Parylene - Common Halogenated Parylenes

Common Halogenated Parylenes

Parylene N can be derivatized with respect to its main-chain phenyl ring and its aliphatic carbon bonds. The most common parylene is parylene C (one chlorine group per repeat unit, as shown above) followed by parylene D (two chlorine groups per repeat unit); both chlorine groups are on the main-chain phenyl ring. Because of its higher molecular weight parylene C has a higher threshold temperature, 90 °C, and therefore has a much higher deposition rate, while still possessing a high degree of conformality. It can be deposited at room temperature while still possessing a high degree of conformality and uniformity and a moderate deposition rate >1 nm/s in a batch process. As a moisture diffusion barrier, the efficacy of coatings scales non-linearly with their density. Halogen atoms such as F, Cl and Br add much density to the coating and therefore allow the coating to be a better diffusion barrier. In that regard parylene D is a better diffusion barrier compared to parylene C; however, parylene D suffers from poor across-the-chamber uniformity and conformality at room temperature due to its high molecular weight (135 °C threshold temperature), as a result it is used much less than parylene C.

There are a couple of fluorinated parylenes commercially available, parylene AF-4 (generic name, aliphatic fluorination 4 atoms) and parylene VT-4 (generic name, fluorine atoms on the aromatic ring) . Parylene AF-4 is very expensive due to its inefficient wet chemical synthesis of its precursor and its inefficient deposition due to its low polarizability. Polarizability ultimately determines how strongly the intermediate chemistry interacts with the surface and polarizability strongly correlates with molecular weight of the intermediate except in the case of the fluorinated chemistries. Parylene AF-4 is a PTFE (Teflon) analogue in the sense that its aliphatic chemistry has the repeat unit -CF₂- and as a result has superior oxidative and UV stability. In contrast, parylene VT-4 (sometimes called just parylene F) has the aliphatic -CH₂- chemistry and therefore has poor oxidative and UV stability. Parylene AF-4 has been used to protect outdoor LED displays and lighting from water, salt and pollutants successfully.

The standard Gorham process regardless of the cyclophane starting chemistry is shown above for parylene AF-4. The octafluoroparacyclophane is generally sublimed below <100 °C via different configurations. The cyclophane is transported to a pyrolysis zone where it is 'cracked' to the p-xylylene intermediate. This temperature is generally 700 °C, higher than the temperature (650 °C) used to crack the hydrocarbon cyclophane since the -CF₂-CF₂- bond is stronger than the -CH₂-CH₂- bond. This resonance-stabilized intermediate is transported to a room temperature deposition chamber where polymerization is able to occur under low pressure (1–100 mTorr) conditions. The threshold temperature of parylene AF-4 is very close to room temperature (30–35 °C), as a result, its deposition efficiency is poor.

More recently an alternate route to parylene AF-4 was developed as shown above. The advantage to this process is the low cost of synthesis for the precursor. The precursor is also a liquid and can be delivered by standard methods developed in the Semiconductor Industry, such as with a vaporizer or vaporizer with a bubbler. Originally the precursor was just thermally cracked to yield the same intermediate as that produced from the cyclophane; however, with the use of catalysts the 'cracking' temperature can be lowered resulting in less char in the pyrolysis zone and a higher quality polymer thin film. By either method free radical bromine is given off as a by-product and is easily converted to hydrogen bromide, which has to be properly processed or equipment damage will occur.