectra. The information that can be extracted from the SIMS spectra is, however, more detailed in terms of the structure of surface molecules and is also more surface-sensitive. For instance, the high degree of aromaticity in the surface of the film, as seen very clearly in the SIMS spectra (e.g. at +117 amu), can be confirmed only with difficulty by XPS, because the only feature indicating aromaticity is the ji-ji transition, which is not clearly seen in Fig. 1. 

High-resolution TOFSIMS. The above experiment was repeated with a very thin film of SAAPS on zinc by decreasing the time of deposition. Since in TOFSIMS the sampling depth is of the order of two molecular layers [12], the spectra obtained from thick films as described above can be expected to be largely independent of the substrate. Orientation effects as a result of different acid—base properties of the substrate can therefore be studied only with very thin films. 

Figure 4. High-resolution positive (a) and negative (b) TOFSIMS spectra of a thin film of SAAPS deposited on an alkaline-deaned zinc substrate. For conditions see text.

As an interim conclusion from the data presented so far, it can be stated that by TOFSIMS a description can be given, albeit in a qualitative mode only, of the structure of the various molecules adsorbed on a metallic substrate. This information is totally different from that available from XPS analyses and is therefore highly complementary. 

Alkaline-cleaned substrate. After alkaline cleaning, the steel substrate was completely devoid of aliphatic fatty acids, as was verified by TOFSIMS. The freshly cleaned substrate was immediately immersed in the 1% silane solution and analyzed. The wettability of the cleaned steel by the solution was considerably improved by the alkaline cleaning procedure. The spectra obtained with a film deposited from a pH 10.5 solution are shown in Fig. 7. A rigorous peak identification procedure was applied here. The masses of all major peaks could be... 

Static SIMS, especially high-mass-resolution TOFSIMS, is a powerful tool for the study of silane films. The accurate mass determination capability of high-mass-resolution SIMS analysis of organic materials has demonstrated that many peaks from organic materials consist of two or more components. Hence ion identification in SIMS with low mass resolution may be ambiguous. 

The nature of the film formed by y-APS on steel is strongly dependent on the pretreatment of the steel surface. This in itself is not surprising and has been documented in the literature. However, the TOFSIMS delta indicate that methanol-cleaned, mechanically polished steel is passivated because it has formed iron soaps by reaction with fatty acids in the polishing process. 

The accurate mass determination capability of TOFSIMS indicated that the ions formed from y-APS sometimes have an unexpected composition. For instance, many odd-numbered N-containing ions were positively detected. It thus seems that ion formation from such highly oriented monomeric molecules is different from that of nonoriented polymeric surfaces. 

For a very thorough web presentation regarding SIMS see (a) http //www.eaglabs.com/ en-US/presentations/TOFSIMS/Rresentation Files/index.html (b) http //www.eaglabs.com/en-US/ research/research.html (other links to SIMS theory, applications, presentations)... 

Time-of-flight SIMS TOFSIMS SNMS also matrix effects 28... 

The type of information provided by the techniques listed in the tables also varies greatly. Many spectroscopic techniques give qualitative and/or quantitative elemental composition. The vibrational techniques, however, generally provide information on the molecular structure. SIMS, especially in the static mode (SSIMS or TOFSIMS), can yield information on molecular structures and even orientation of monolayers [5-10]. This is particularly useful for the study of the absorption of coupling agents on metals or to determine the effects of plasma treatments on polymer surfaces [11]. TOFSIMS instruments also have capabilities for determining the two-dimensional distributions of elements or molecular species at the surface, similar to the capabilities (for elements only) offered by AES and EDXA or WDXA. 

TOFSIMS is rapidly gaining popularity as a tool for studying chemistry and orientation at organic surfaces, sueh as polymers and polymer blends. The problem of surface... 

In the example shown in Fig. 7, a thin film of plasma-polymerized trimethylsilane had been deposited on cold-rolled steel as a pretreatment for improved adhesion and corrosion [30]. The film thickness was determined by ellipsometry to be 500 A. The composition was characterized by AES, XPS, and TOFSIMS. AES gave information on the bulk composition, surface enrichment, and interfacial oxide (Fig. 7b). Note that the C/Si ratio of the bulk of the film, after equilibrium sputtering conditions have been reached, is approximately 3, i.e., identical to that of the monomer from which the film was... 

Table 3 TOFSIMS Peak Identification of Silane Film on Steel... 

The final example is a combined application of TOFSIMS and XPS, which was used to characterize the interface between a metal and a polymer system [33]. The... 

X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToFSIMS) are well established methods for the chenodcal characterisation of surfaces. The techniques combine well, where the quantitative elemental and chemical state information provided by XPS compliments the chemical and molecular structure information provided by ToFSIMS. The techniques probe only from the outermost few nanometres, which define and control the surface properties of many materials. 

---