Parylene is the common name of a polymer whose backbone consists of para-benzenediyl rings −− connected by 1,2-ethanediyl bridges −−−. It can be obtained by polymerization of para-xylylene ==.
The name is also used for several polymers with the same backbone, where some hydrogen atoms are replaced by other functional groups. Some of these variants are designated in commerce by letter-number codes such as "parylene C" and "parylene AF-4". Some of these names are registered trademarks in some countries.
Coatings of parylene are often applied to electronic circuits and other equipment as electrical insulation, moisture barriers, or protection against corrosion and chemical attack (conformal coating). They are also used to reduce friction and in medicine to prevent adverse reactions to implanted devices. These coatings are typically applied by chemical vapor deposition in an atmosphere of the monomer para-xylylene.
Parylene is considered a "green" polymer because its polymerization needs no initiator or other chemicals to terminate the chain; and the coatings can be applied at or near room temperature, without any solvent.
History Parylene was discovered in 1947 by Michael Szwarc as one of the thermal decomposition products of para-xylene −− above 1000 °C. Szwarc identified para-xylylene as the precursor by observing that reaction with iodine yielded para-xylylene di-iodide as the only product. The reaction yield was only a few percent.
A more efficient route was found in 1965 by William F. Gorham at Union Carbide. He deposited parylene films by the thermal decomposition of [2.2]paracyclophane at temperatures exceeding 550 °C and in vacuum below 1 Torr. This process, referred to as vacuum deposition polymerization (VDP), did not require a solvent and resulted in chemically resistant films free from pinholes. Union Carbide commercialized a parylene coating system in 1965.
Union Carbide went on to undertake research into the synthesis of numerous parylene precursors, including parylene AF-4, throughout the 1960s into the early 1970s. Union Carbide purchased NovaTran (a parylene coater) in 1984 and combined it with other electronic chemical coating businesses to form the Specialty Coating Systems division. The division was sold to Cookson Electronics in 1994.
There are parylene coating service companies located around the world, but there is limited commercial availability of parylene. The [2.2]paracyclophane precursors can be purchased for parylene N, C, D, AF-4 and VT-4. Parylene services are provided for N, C, AF-4, VT-4 and E (copolymer of N and E).
Varieties
Parylene N Parylene N is the un-substituted polymer obtained by polymerization of the para-xylene intermediate.
Chlorinated parylenes
Derivatives of parylene can be obtained by replacing hydrogen atoms on the phenyl ring or the aliphatic bridge by other functional groups. The most common of these variants is parylene C, which has one hydrogen atom in the aryl ring replaced by chlorine. Another common variant is parylene D, with two such substitutions on the ring.
Parylene C is the most used variety, due to its low cost of its precursor and to the balance of its properties as dielectric and moisture barrier properties and ease of deposition. A major disadvantage for many applications is its insolubility in any solvent at room temperature, which prevents removal of the coating when the part has to be re-worked.
Parylene C is also the most commonly used because of its relatively low cost. It can be deposited at room temperature while still possessing a high degree of conformality and uniformity and a moderate deposition rate in a batch process.
Also, the chlorine on the phenyl ring of the parylene C repeat unit is problematic for RoHS compliance, especially for the printed circuit board manufacture. Moreover, some of the dimer precursor is decomposed by breaking of the aryl-chlorine bond during pyrolysis, generating carbonaceous material that contaminates the coating, and hydrogen chloride that may harm vacuum pumps and other equipment. The chlorine atom leaves the phenyl ring in the pyrolysis tube at all temperatures; however, optimizing the pyrolysis temperature will minimize this problem. The free-radical (phenyl radical) generated in this process is not resonance-stabilized and mitigates the deposition of a parylene-like material on the downside of the pyrolysis tube. This material becomes carbonized and generates particles in situ to contaminate clean rooms and create defects on printed-circuit boards that are often called "stringers and nodules". Parylene N and E do not have this problem and therefore are preferred for manufacturing and clean room use.
Fluorinated parylenes
Another common halogenated variant is parylene AF-4, with the four hydrogen atoms on the aliphatic chain replaced by fluorine atoms. This variant is also marketed under the trade names of parylene SF (Kisco) and HT parylene (SCS). The −− unit that comprises the ethylene chain is the same as the repeating unit of PTFE (Teflon), consistent with its superior oxidative and UV stability. Parylene AF-4 has been used to protect outdoor LED displays and lighting from water, salt and pollutants successfully.
Another fluorinated variant is parylene VT-4 (also called parylene F), with fluorine substituted for the four hydrogens on the aryl ring. This variant is marketed by Kisco with the trademark Parylene CF. Because of the aliphatic −CH2− units, it has poor oxidative and UV stability, but still better than N, C, or D.
Alkyl-substituted parylenes
The hydrogen atoms can be replaced also by alkyl groups. Substitution may occur on either the phenyl ring or the ethylene bridge, or both.
Specifically, replacement of one hydrogen on the phenyl ring by a methyl group or an ethyl group yields parylene M and E respectively.
These substitutions increase the intermolecular (chain-to-chain) distance, which makes the polymer more soluble and permeable. For example, compared to parylene C, parylene M was shown to have a lower dielectric constant (2.48 vs. 3.2 at 1 kHz). Parylene E had a lower tensile modulus (175 kpsi (1.21 GPa) vs. 460 kpsi (3.17 GPa)), a lower dielectric constant (2.34 vs. 3.05 at 10 kHz), slightly worse moisture barrier properties (4.1 vs. 0.6 g·mil/(atom·100 in2·24 hr) (11 vs. 1.6 kg·m·pmol−1·m−2·s−1)), and equivalent dielectric breakdown 5–6 kV/mil for a 1-mil coating) but better solubility. However, the copolymer of parylene N and E has equivalent barrier performance of parylene C.
Replacement of one hydrogen by methyl on each carbon of the ethyl bridge yields parylene AM-2, (not to be confused with an amine-substituted variant trademarked by Kisco). The solubility of parylene AM-2 is not as good as parylene E.
Reactive parylenes
While parylene coatings are mostly used to protect an object from water and other chemicals, some applications require a coating that can bind to adhesives or other coated parts, or immobilize various molecules such as dyes, catalysts, or enzymes.
These "reactive" parylene coatings can be obtained with chemically active substituents. Two commercially available products are parylene A, featuring one amine substituent − in each unit, and parylene AM, with one methylene amine group − per unit. Both are trademarks of Kisco.
Parylene AM is more reactive than the A variant. The amine of the latter, being adjacent to the phenyl ring, is in resonance stabilization and therefore less basic. However, parylene A is much easier to synthesize and hence cheaper.
Another reactive variant is parylene X, which features an ethinyl group − attached to the phenyl ring in some of the units. This variant, which contains no elements other than hydrogen and carbon, can be cross-linked by heat or with UV light and can react with copper or silver salts to generate the corresponding metalorganic complexes Cu-acetylide or Ag-acetylide. It can also undergo "click chemistry" and can be used as an adhesive, allowing parylene-to-parylene bonding without any by-products during processing. Unlike most other variants, parylene X is amorphous (non-crystalline).
Colored parylenes
It is possible to attach a chromophore directly to the [2.2]paracyclophane base molecule to impart color to parylene.
Parylene-like copolymers
Copolymers and nanocomposites (SiO2/parylene C) of parylene have been deposited at near-room temperature previously. With strongly electron withdrawing comonomers, parylene can be used as an initiator to initiate polymerizations, such as with N-phenyl maleimide. Using the parylene C/SiO2 nanocomposites, parylene C could be used as a sacrificial layer to make nanoporous silica thin films with a porosity of >90%.
Properties
Transparency and crystallinity
Parylene thin films and coatings are transparent; however, they are not amorphous except for the alkylated parylenes, e.g. parylene E. As a result of this semi-crystallinity, they scatter light. Parylene N and C have a low degree of crystallinity; however, parylene VT-4 and AF-4 are highly crystalline ~60% in their as-deposited condition (hexagonal crystal structure) and therefore are generally not suitable as optical materials.
Parylene C will become more crystalline if heated at elevated temperatures until its melting point at 270 °C.
Parylene N has a monoclinic crystal structure in its as-deposited condition and it does not appreciably become more crystalline until it undergoes a crystallographic phase transformation at ~220 °C to hexagonal, at which point it becomes highly crystalline like the fluorinated parylenes. It can reach 80% crystallinity at anneal temperatures up to 400 °C, after which point it degrades.
Mechanical and chemical
Parylenes are relatively flexible (0.5 GPa for parylene N), except for cross-linked parylene X (1.0 GPa), and have poor oxidative resistance (~60–100 °C, depending on failure criteria) and UV stability, except for parylene AF-4. However, parylene AF-4 is more expensive due to a three-step synthesis of its precursor with low yield and poor deposition efficiency. Their UV stability is so poor that parylene cannot be exposed to regular sunlight without yellowing.
Nearly all the parylenes are insoluble at room temperature, except for the alkylated parylenes, one of which is parylene E, and the alkylated-ethynyl parylenes. This lack of solubility has made it difficult to re-work printed circuit boards coated with parylene.
Permeability
As a moisture diffusion barrier, the efficacy of halogneated parylene 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; however, if parylenes are used as a diffusion barrier against water then the apolar chemistries such as parylene E are much more effective. For moisture barriers the three principal material parameters to be optimized are: coating density, coating polarity (olefin chemistry is best) and a glass-transition temperature above room temperature and ideally above the service limit of the printed-circuit board, device or part. In this regard parylene E is a best choice although it has a low density compared to, for example, parylene C.
Industry specifications