Secondary ion positive

Ar, Cs, Ga or other elements with energies between 0.5 and 10 keV), energy is deposited in the surface region of the sample by a collisional cascade. Some of the energy will return to the surface and stimulate the ejection of atoms, ions and multi-atomic clusters (figure Bl.25.8). In SIMS, secondary ions (positive or negative) are detected directly with a mass spectrometer. 

What is considered as interference in LEISS can in fact be used as a basis for a direct analysis of the surface composition. Surface sputtering creates secondary ions (positive or negative). whose mass/charge ratios are characteristic of the elements that constitute the surface (Fig. 6.2). 

With a side-chain of three or more carbon atoms in the 5-position an intense peak is obtained at m-(R-14), resulting from /3 cleavage of the side-chain. Thus the highest intensity peak for 2,4-dimethyl-5-propylthiazole occurs at m/e 126 (m-29). Secondary ions have been demonstrated by Clark (115), especially at m/e 45. 

A big step forward came with the discovery that bombardment of a liquid target surface by abeam of fast atoms caused continuous desorption of ions that were characteristic of the liquid. Where this liquid consisted of a sample substance dissolved in a solvent of low volatility (a matrix), both positive and negative molecular or quasi-molecular ions characteristic of the sample were produced. The process quickly became known by the acronym FAB (fast-atom bombardment) and for its then-fabulous results on substances that had hitherto proved intractable. Later, it was found that a primary incident beam of fast ions could be used instead, and a more generally descriptive term, LSIMS (liquid secondary ion mass spectrometry) has come into use. However, note that purists still regard and refer to both FAB and LSIMS as simply facets of the original SIMS. In practice, any of the acronyms can be used, but FAB and LSIMS are more descriptive when referring to the primary atom or ion beam. 

Like the chemical composition of the primary beam, the chemical nature of the sample affects the ion yield of elements contained within it. For example, the presence of a large amount of an electronegative element like oxygen in a sample enhances the positive secondary ion yields of impurities contained in it compared to a similar sample containing less oxygen. 

Static SIMS entails the bombardment of a sample surface with an energetic beam of particles, resulting in the emission of surface atoms and clusters. These ejected species subsequendy become either positively or negatively charged and are referred to as secondary ions. The secondary ions are the actual analytical signal in SIMS. A mass spectrometer is used to separate the secondary ions with respect to their charge-to-mass ratios. The atomic ions give an elemental identification (see... 

Figure 1 shows a positive static SIMS spectrum (obtained using a quadrupole) for polyethylene over the mass range 0—200 amu. The data are plotted as secondary ion intensity on a linear y-axis as a function of their chaige-to-mass ratios (amu). This spectrum can be compared to a similar analysis from polystyrene seen in Figure 2. One can note easily the differences in fragmentation patterns between the...

Figure 5 Plot of positive CF3 secondary ion intensity versus ellipsometric thickness from a set of perfluoropolyether standards.

The use of chemical mapping is demonstrated in the following example involving the delamination of a silicone primer and polytetrafluoroethylene (PTFE) material. The positive mass spectrum acquired from the delaminated interface contains peaks known to be uniquely characteristic of PTFE (CF3 at mass 69) and the silicone primer (Si(CH3)3 at mass 73). Figures 6 and 7 are secondary ion im es of the CF3 and (Si(CH3)3 fragments taken from a 1-mm area of the delaminated interface. These maps clearly indicate that the PTFE and the silicone primer exist in well-defined and complementary areas. 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

Nearly all these techniques involve interrogation of the surface with a particle probe. The function of the probe is to excite surface atoms into states giving rise to emission of one or more of a variety of secondary particles such as electrons, photons, positive and secondary ions, and neutrals. Because the primary particles used in the probing beam can also be electrons or photons, or ions or neutrals, many separate techniques are possible, each based on a different primary-secondary particle combination. Most of these possibilities have now been established, but in fact not all the resulting techniques are of general application, some because of the restricted or specialized nature of the information obtained and others because of difficult experimental requirements. In this publication, therefore, most space is devoted to those surface analytical techniques that are widely applied and readily available commercially, whereas much briefer descriptions are given of the many others the use of which is less common but which - in appropriate circumstances, particularly in basic research - can provide vital information. 

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]. 

Positive secondary ions (SI" ) are repelled by an electrode in e-beam SNMS or completely (in DBM) or widely (in HFM, see below) re-attracted by the sample surface in HF-plasma SNMS. Ionized plasma species (Ar", contamination) are suppressed by energy filtering. 

In secondary ion mass spectrometry (SIMS), a beam of energetic primary ions is focused onto the surface of a solid. Some of the ions are reflected but most of the energy of the primary ions is dissipated in the surface by binary collisions that cause neutrals, excited neutrals, and ions (positive and negative) to be ejected or sputtered from the surface. The secondary ions can be analyzed by a mass spectrometer to provide information about the surface composition of the solid. 

The apparatus for the PFAM film coating on the slider surface is shown in Fig. 1 (a). The film thickness was measured by the TOF-SIMS as shown in Fig. 1 (b). It used a pulsed primary Ga+ ion beam to impact the surface of the PFAM film with an inset energy of 15 keV, an extractor current of 2 fj,A, beam current of 600 pA, a pulse width of 17.5 ns, and a frequency of 10 kHz, respectively. The positive TOF-SIMS spectra on the slider surface is shown in Fig. 2 where the peaks at m/z 31, 50, 69, 100, and 131 in Fig. 2(a) correspond to the positive secondary ion fragments of CF+, CFj, C2F4, and C3F5, respectively. The peak at m/z 469 apparent in Fig. 2(b) corresponds to the ion C12H7F 15O2H+ which is the characteristic ion of PFAM molecules. Therefore, the positive TOF-SIMS spectra demonstrates the existence of PFAM film [24,25]. The thickness of the PFAM film can be determined... 

Mass Spectrometry. Mass spectrometry holds great promise for low-level toxin detection. Previous studies employed electron impact (El), desorption chemical ionization (DCI), fast atom bombardment (FAB), and cesium ion liquid secondary ion mass spectrometry (LSIMS) to generate positive or negative ion mass spectra (15-17, 21-23). Firm detection limits have yet to be reported for the brevetoxins. Preliminary results from our laboratory demonstrated that levels as low as 500 ng PbTx-2 or PbTx-3 were detected by using ammonia DCI and scans of 500-1000 amu (unpublished data). We expect significant improvement by manipulation of the DCI conditions and selected monitoring of the molecular ion or the ammonia adduction. 

Imaging SIMS, used for spatially resolved elemental analysis. A focussed ion beam is rastered over the surface so that each point on the target is individually bombarded in turn, so that secondary ion emission is localized. The intensity of a particular secondary ion is monitored for each position of the primary beam and the result shown at the corresponding point of a synchronized oscilloscope display or computerized data system. In this way, pixel by pixel across the sample surface and in depth as the material is removed, three-dimensional information on the sample composition may be obtained. 

The exact mass determination of emitted secondary ions yields informations about the molecular structure of the thick film. In the low mass area the fragmentation of the quasimolecular cation m/e = 163, i.e. (EtO)3Si+ derived from the alkoxysilane monomer gives rise to the positive ions m/e = 135, 119, 107, 91, 79, 63. The resulting fragmentation pattern can be described as follows ... 

Figure 2. Proposed structures for selected positive secondary ions 1, 2, 3 and 4 (see Figure 1)...

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