Precursor species

Plasmas can be used in CVD reactors to activate and partially decompose the precursor species and perhaps form new chemical species. This allows deposition at a temperature lower than thermal CVD. The process is called plasma-enhanced CVD (PECVD) (12). The plasmas are generated by direct-current, radio-frequency (r-f), or electron-cyclotron-resonance (ECR) techniques. Eigure 15 shows a parallel-plate CVD reactor that uses r-f power to generate the plasma. This type of PECVD reactor is in common use in the semiconductor industry to deposit siUcon nitride, Si N and glass (PSG) encapsulating layers a few micrometers-thick at deposition rates of 5—100 nm /min. 

Many materials have been deposited by PECVD. Typically, the use of a plasma allows equivalent-quaUty films to be deposited at temperatures several hundred degrees centigrade lower than those needed for thermal CVD techniques. Often, the plasma-enhanced techniques give amorphous films and films containing incompletely decomposed precursor species such as amorphous siUcon (i -Si H) and amorphous boron (i -B H). 

These reactions proceed at lower temperature (250-750°C) than those based on the methyl-radical mechanism reviewed above. The halogen reaction mechanism is still controversial and the optimum precursor species are yet to be determined.P9] To proceed, the reactions must be highly favored thermodynamically. This is achieved when the reaction products are solid carbon and stable gaseous fluorides or chlorides (HF, HCl, SFg). 

The catalysts had to be free of residual impurities such as chloride ions, nitrates, solvent, or other precursor species. 

Silver salts are also employed to create more effective chiral catalysts by exchange of counter anions. For example, in the Mizoroki-Heck reaction of alkenyl or aryl halides, silver salts are employed to form effective chiral Pd intermediates by abstracting a halide group from the Pd11 precursor species (Scheme 53).227,228... 

In this paper, we report about a recent in situ SANS study of the synthesis of the SBA-15 materials [5]. Our results demonstrate that SANS (using D20 instead of H20 as a solvent) is a powerful tool for the determination of size and shape of hybrid inorganic-organic micelles during the first synthesis step when the precursor species undergo hydrolysis and form an inorganic framework around the copolymer micelles. 

Reagent selection, solvent, and choice of solution reaction conditions serve to define the nature of the solution precursor species that are formed and that will be used for him deposition. The characteristics of these solution species serve to define him processing characteristics, including aspects such as cracking tendency,50 organic burn-out temperature,48 and for crystalline films, crystallization temperature.49,51... 

These reactions have been studied in detail for materials such as silica, and understanding of reaction mechanisms, as well as of the role of the precursor and catalyst (acid or base), has been well documented.63,64 Similar studies have been carried out in other material systems, most notably, lead zirconate titanate [Pb(Zr,Ti)03 PZT].52,65-68 For multicomponent (mixed-metal) systems such as those noted, prehydrolysis of less reactive alkoxides is sometimes employed to improve solution compositional uniformity. Other synthetic strategies to achieve molecular level mixing of reagents have also been employed. Here, synthesis of mixed-metal alkoxides has been a focus of investigators.40-42 A key point is to restrict the amount of water and to control how it is added to form solubalizable precursor species, rather than to induce precipitation.1,52,69,70... 

In this instance, the resulting species possess both alkoxide and acetate ligands. Because the acetate species may be (and typically are) bidentate in nature,77 as well as sterically larger than short chain alkoxy groups, they are less susceptible to attack by water, and the hydrolysis and condensation reactions are slowed. Because of the reduced sensitivity toward hydrolysis, the nature of the precursor species resulting from these processes is desirable for film formation (to be discussed in Sections 2.5 and 2.6). 

An overview of the precursors, process chemistry, and relative advantages and disadvantages of the three principal methods of inorganic electronic thin film preparation is shown in Table 2.1. Generally, sol-gel methods offer the greatest control over the nature of the solution precursor species, but they have involved... 

General Aspects and Heat Treatment Effects. After him formation, for most of the hlms aimed at electronic applications (other than amorphous oxides, such as Si02), the hlms are subjected to a heat treatment process for removal of residual organic species (entrapped solvent as well as the organic constituents associated with the precursor species), densihcation (elimination of residual porosity and structural free volume in the him), and crystallization. 

Numerous investigators have attempted to control the precursor structure and related solution chemistry effects with varying degrees of success, to influence subsequent processing behavior, such as crystallization tempera-ture.40-42,78,109 110 Particular attention has been given to precursor characteristics such as structural similarity to the desired product and the chemical homogeneity of the precursor species. For multicomponent films, this latter factor is believed to influence the interdiffusional distances associated with the formation of complex crystal structures, such as perovskite compounds. Synthetic approaches have been geared toward the preparation of multimetal species with cation stoichiometry identical to that of the desired crystalline phase.40 42 83 84... 

Table 3.28. Equilibrium bond lengths Rxy and Ryz of linear triatomic anions XYZ-, including binding energy AE and bond-length increase AR with respect to X—Y + Z- or X- + Y—Z precursor species...

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