Parylene N
Parylene N is a polymer manufactured (chemical vapor deposited) from the p-xylylene intermediate. The p-xylylene intermediate is commonly derived from paracyclophane. The latter compound can be synthesized from p-xylene involving several steps involving bromination, amination and Hofmann elimination.
Parylene N is an non-substituted molecule. Heating paracyclophane under low pressure (0.001 – 0.1 Torr) conditions gives rise to the p-xylylene intermediate, which polymerizes when physisorbed on a surface. The p-xylylene intermediate has two quantum mechanical states, the benzoid state (triplet state) and the quinoid state (singlet state). The triplet state is effectively the initiator and the singlet state is effectively the monomer. The triplet state can be de-activated when in contact with transition metals or metal oxides including Cu/CuOx. Many of the parylenes exhibit this selectivity based on quantum mechanical deactivation of the triplet state, including parylene X. However, like any selective process there is a 'selectivity' window based on mostly deposition pressure and deposition temperature for the parylene polymers. What is more, the intermediate, p-xylylene has a low reactivity and therefore a small 'sticking coefficient' and as a result parylene N produces a highly conformal thin film or coating.
The deposition of parylene N is a function of a two-step process. First, physisorption needs to take place, which is a function of deposition pressure and temperature. The physisorption has inverse Arrhenius kinetics, in other words it is stronger at lower temperatures than higher temperatures. All the parylenes have a critical temperature called the threshold temperature above which practically no deposition is observed. The closer the deposition temperature is to the threshold temperature the weaker the physisorption. Once physisorption occurs, the p-xylylene intermediate needs to react with itself (2nd step) for polymerization to occur. For parylene N, its threshold temperature is 40 °C.
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