Other Sputtering Models

The various forms of sputtering not described within the context of knock-on sputtering include the following  

Energetic large cluster ion impact on organic substrates, with examples of 

Energetic dense atomic and small molecular ion impact on organic and/or dense matrices, with examples of primary ion species including, but not limited to In , Ann , Bi , and SEj . 

Low-energy singly and multiply charged inert gas ion impact on nonmetallic matrices such as LiF, NaCl, AI2O3, and Si02. 

The inability of knock-on mechanisms, inclusive of the linear cascade model, to effectively predict sputter yields in the cases described earlier arises from the fact that such mechanisms describe sputter yields as arising from many individual momentum transfer processes occurring in a linear sequence. However, as outlined in Section 3.2, ejection of atoms/ions or molecules from a solid surface can also occur through  

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Molecular dynamics simulations have yielded a great deal of information about the sputtering process. First, they have demonstrated that for primary ion energies of a few keV or less, the dynamics which lead to ejection occur on a very short timescale on the order of a few hundred femtoseconds. This timescale means that the ejection process is best described as a small number of direct collisions, and rules out models which rely on many collisions, atomic vibrations and other processes to reach any type of steady state . Within this same short-timescale picture, simulations have shown that ejected substrate atoms come from very near the surface, and not from subsurface regions. 

Many attempts have been made to quantify SIMS data by using theoretical models of the ionization process. One of the early ones was the local thermal equilibrium model of Andersen and Hinthome [36-38] mentioned in the Introduction. The hypothesis for this model states that the majority of sputtered ions, atoms, molecules, and electrons are in thermal equilibrium with each other and that these equilibrium concentrations can be calculated by using the proper Saha equations. Andersen and Hinthome developed a computer model, C ARISMA, to quantify SIMS data, using these assumptions and the Saha-Eggert ionization equation [39-41]. They reported results within 10% error for most elements with the use of oxygen bombardment on mineralogical samples. Some elements such as zirconium, niobium, and molybdenum, however, were underestimated by factors of 2 to 6. With two internal standards, CARISMA calculated a plasma temperature and electron density to be used in the ionization equation. For similar matrices, temperature and pressure could be entered and the ion intensities quantified without standards. Subsequent research has shown that the temperature and electron densities derived by this method were not realistic and the establishment of a true thermal equilibrium is unlikely under SIMS ion bombardment. With too many failures in other matrices, the method has fallen into disuse. 

Following these studies, a microstructure of sputter-deposited ZnO films on polycrystalline CdS substrates is outlined in Fig. 4.21. The different evolution of the Zn 2p and O Is binding energies can consequently be attributed to the amorphous ZnO nucleation layer with a different chemical bonding between Zn and O. The model is also valid for polycrystalline In2S3 and Cu(In,Ga)Se2 substrates and for deposition of (Zn,Mg)0 films, as these show the same behavior (see Figs. 4.20 and 4.24). It is not clear whether an amorphous nucleation layer occurs also when the ZnO is deposited by other techniques as MBE, CVD, or PLD, as no data are available for such interfaces. In addition, the influence of the polycrystallinity of the substrates is not clear so far. 

The most commonly used methods for the preparation of ultrathin oxide films are (1) direct oxidation of the parent metal surface, (2) preferential oxidation of one metal of choice from a suitable binary alloy, and (3) simultaneous deposition and oxidation of a metal on a refractory metal substrate. The detailed procedures for (1) and (2) are discussed elsewhere [7,56,57] procedure (3) is discussed here in detail. Preparation of a model thin-film oxide on a refractory metal substrate (such as Mo, Re, or Ta) is usually carried out by vapor-depositing the parent metal in an oxygen environment. These substrate refractory metals are typically cleaned by repeated cycles of Ar sputtering followed by high-temperature annealing and oxygen treatment. The choice of substrate is critical because film stoichiometry and crystallinity depend on lattice mismatch and other interfacial properties. Thin films of several oxides have been prepared in our laboratories and are discussed below. 

We mention some other systems that have fractal structures. For example, using sputtering regimes that correspond to the diffusional aggregation model [82], thin films consisting of metallic fractal clusters can be obtained. Fractal structures are also characteristic of percolation clusters near the percolation threshold, as well as certain binary solutions and polymer solutions. The dielectric properties of all these systems can be predicted using the above fractal model. 

LEI has been applied successfully to the trace determination of T1 [674] for certification purposes, and for combinations with laser evaporation and all other atomization techniques represents a powerful approach to detection. Laser photoionization and galvanic detection have been applied to hollow cathode dark space diagnostics [675]. Photoionization is produced to measure the dark space widths of linear field distributions directly. A theoretical model has been developed and its predictions verified with experimental findings for a uranium hollow cathode discharge operated in neon or xenon. Variations in the ground-state densities of sputtered neutrals have also been measured. 

Hence at least one of the basic assumptions used in the set of models for quantitative XPS had to be incorrect in the present case. Most likely the XPS signals for Ti and V are attenuated by a third compound. The only other material present is silica from the support. No model covering this combination of compounds is included in the standard set. The new Ti/V/Si model had to be proven with another analytical technique. Sputtering of the possible silica overlayer in a LEIS experiment was chosen. This technique has a surface sensitivity superior to XPS, is sensitive to essentially only the topmost atomic layer, but is not capable of separating the Ti and V signals. LEIS proved that the main fraction of Ti/V was buried under a thin layer of silica. 

The view described above is exactly what would be expected if the experiment were performed under ultra-high-vacuum conditions and if cesium atoms were sputtered onto a silver surface. It is clear, however, that these conditions do not correspond to what is depicted in Fig. 37. There are important differences between the NMR experiments represented in Fig. 37 and one based on a high-vacuum Cs-sputtered system, the most obvious being the difference between cesium atoms and cesium ions. This suggests that cesium ion pairs in the experimental system represented in Fig. 37 play the role of the cesium atom at the silver surface in the model of Fig. 38. Whether these ion pairs involve oxygen atoms at the surface or anions from the impregnation solution has not been resolved. Furthermore, from the results of SEDOR [74,77] experiments, it is clear that the cesium is not directly involved with ethylene. More research is needed on this and other systems before these results are completely understood. As can be easily seen, the alkali metal salts play several roles in ethylene oxide production the picture is equally intriguing in HDS systems. 

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