Parylene C vs Parylene F

Parylene C is the most widely used parylene type for conformal coatings.  It is classified as a poly-monochoro para-xylene, produced from dimer material, with one chlorine group per repeat unit on its main-chain phenyl ring. As a conformal coating, Type C can be deposited at room temperature via CVD.  The resulting film exhibits low chemical, moisture, and vapor permeability, making it particularly useful where protection is needed from corrosive gases.  Parylene C’s alliance of electrical and physical properties distinguish it uses from those Parylene F, a consequence of their different chemical composition; F has a fluorine atom on its benzene ring, in contrast to C’s chlorine atom.

In comparison to Type F, Parylene C demonstrates a lesser throw-capability, generating a commensurately lower reduction in crevice penetration-activity.  At the same time, Type C has a faster rate of deposits on substrates.  Unlike C, which depends upon standard CVD processes, Type F is deposited using the Gorham method, with the cyclic dimer octofluoro-[2.2|paracyclophane].  A problem interfering with wider scale adaptation of Parylene F is difficulty synthesizing dimer.  Low availability of F dimer inhibits it commercial viability, although researchers actively seek alternatives to dimer synthesis.

Polarizability is the tendency of an atom’s electron cloud to be distorted from its normal shape by an external electric field.  Because it is influential in establishing the degree to which a given surface interacts with a designated compound’s chemistry, polarizability is well-correlated with its molecular weight, except in the case of the fluorinated chemistries.  In contrast to C, Parylene F is fluorinated, characterized by fluorine atoms on its aromatic ring; its use can significantly lower coating capacitation, reducing a coated surface’s electrical charge during operation.

In comparison to Parylene C, Parylene F has a lower dielectric constant and better thermal stability, enhancing its functionality for inner layer dielectric (ILD) applications.  This capacity makes F potentially very useful for ultra large scale integration (ULSI), wherein one million or more circuit elements are situated on a single chip; these properties would support F’s expanded MEMS and NT applications, if its synthesis were more readily achieved.

F’s superior thermal stability is attributed to its aliphatic C-F bond, compared to Type C’s C-C bond.  Possessing aliphatic -CH₂– chemistry, F therefore has poor oxidative and UV stability.  At the same time, F exhibits a higher coating density and significantly more penetrating power than C, although its refractive index is slightly lower.

A further comparison of essential properties of parylene Types F and C is provided in Table 1

Table 1:  Properties of Parylene F and C Compared

Both Parylene F and C are largely impervious to the effects of corrosive chemicals and exhibit low levels of trace metal contamination.  Parylene C’s faster deposition rate and lower processing cost render it readily applicable for a more extensive range of uses than other parylene types, including F, explaining C’s popularity.  However, as the evidence suggests, both F and C have their own recommended applications.  Because of their unique chemical formulation, silicone coatings need to be considered within the context of all parylene types.

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