Deposition mechanism, LCVD

The photon-emitting species are vitally important in luminous chemical vapor deposition (LCVD) process, and the location of the luminous gas phase indicates where actions occur within the interelectrode space. On the other hand, whether any particular photon-emitting species is primarily responsible for LCVD is a dilferent issue. As is described in Chapter 5 for the growth and deposition mechanisms, many chemically reactive species, such as various forms of free radicals that do not emit photons, are major reactive species that carry the growth reactions. No single species could be identified as the precursor or chemically reactive species for the process. 

In luminous chemical vapor deposition (LCVD), irradiation with photons at various energy levels and the deposition of materials that contain free radicals occur concurrently. The balance of these processes is closely related to the chemical structure of the monomer and its characteristic behavior in the luminous gas phase. The investigation of this balance by ESR provides valuable information pertinent to the growth and the deposition mechanisms in LCVD. The details of this aspect are described in Chapter 7, which deals the chemical structure of the monomer (starting material for LCVD). 

However, there are indications that some structures are favorable for the material deposition more than others. According to the growth and the deposition mechanisms described in Chapter 5, the most important feature of organic molecules, which determines the ease of material deposition in LCVD, is the presence or absence of chemical structures that split to form diradicals by the energy transfer reaction with electrons or excited species in the luminous gas phase. The double bonds, triple bonds, and cyclic structures are the major chemical structures that create the reactive species that could proceed via cycle II of the growth and the deposition mechanisms (Figure 5.3). Without these structures, molecules depend on hydrogen abstraction or the detachment of simple stable species such as HF and HCl to create free radicals. 

The formation of powders in luminous gas phase has significant implications in the processing of LCVD. In an attempt to obtain a uniform nanofilm on a substrate, the powder formation in gas phase ruins the product. On the other hand, the analysis of powder formation as a function of operational parameters provides important information pertinent to the growth mechanism and the deposition mechanism of LCVD, by which growing species deposit on the surface. 

Because powder formation depends on the polymer deposition portion of the polymerization-deposition mechanisms of LCVD, its dependence on operational parameters such as the flow rate and system pressure is not necessarily the same in... 

Powder formation in an LCVD system is a reflection of the polymer deposition mechanism. The size and number of particles may be taken as a measure of the polymerization-deposition mechanism or the status of an LCVD system. At one extreme is exclusive powder formation, as reported by Liepins and Sakaoku [7] at the opposite extreme is the formation of a continuous film in which no visible particles can be found. Even in the latter case, however, the work of Havens et al. [12] involving the use of small-angle X-ray scattering indicates that detectable domains... 

The terms luminous chemical vapor deposition and plasma polymerization are used synonymously in this book. Dealing with mechanism of reactions that lead to formation of solid deposition, PP is used according to the traditional use of the term. When dealing with the formation of reactive species and other operation and processing aspects, LCVD is preferentially used. 

In LCVD, polymerization and deposition are inseparable components of the polymerization ieposition mechanism. None of the reactions considered in Figure 5.3 is a polymerization by itself. While repeating the steps via cycle I or cycle II, the species varying in size, depending on how many cycles has been progressed, deposit on the substrate surface. 

Because of the unique growth mechanism of material formation, the monomer for plasma polymerization (luminous chemical vapor deposition, LCVD) does not require specific chemical structure. The monomer for the free radical chain growth polymerization, e.g., vinyl polymerization, requires an olefinic double bond or a triple bond. For instance, styrene is a monomer but ethylbenzene is not. In LCVD, both styrene and ethylbenzene polymerize, and their deposition rates are by and large the same. Table 7.1 shows the comparison of deposition rate of vinyl compounds and corresponding saturated vinyl compounds. 

Some monomers show a more or less anticipated decrease in polymer deposition rates based on the concept that a pulsed discharge decreases the initiation rate, but some monomers show dramatically increase deposition rates. The most significant effect of pulsed discharge, however, can be seen in the concentration of dangling bonds, which reflects the unique mechanisms of material formation in LCVD. 

The water transport mechanism changes from the flow mechanism in porous membrane to the diffusive transport in nonporous homogeneous membrane due to the deposition of a homogeneous LCVD layer that fills the pore, i.e., water transport changes from bulk flow to diffusive flow when pores are covered by LCVD film. 

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