![P-xylylene](https://upload.wikimedia.org/wikipedia/commons/7/74/P-xylylene.png)
Parylene is the common name of a
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
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
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
atoms are replaced by other
functional groups
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest ...
. 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
trademark
A trademark (also written trade mark or trade-mark) is a type of intellectual property consisting of a recognizable sign, design, or expression that identifies products or services from a particular source and distinguishes them from othe ...
s in some countries.
Coatings of parylene are often applied to
electronic circuits and other equipment as
electrical insulation
An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current ...
, moisture barriers, or protection against
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
and chemical attack. They are also used to reduce
friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of ...
, 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
In chemistry, a monomer ( ; '' mono-'', "one" + ''-mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization.
Classification
...
''para''-xylylene.
Parylene is considered a
"green" polymer because its polymerization needs no
initiator
An initiator can refer to:
* A person who instigates something.
* Modulated neutron initiator, a neutron source used in some nuclear weapons
** Initiator, an Explosive booster
** Initiator, the first Nuclear chain reaction
* Pyrotechnic initiato ...
or other chemicals to terminate the chain; and the coatings can be applied at or near room temperature, without any
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
.
History
Parylene was discovered in 1947 by
Michael Szwarc Michael Szwarc (9 June 1909, Będzin, Poland – 4 August 2000, San Diego, California) was a British and American polymer chemist who discovered and studied ionic living polymerization.
Biography
Michael Mojżesz Szwarc was born into a Polis ...
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 .2paracyclophane at temperatures exceeding 550 °C and in vacuum below 1 Torr. This process 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 .2aracyclophane 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''-xylylene intermediate.
Chlorinated parylenes
Derivatives of parylene can be obtained by replacing hydrogen atoms on the phenyl
In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen ...
ring or the aliphatic
In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane, ...
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
Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
. 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
The Restriction of Hazardous Substances Directive 2002/95/EC (RoHS 1), short for Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, was adopted in February 2003 by the European Unio ...
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
The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...
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
ScS (Sofa Carpet Specialist), is a home furnishings retailer in the United Kingdom, specialising in sofas, carpets and flooring, dining and occasional furniture.
History
ScS was established in Sunderland, Tyne and Wear in 1894, as a family own ...
). 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 group
In organic chemistry, an alkyl group is an alkane missing one hydrogen.
The term ''alkyl'' is intentionally unspecific to include many possible substitutions.
An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloalk ...
s. 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
In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in ma ...
or an ethyl group group yields parylene M and parylene E, respectively.
Repeat unit of payrlene M.jpg, Parylene M
Repeat unit of parylene E.jpg, Parylene E
Repeat unit of parylene AM-2.jpg, Parylene AM-2
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 kpi vs. 460 kpsi), 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-100in2-24hr), and equivalent dielectric breakdown 5-6 kvol/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
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen
Hydrogen is the chemical element wi ...
-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
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen
Hydrogen is the chemical element wi ...
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 it costs less.
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-link
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
ed by heat or with UV light, and can react with copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
or silver
Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
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 .2aracyclophane base molecule to impart color to parylene.
Parylene-like copolymers
Copolymer
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
s[ and nanocomposites (SiO2/parylene C)][ of parylene have been deposited at near-room temperature previously; and with strongly electron withdrawing comonomers, parylene can be used as an initiator to initiate polymerizations, such as with N-phenyl ]maleimide
Maleimide is a chemical compound with the formula H2C2(CO)2NH (see diagram). This unsaturated imide is an important building block in organic synthesis. The name is a contraction of maleic acid and imide, the -C(O)NHC(O)- functional group. Malei ...
. 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. i.e. parylene E. As a result, of the coatings being semi-crystalline, 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 crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic sys ...
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 (parylene N 0.5 GPa)[ except for cross-linked parylene X (1.0 GPa)][ and they 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.
Coating process
Parylene coatings are generally applied by chemical vapor deposition in an atmosphere of the monomer ''para''-xylylene or a derivative thereof. This method has one very strong benefit, namely it does not generate any byproducts besides the parylene polymer, which would need to be removed from the reaction chamber and could interfere with the polymerization.
Parts to be coated need to be clean in order to ensure good adherence of the film. Since the monomer diffuses, areas that are not to be coated must be hermetically sealed, without gaps, crevices or other openings. The part must be maintained in a relatively narrow window of pressure and temperature.[
The process involves three steps: generation of the gaseous monomer, adsorption on the part's surface, and polymerization of adsorbed film.
]
Polymerization
Polymerization of the adsorbed ''p''-xylylene monomer requires a minimum threshold temperature. For parylene N, its threshold temperature is 40 °C.
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.
Polymerization may proceed by a variety of routes that differ in the transient termination of the growing chains, such as a radical group – or a negative anion group :
]
Physisorption
The monomer polymerizes only after it is physically adsorbed ( physisorbed) on the part's surface. This process has inverse Arrhenius kinetics, meaning that it is stronger at lower temperatures than higher temperatures. There is critical threshold temperature above which there is practically no physisorption, and hence no deposition. The closer the deposition temperature is to the threshold temperature the weaker the physisorption. Parylene C has a higher threshold temperature, 90 °C, and therefore has a much higher deposition rate, greater than 1 nm/s, while still yielding fairly uniform coatings.[ In contrast, the threshold temperature of parylene AF-4 is very close to room temperature (30–35 °C), as a result, its deposition efficiency is poor.][
An important property of the monomer is the so-called 'sticking coefficient', that expresses the degree to which it adsorbs on the polymer. A lower coefficient results more uniform deposition thickness and a more conformal coating.
Another relevant property for the deposition process is polarizability, which determines how strongly the monomer interacts with the surface. Deposition of halogenated parylenes strongly correlates with molecular weight of the monomer. The fluorinated variants are an exception: the polarizability of parylene AF-4 is low, resulting in inefficient deposition.
]
Monomer generation
From the cyclic dimer
The ''p''-xylylene monomer is normally generated during the coating process by evaporating the cyclic
Cycle, cycles, or cyclic may refer to:
Anthropology and social sciences
* Cyclic history, a theory of history
* Cyclical theory, a theory of American political history associated with Arthur Schlesinger, Sr.
* Social cycle, various cycles in s ...
dimer
Dimer may refer to:
* Dimer (chemistry), a chemical structure formed from two similar sub-units
** Protein dimer, a protein quaternary structure
** d-dimer
* Dimer model, an item in statistical mechanics, based on ''domino tiling''
* Julius Dimer ...
.2'para''-cyclophane
In organic chemistry, a cyclophane is a hydrocarbon consisting of an aromatic unit (typically a benzene ring) and a chain that forms a bridge between two non-adjacent positions of the aromatic ring. More complex derivatives with multiple aromati ...
at a relatively low temperature, then decomposing the vapor at 450–700 °C and pressure 0.01–1.0 torr
The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ).
Historically, one torr was intended to be the same as one " millimeter of merc ...
. This method (Gorham Process) yields 100% monomer with no by-products or decomposition of the monomer. [
The dimer can be synthesized from ''p''-xylene involving several steps involving ]bromination
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polyme ...
, amination and Hofmann elimination
Hofmann elimination is an elimination reaction of an amine to form alkenes. The least stable alkene (the one with the least number of substituents on the carbons of the double bond), called the Hofmann product, is formed. This tendency, known as ...
.[
The same method can be used to deposit substituted parylenes. For example, parylene C can be obtained from the dimeric precursor dichloro .2'para''-cyclophane, except that the temperature must be carefully controlled since the ]chlorine
Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
-aryl
In organic chemistry, an aryl is any functional group or substituent derived from an aromaticity, aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar ...
bond breaks at 680 °C.
The standard Gorham process[ is shown above for parylene AF-4. The octafluoro .2'para''-cyclophane precursor dimer can be sublimed below <100 °C and cracked at 700-750 °C, higher than the temperature (680 °C) used to crack the unsubstituted 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 occurs under low pressure (1–100 mTorr) conditions.][
]
From substituted ''p''-xylenes
Another route to generation of the monomer is to use a ''para''-xylene precursor with a suitable substituent on each methyl group
In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in ma ...
s, whose elimination generates ''para''-xylylene.
Selection of a leaving group may consider its toxicity (which excludes sulfur and amine-based reactions), how easily it leaves the precursor, and possible interference with the polymerization. The leaving group can either be trapped before the deposition chamber, or it can be highly volatile so that it does not condense in the latter.[
For example, the precursor α,α'-dibromo-α,α,α',α'-tetrafluoro-''para''-xylene yields parylene AF-4 with elimination of ]bromine
Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
.[
:
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, vaporizer with a ]bubbler
A drinking fountain, also called a water fountain or water bubbler, is a fountain designed to provide drinking water. It consists of a basin with either continuously running water or a tap. The drinker bends down to the stream of water and s ...
, or a mass-flow controller. Originally the precursor was just thermally cracked,[ but suitable ]catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
s lower the pyrolysis temperature, resulting in less char residue and a better coating.[ By either method an atomic bromine ]free-radical
In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron.
With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spon ...
is given off from each methyl end, which can be converted to hydrogen bromide
Hydrogen bromide is the inorganic compound with the formula . It is a hydrogen halide consisting of hydrogen and bromine. A colorless gas, it dissolves in water, forming hydrobromic acid, which is saturated at 68.85% HBr by weight at room tempe ...
and removed from monomer flow. Special precautions are needed since bromine and HBr are toxic and corrosive towards most metals and metal alloys, and bromine can damage viton O-ring
An O-ring, also known as a packing or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a round cross-section, designed to be seated in a groove and compressed during assembly between two or more par ...
s.
A similar synthesis for parylene N uses the precursor α,α'-dimethoxy-''p''-xylene.[ The methoxy group – is the leaving group; while it condenses in the deposition chamber, it does not interfere with the deposition of the polymer.][ This precursor is much less expensive than .2'para''-cyclophane. Moreover, being a liquid just above room temperature, this precursor can delivered reliably using a mass-flow controller; whereas the generation and delivery of the gaseous monomer of the Gorham process are difficult to measure and control.][
The same chemistry can generate parylene AM-2 can be generated from the precursor α,α'-dimethyl-α,α'-dimethoxy-''p''-xylene.
Another example of this approach is the synthesis of parylene AF-4 from α,α'-diphenoxy-α,α,α',α'-tetrafluoro-''para''-xylene. In this case, the leaving group is ]phenoxy
In chemistry, the alkoxy group is an alkyl group which is Single bond, singularly bonded to oxygen; thus . The range of alkoxy groups is vast, the simplest being methoxy (). An ethoxy group () is found in the organic compound ethyl phenyl ethe ...
–, which can be condensed before the deposition chamber.[
]
Characteristics and advantages
Parylenes may confer several desirable qualities to the coated parts. Among other properties, they are
*Hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, t ...
, chemically resistant, and mostly impermeable to gases (including water vapor) and inorganic and organic liquids (including strong acids and bases).
*Good electrical insulator
An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current ...
with a low dielectric constant (average in-plane and out-of-plane: 2.67 parylene N and 2.5 parylene AF-4, SF, HT)[
*Stable and accepted in biological tissues, having been approved by the ]US FDA
The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food s ...
for various medical applications.
*Dense and pinhole free, for thickness above 1.4 nm[
* Homogeneous and uniformly thick, even within cavities.
*Stable to oxidation up to 350 °C (AF-4, SF, HT)
*Low coefficient of friction (AF-4, HT, SF)
Since the coating process takes place at ambient temperature in a mild vacuum, it can be applied even to temperature-sensitive objects such as dry biological specimens. The low temperature also results in low intrinsic stress in the thin film. Moreover, the only gas in the deposition chamber is the monomer, without any solvents, catalysts, or byproducts that could attack the object.
Parylene AF-4 and VT-4 are both fluorinated and as a result very expensive compared to parylene N and C, which has severely limited their commercial use, except for niche applications.
]
Applications
Parylene C and to a lesser extent AF-4, SF, HT (all the same polymer) are used for coating printed circuit boards (PCBs) and medical device
A medical device is any device intended to be used for medical purposes. Significant potential for hazards are inherent when using a device for medical purposes and thus medical devices must be proved safe and effective with reasonable assura ...
s. There are numerous other applications as parylene is an excellent moisture barrier. It is the most bio-accepted coating for stents, defibrillators, pacemakers and other devices permanently implanted into the body.[
]
Molecular layers
The classic molecular layer chemistries are self-assembled monolayer
Self-assembled monolayers (SAM) of organic molecules are molecular assemblies formed spontaneously on surfaces by adsorption and are organized into more or less large ordered domains. In some cases molecules that form the monolayer do not interact ...
s (SAMs). SAMs are long-chain alkyl chains, which interact with surfaces based on sulfur-metal interaction (alkylthiolates)[ or a sol-gel type reaction with a hydroxylated oxide surface (trichlorosilyl alkyls or trialkoxy alkyls).][ However, unless the gold or oxide surface is carefully treated and the alkyl chain is long, these SAMs form disordered monolayers, which do not pack well.][ This lack of packing causes issues in, for example, ]stiction
Stiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact. The term is a portmanteau of the words ''static'' and ''friction'', and is perhaps also influenced by the verb '' to stick''.
Any ...
in MEMS
Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
devices.[
The observation that parylenes could form ordered molecular layers (MLs) came with ]contact angle
The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liq ...
measurements, where MLs thicker than 10 Å had an equilibrium contact angle of 80 degrees (same as bulk parylene N) but those thinner had a reduced contact angle.[ This was also confirmed with electrical measurements (bias-temperature stress measurements) using metal-insulator-semiconductor capacitors (MISCAPs).][ In short, parylene N and AF-4 (those parylenes with no functional groups) are pin-hole free at ~14 Å. This results because the parylene repeat units possess a phenyl ring and due to the high electronic polarizability of the phenyl ring adjacent repeat units order themselves in the XY-plane. As a result of this interaction parylene MLs are surface independent, except for transition metals, which de-activate the triplet (benzoid) state and therefore the parylenes cannot be initiated. This finding of parylenes as molecular layers is very powerful for industrial applications because of the robustness of the process and that the MLs are deposited at room temperature. In this way parylenes can be used as diffusion barriers and for reducing the polarizability of surface (de-activation of oxide surfaces). Combining the properties of the reactive parylenes with the observation that they can form dense pin-hole-free molecular layers, parylene X has been utilized as a genome sequencing interface layer.
One caveat with the molecular layer parylenes, namely they are deposited as oligomers and not high polymer.][ As a result, a vacuum anneal is needed to convert the oligomers to high polymer. For parylene N that temperature is 250 °C, whereas it is 300 °C for payrlene AF-4.
]
Typical applications
Parylene films have been used in various applications, including[
*Hydrophobic coating (moisture barriers, e.g., for biomedical hoses)
*Barrier layers (e.g., for filter, diaphragms, valves)
*Microwave electronics (e.g., protection of PTFE dielectric substrates from oil contamination)
*Implantable medical devices
*Sensors in rough environment (e.g., automotive fuel/air sensors)
*Electronics for space travel and defense
*Corrosion protection for metallic surfaces
*Reinforcement of micro-structures
*Protection of plastic, rubber, etc., from harmful environmental conditions
*Reduction of ]friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of ...
, e.g., for guiding catheters, acupuncture needles and microelectromechanical systems.
See also
Conformal coating
Conformal coating is a protective coating of thin polymeric film, applied to printed circuit boards (PCB). The coating is named conformal since it ''conforms'' to the contours of the PCB. Conformal coatings are typically applied at 25-250 μm to ...
References
[C. Chiang, A. S. Mack, C. Pan, Y.-L. Ling, D. B. Fraser ''Mat. Res. Soc. Symp. Proc.'' vol. 381, 123 (1995).]
[D.M. Dobkin, S. Mokhtari, M. Schmidt, A. Pant, L. Robinson, Mechanisms of Deposition of SiO2 from TEOS and Related Organosilicon Compounds and Ozone" J. Electrochem. Soc. 142(7), 2332-40 (1995).]
[Horn, Sean "The Parylene Deposition Process: Pre-Deposition" https://www.paryleneconformalcoating.com/#TheParyleneDepositionProcess]
[Lee, Chung J.; Wang, Hui; Foggiato, Giovanni Antonio, , Issue date: October 31, 2000.]
[Lee, Chung J., , Issue date: March 9, 2004.]
[Mattox, D. M]
The foundations of vacuum coating technology
, Springer, 2003
Google books
/ref>
[SCS Coatings History]
. Scscoatings.com. Retrieved on 2012-06-04.
[J.J. Senkevich (2011): "CVD of Poly(α,α'-dimethyl-''p''-xylylene and Poly(α,α,α',α'-tetramethyl-''p''-xylylene)-co-poly(''p''-xylylene) from Alkoxide Precursors I: Optical Properties and Thermal Stability". Chemical Vapor Deposition, volume 17, pages 235-240.]
[{{cite journal, author=Z. Yapu, journal= Acta Mechanica Sinica, volume=19, page= 1, year=2003, title=Stiction and anti-stiction in MEMS and NEMS, doi=10.1007/BF02487448, issue=1, bibcode = 2003AcMSn..19....1Z , s2cid= 110540114]
Polymers