Chemical vapor deposition plasma polymerization

The unique capabilities of chemical vapor deposition are clearly demonstrated by the thin films belonging to this class of polymers. Yasuda et al first studied and reported the polymerization of organic compounds in glow discharge. Polymerization of organic compounds in the presence of plasma is quite different from the conventional chemically or radiatively initiated polymerization. For instance, polymerization of styrene in conventional polymerization can be done using several means of initiation such as radiation, pyrolysis induced, etc., to create the free radical species. But the propagation is... 

Any chemical reaction that yields polymeric material can be considered polymerization. However, polymerization in the conventional sense, i.e., yielding high enough molecular weight materials, does not occur in the low-pressure gas phase (without a heterogeneous catalyst). With a heterogeneous catalyst, polymerization is not a gas phase reaction. Therefore, the process of material deposition from luminous gas phase in the low-pressure domain might be better represented by the term luminous chemical vapor deposition (LCVD). Plasma polymerization and LCVD (terms explained in Chapter 2) are used synonymously in this book, and the former... 

The material deposition that occurs in the low-pressure electrical discharge has been discussed under various terminologies such as plasma polymerization (PP), plasma-enhanced chemical vapor deposition (PECVD), plasma-assisted chemical vapor deposition (PACVD), plasma chemical vapor deposition (PCVD), and so forth [1]. However, none of these terminologies seems to represent the phenomenon adequately. The plasma aspect in the low-pressure discharge is remote, although it plays a key role in creating the environment from which material deposition occurs to the extent that no chemical reaction occurs without the plasma. In this sense, PECVD and PACVD could be out of the context in many cases in which nothing happens without plasma. In such cases, PP or PCVD would describe the phenomenon better. If the substrate was not heated substantially above the ambient temperature, the use of PECVD or PACVD should be avoided. 

In order to find the domain of LCVD, it is necessary to compare various vacuum deposition processes chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma chemical vapor deposition (PCVD), plasma-assisted CVD (PACVD), plasma-enhanced CVD (PECVD), and plasma polymerization (PP). All of these terms refer to methods or processes that yield the deposition of materials in a thin-film form in vacuum. There is no clear definition for these terms that can be used to separate processes that are represented by these terminologies. All involve the starting material in vapor phase and the product in the solid state. 

Comparing the terms plasma chemical vapor deposition and luminous chemical vapor deposition, the dilference exists in the meaning of plasma and luminous gas and its implications to the nature of chemical reactions that occur in the gas phase. Without referring the details of the difference, however, the process could be described either plasma polymerization (plasma CVD) or luminous CVD in all practical purposes. 

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. 

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. 

If plasma is not an adequate term to describe low-pressure glow discharge, what term can be used to describe glow discharge Similarly, if polymerization is not a proper term to describe the material formation that takes place in the glow discharge, what term represents the process better than polymerization This book explains why luminous chemical vapor deposition (LCVD) is chosen to replace plasma polymerization. 

Because of the high rate of one-directional movement of the excited species of Ar, the chemical vapor deposition does not yield as well-developed a layer as that which can be obtained with plasma polymerization... 

Plasmas are also used for the low temperature deposition of thin solid films, for example amorphous hydrogenated silicon, diamond, and a host of other materials. Since the fundamentals of plasma physics and chemistry are the same for both plasma etching and plasma assisted chemical vapor deposition (PECVD), the latter will only be discussed briefly in Section 6.6. A review of PECVD can be foxmd in [14]. Sputtering is discussed by Chapman [15], and plasma polymerization is covered by Yasuda [16]. 

For forming a protective layer, dependent on the nature of the material, a vacuum vapor deposition method, a sputtering method, a plasma polymerization method, a chemical vapor deposition method or a coating method, can be applied. ... 

More recently, an approach for low permeability materials is to deposit parylene-C into the poly-(dimethylsiloxane). The base matrix is coated with parylene-C by chemical vapor deposition polymerization in the usual way [78]. Then the parylene-C on the surface is removed by oxygen plasma etching and only what is in the pores of the matrix remains there. 

Plasma treatment is a method of modifying the chemistry and often the topography of a surface. It uses a highly ionized, activated gas to react with the molecules of a surface. The plasma gas can vary from an inert gas, such as argon or helium, which would be expected to cause the species at the surface to react with one another, or a polymeric monomer, which could polymerize on the surface and create a thin plasma-treated layer. Plasmas can also be employed to clean surfeces before modification. Chemical vapor deposition is a technique in which the sample is exposed to a vapor that reacts with the surface to modify it. 

The techniques of graft polymerization have been developed for liquid as well as gas phase. One of the polymerization techniques comprises impregnation of cellulose by a solution of conjugated monomers, followed by soaking in a relevant initiator. Chemical vapor deposition (CVD) of poly(3,4-ethylenedioxythiophene) is another, solvent-less technique, used for thin layer deposition of conjugated polymer on fibers [33]. Three CVD methods have been developed physical vaporization CVD [34, 35], plasma-enhanced CVD [36], and thermally initiated CVD [37, 38]. However, low conductivity is achieved when dopants are not used. 

Table 7.2 summarizes various subtypes of CVD polymerization processes. These methods differ in the means by which the CVD chemistry is driven (plasma, thermal, or UV). For hot wire chemical vapor deposition (HWCVD) and initiated chemical vapor deposition (iCVD), no plasma excitation or UV exposure is utilized during the polymerization, eliminating the possibility for forming defects in the films via these... 

Figure 2 presents the most common plasma-based surface modification techniques for biomedical applications, described in more detail later plasma assisted chemical vapor deposition or PACVD (RF, MW), physical vapor deposition or PVD (sputtering, cathodic arc), plasma polymerization and grafting, plasma-based thermochemical treatments (e.g. plasma nitriding), ion implantation, plasma immersion ion implantation or PHI, and plasma spraying. Each technique has unique advantages and applications, and the choice of the more adequate technique often depends on the... 

Susut C, Timmons DF. (2005) Plasma enhanced chemical vapor depositions to encapsulate crystals in thin polymeric films A new approach to controlling drug release rates. Int J Pharm 288 253—261. 

Nowadays research efforts are mainly directed to the synthesis of microporous and dense inorganic membranes. The multilayer casting method based on sol-gel technology is not the sole approach for the preparation of these membranes. CVD and hydrothermal synthesis are currently used as well as the sol-gel process. CVI (chemical vapor infiltration) for support infiltrated membranes, PE-CVD (plasma enhanced chemical vapor deposition) for surface modification of existing membranes, growing of zeolite membrane layers, pyrolysis of polymeric preciusors have been described as alternative preparation methods... 

---