Adhesion
The majority of parylene used is deposited as passivation coatings to passivate the part or device towards moisture, chemical attack or as a dielectric insulator. This in turn often means parylene is coated over complex topographies with many different surface chemistries. If one considers a solid-state material, those materials have three fundamental surfaces when exposed to ambient conditions: 1) noble metal surfaces, 2) metal-oxide forming surfaces, and 3) organic surfaces, e.g. polymeric.
Polymeric surfaces generally only possess dispersion forces but may contain functional groups able to bond to adhesion promoters. If parylene is bonded to a printed circuit board (PCB) then often the mechanical tie-points allow parylene to exhibit good adhesion as opposed to bonding through covalent links (chemical bonding). Sometimes plasma methods are effective in the promotion of adhesion between parylene and polymeric surfaces but these techniques are not trivial to employ. The third surface, metal-oxide forming surfaces, generally possess a hydroxyl-terminated surface, M-OH, where M is a metal such as aluminum or chromium. This termination group has the ability to react with commercially available silanes such as A-174 (methacryloxypropyltrimethoxysilane), which is the common adhesion promoter for the parylene polymers.
The A-174 silane can be vapor delivered in situ or bonded via wet chemical baths. In all cases one half of the molecule binds to metal oxide forming surface through sol-gel chemistry (hydrolysis and condensation) and the other half co-polymerizes with parylene via a free radical addition reaction. In all cases the A-174 silane molecule 'lies down' on the surface and forms self-limited molecular layers of less than 1.0 nm. If thick layers are observed then the silane bath has started to 'polymerize' and a new bath should be started. Vapor phase silylation never yields more than a sub-monolayer of silane on the part being coated; and therefore this problem is circumvented.
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