Deposition primary steps

Chemical Characteristics. Because etching or deposition processes are merely chemical reactions that yield a volatile or involatile product, respectively, the overall process can be broken down into the following six primary steps ... 

The carbosilanes described in Section II,A, 1 and 2 are formed from Si(CH3)4 and the methyl chlorosilanes by gas phase thermal decomposition at 700°C. Under these conditions it is reasonable to assume that radical mechanisms are operative. Kinetic measurements of the thermal decomposition of Si(CH3)4 in a static system have shown (68) that, on heating to 700°C in the gas phase for several hours, hydrogen and methane are formed while Si and C are deposited. The primary step in the decomposition is assumed to be ... 

Very limited mechanistic studies of these reactions indicate that electron impact induced processes, such as dissociation and ionization, are the primary steps This conclusion is likely to apply to most systems since the contribution of uncharged radicals to the overall reaction rates is small in weak discharges at low temperatures However, caution is advisable in considering any mechanistic interpretations of such complicated processes. For example, more recent results show that ion- and electron-impact induced process taking place on the surface of the deposit are those actually controlling the deposition rate of microcrystalline silicon 55.i59.i60)... 

The room-temperature mercury 6( Pj) photosensitization of disilane produces H2, SiH4, SisHs, Si4Hjo and a solid deposit. Mechanistic studies show that the sole primary step is ... 

A well-known practical example of homoepitaxy is the deposition, by CVD vapor phase epitaxy, of a single crystal Si film on a Si wafer. This is one of the primary steps in the manufacture of bipolar transistors and some metal-oxide semiconductor (MOS) transistors. Explain why an epitaxial Si film on Si is deposited while the Si substrate alone could well suffice as the material for the required electronic and mechanical response ... 

Although surface-cooled types of MSMPR crystalhzers are available, most users prefer crystallizers employing vaporization of solvents or of refrigerants. The primary reason for this preference is that heat transferred through the critical supersaturating step is through a boil-ing-hquid-gas surface, avoiding the troublesome solid deposits that can form on a metal heat-transfer surface. 

The primary goal of the researchers has been to produce Q-dots possessing all of the attributes of the Q-dots prepared using liquid-phase synthetic methods (that is adjustability of the nanocrystal identity and diameter and size monodispersity) and also the technological utility of Q-dots prepared by MBE (specifically, the deposition of nanocrystals with a defined orientation and an electrical output contact). It was shown that the E/C-synthesized 5-CuI and CdS Q-dots were indeed epitaxial with narrow size distribution and strong photoluminescence tunable by the particle size. Qne of the advantages of the E/C method is that it can be made size selective. The key point is that the size as well as the size dispersion of product nanoparticles are directed actually by the corresponding properties of the metal nanoparticles therefore the first deposition step assumes special importance. 

These primary electrochemical steps may take place at values of potential below the eqnilibrinm potential of the basic reaction. Thns, in a solntion not yet satnrated with dissolved hydrogen, hydrogen molecnles can form even at potentials more positive than the eqnilibrinm potential of the hydrogen electrode at 1 atm of hydrogen pressnre. Becanse of their energy of chemical interaction with the snbstrate, metal adatoms can be prodnced cathodically even at potentials more positive than the eqnilibrinm potential of a given metal-electrolyte system. This process is called the underpotential deposition of metals. 

The first step in the preparation of Pt-group metals is the dissolution of the raw materials (primary deposits, ores, metal scraps, supported catalysts, etc.). According to the characteristics of their content in the different mixed noble metals, different dissolution methods are used. Typical processes are dissolution in... 

Nucleation of the chalcogenide is much simpler in this process, since a solid phase—the metal hydroxide (or other solid phase)—is already present and the process proceeds by a substitution reaction on that solid phase. In this case, the initial step in the deposition is adhesion of the hydroxide to the substrate. This hydroxide is then converted into, e.g., CdS, forming a primary deposit of CdS clusters. More Cd(OFI)2 and, as the reaction proceeds, CdS and partially converted hydroxide diffuses/convects to the substrate, where it may stick, either to uncovered substrate (in the early stages of deposition) or to already deposited material. This is essentially the same process as aggregation, described in Chap-... 

For nearly half a century the elastic fibre has been considered a primary site of vascular calcification326, 334 336), Calcium deposits in the arterial wall commonly take the form of phosphate salts which are intimately intermeshed with the protein of the elastic fibre327. Much effort has been directed toward identifying the mechanism for the initiation of focal calcification. It has been proposed that the initial step is ionic calcium binding to the uncharged peptide carbonyl oxygens of the elastic fibre protein328, 329. ... 

The primary source of aluminum is the mineral bauxite, which is found in large deposits in Australia, Jamaica, and Sumatra. Transportation costs are high, and many steps are required to process the ore. Because bauxite ore is usually contaminated with the oxides of iron, titanium, and silicon, pretreatment is required before extraction of the metal can begin. The crude bauxite must be crushed, washed, and dried in a kiln to free it from silicate clays, which interfere with later purification steps. 

Fractionation and Chemistry of Citrus Pectic Polysaccharides. Pectic polysaccharides, commonly known as pectin, appear early in plant cell-wall formation. A series of complex biochemical steps results in the formation of cell plates followed first by its growth in area (primary cell wall) then in thickness (secondary cell wa.ll). Exclusive of randomly oriented cellulose fibrils, primary cell wall is composed mainly of pectic polysaccharides (34). These pectic polysaccharides are rich in D-galacturonic acid, D-galactose and L-arabinose residues. With growth in thickness of cell wall (secondary cell wall),there appears to be a replacement of pectic polysaccharide deposition with polysaccharides rich in D-glucuronic acid or 4-0-methyl-D-glucuronic acid,... 

The next step in the experiment will be to incorporate mass analysis of material sputtered from the primary surface in order to reject neutrals and to be more selective in what is deposited on the secondary surface. It is hoped that catalytically useful materials, such as mass-selected small metal clusters, may eventually be deposited on surfaces. Furthermore, it may be possible to transfer reactive organic species (such as those in Table IV) to create new materials through control of the potential between the two surfaces. 

---