Parylene polymerization

Although Gorham process is the most popular method of parylene polymerization, there are reports of at least two other polymerization schemes for parylenes. Figure 3 shows the different polymerization schemes used for CVP of parylene thin films. However, in this paper, only the Gorham process will be discussed, the details of alternate polymerization schemes can be found elsewhere. ... 

Parylene polymerization Active species is created in a separate chamber by thermal cracking Substrate in a separate deposition chamber Ts < Tv AE < 0... 

Plasma polymerization is a vapor phase-activated (by plasma) A/D-coupled CVD. Parylene polymerization is a vapor phase-activated (by thermal process) A/D-decoupled CVD. Cascade arc torch (CAT) polymerization is a vapor phase-activated (by plasma) A/D-decoupled CVD. General CVD is a surface-activated (by thermal process) A/D-coupled CVD. HWCVD is a hot wire activated A/D-decoupled CVD. Coupled and decoupled refer only to the spatial separation. 

In the chain growth free radical polymerization of a vinyl monomer (conventional polymerization), the growth reaction is the repeated reaction of a free radical with numbers of monomer molecules. According to the termination by recombination of growing chains, 2 free radicals and 1000 monomer molecules leads to a polymer with the degree of polymerization of 1000. In contrast to this situation, the growth and deposition mechanisms of plasma polymerization as well as of parylene polymerization could be represented by recombinations of 1000 free radicals (some of them are diradicals) to form the three-dimensional network deposit via 1000 kinetic... 

Polymerization in gas phase must cope with larger entropy change than polymerization in liquid phase. Therefore, polymerization of gas phase monomers such as olefins is carried out in superatmospheric pressure and/or in the presence of heterogeneous catalyst. Polymerization in gas phase in low pressure (in vacuum) does not occur easily due to the limitation of the ceiling temperature of polymerization, and there are only few cases in which the deposition of polymeric material from gas phase starting material occurs in vacuum. Those main exceptional cases are plasma polymerization and parylene polymerization. 

In parylene polymerization, the thermal cracking of the dimer (starting material) creates monomeric diradicals. All starting materials are converted to the reactive species, i.e., diradicals. No specific initiator for the chemical structure of starting material is formed. The situation is close to that of plasma polymerization. The comparison of plasma polymerization and radiation polymerization, and the comparison of the two vacuum deposition polymerizations (parylene polymerization and plasma polymerization) enable us to construct an overall view of material formation in the luminous gas phase. 

So far as the growth mechanism is concerned, parylene polymerization is the closest kin of plasma polymerization. It is advantageous to go into a little details of parylene polymerization in order to understand how vacuum phase deposition of material occurs starting from a vapor phase material. 

The process of parylene polymerization is presented schematically in Figure 5.2 using parylene N, unsubstituted poly(para-xylylene). Parylene dimer is heated until it sublimes. The dimer vapor passes through a high temperature pyrolysis zone where it cracks and becomes monomer vapor, i.e., monomer is created in vacuum. The monomer polymerizes and deposits in the deposition chamber, which is usually at room temperature. Parylene polymerization completed in a vacuum is a process involving no solvents, no curing, and no liquid phase. Its use essentially eliminates concern about the operator s health and safety, air pollution, and waste disposal. 

The common denominator features of plasma polymerization (LCVD) and parylene polymerization are as follows ... 

Figure 5.2 Schematic diagram of parylene polymerization steps.

Parylene polymerization proceeds with well-defined chemical species, whereas plasma polymerization proceeds via variety of not-well-defined chemical species, which are created in the luminous gas phase. The reactive species for parylene polymerization is para-xylylene, which has features of (1) difunctional (e.g., diradicals), (2) reactive but relatively stable, and (3) highly selective reactivity (see Fig. 2.1). The exact nature of reactive species involved in glow discharge polymerization is not well known however, (1) they are not exclusively bifunctional, (2) they are highly reactive, and (3) consequently they have very low selectivity. The difference in the stability or the selectivity of reactive species is reflected in the distinctively different characters of polymer depositions of these two processes. 

The following aspects of parylene polymerization [3-9] seem to have important implications in an effort to understand the growth mechanisms of plasma... 

Conventional polymerization does not occur in gas phase, particularly in vacuum, because of the limitation set by the ceiling temperature of polymerization, and there are only a few cases in which the deposition of polymeric material occurs in vacuum. Those exceptional cases are plasma polymerization and Parylene polymerization, which is also a vacuum polymerization coating process using a gaseous monomer. The common denominators for these two processes are 1) the polymerizations yield solid-state polymer (in the form of film in most cases) from a gas phase monomer in vacuum and 2) the polymer formed by the processes... 

Parylene polymerization proceeds with well-defined chemical species, whereas plasma polymerization proceeds via a variety of not well-defined chemical species, which are created in the luminous gas phase. The reactive species for Parylene polymerization is pora-xylylene, which is bifunctional (i.e., biradicals), moderately reactive and relatively stable, and highly selective in chemical reactivity to its own structure. 

In both the polymerizations, free radicals are the species that are responsible for the formation of bonds in the depositing materials. The growth mechanism, however, is not by the conventional chain-growth free-radical polymerization. In a conventional free-radical chain-growth polymerization, two free radicals and 10,000 monomer molecules yield a polymer with degree of polymerization 10,000, which does not contain free radicals. In contrast to this situation, in plasma polymerization and Parylene polymerization, 10,000 species with free radical(s) recombine to yield a polymer matrix that has an equivalent degree of polymerization, and contains numbers of unreacted free radicals (dangling bonds). 

L. Baldauf, C. Hamann, and L. Libera, Parylene-Polymere, Synthese, Eigenschaften, Bedeutung, Plaste Kautsch. 25/(2), 61-64 (1978). 

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