Specific Process Knowledge/Polymer Processing/Polymer Processing/Coating/Parylene Coater

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Parylene is the common name for a family of poly(p-xylylene) polymers deposited as thin, conformal films by chemical vapour deposition (CVD). Unlike liquid coatings (spray, dip, spin), parylene is deposited from the vapour phase with no solvent, catalyst, or curing step. This gives pinhole-free, uniform-thickness films on virtually any geometry, including sharp edges, crevices, and fine features that liquid coatings cannot reach evenly.

A parylene coater is the integrated CVD system — vaporiser, pyrolysis furnace, deposition chamber, cold trap, and vacuum system — used to run this process.

Chemistry in Brief

The precursor for each parylene type is a cyclic dimer of the corresponding p-xylylene unit (di-para-xylylene, “DPX,” for Parylene N; the chlorinated dimer for Parylene C; and so on). Heating this solid dimer under vacuum sublimes it directly to a vapour without melting. Pyrolysis at high temperature homolytically cleaves the dimer's two methylene–methylene bonds, producing a pair of reactive p-xylylene monomers (diradicals). These monomers do not react with each other in the gas phase because the chamber is kept cold and dilute; instead, they diffuse to any surface at or near room temperature, adsorb, and rapidly propagate into a linear, high-molecular-weight polymer directly on that surface. There is no solvent, no initiator, and no exotherm large enough to damage typical substrates — polymerisation is driven by the radical reactivity of the monomer itself, not by added chemistry. Because polymerisation happens directly on the substrate surface rather than in solution, the resulting film has very high molecular weight, high crystallinity (grade-dependent), and essentially no seams, pinholes, or coating-induced stress from solvent evaporation or shrinkage, which is the root reason parylene outperforms liquid coatings as a barrier film.

Working Principle

The Parylene deposition process consists of four main stages:

1. Sublimation: Solid Parylene dimer is heated (approximately 120–180°C) under vacuum to produce a vapour.
2. Pyrolysis: The vapour passes through a high-temperature furnace (650–750°C), where the dimer molecules split into reactive monomers.
3. Deposition and Polymerisation: The monomer enters a slightly heated vacuum chamber containing the substrate. The monomer condenses and polymerises directly on all exposed surfaces, forming a thin, conformal polymer coating.
4. Cold Trap: Residual monomer and by-products are captured in a cryo-cooled cold trap before reaching the vacuum pump.