Parylene

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Parylene N repeat unit.svg

Parylene is the tradename for a variety of chemical vapor deposited poly(p-xylylene) polymers used as moisture and dielectric barriers. Among them, Parylene C is the most popular due to its combination of barrier properties, cost, and other processing advantages.

Parylene is green polymer chemistry. It is self-initiated (no initiator needed) and un-terminated (no termination group needed) with no solvent or catalyst required. The commonly used precursor, [2.2]paracyclophane, yields 100% monomer above 550 °C in vacuum[1] and the initiator and does not yield any by-products. That said there are alternative precursors to arrive at the parylene polymers that possess leaving groups as opposed to the cyclophane precursor. The most popular using bromine to yield the parylene AF-4 polymer.[2] However, bromine is corrosive towards most metals and metal alloys and Viton O-rings so it is difficult to work with and precautions are needed.

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 devices. 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.[3]

Parylenes are relatively soft (0.25 GPa)[citation needed] except for Parylene X (1.0 GPa)[4] and they have poor oxidative resistance (~115 °C) and UV stability,[5] 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 a 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.[6] This lack of solubility has made it difficult to re-work printed circuit boards coated with parylene.

Copolymers[7] and Nanocomposites (SiO2/parylene C)[8] 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. 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%.[9]

Contents

[edit] Parylene N

[2.2] paracyclophane or commonly known as the 'dimer'

Parylene N is a polymer manufactured (chemical vapor deposited) from the p-xylylene intermediate. The p-xylylene intermediate is commonly derived from [2.2]paracyclophane. The latter compound can be synthesized from p-xylene involving several steps involving bromination, amination and Hofmann elmination.[10]

Parylene N is an unsubstituted molecule. Heating [2.2]paracyclophane under low pressure (0.001 – 0.1 Torr) conditions gives rise to the p-xylylene intermediate[11][12] 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.[13][14] 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 paryelne 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 than the p-xylylene intermediate needs to react with itself (2nd step) for polymerization to occur. For parylene N, its threshold temperature is 40 °C.

[edit] Common halogenated parylenes

Parylene C repeat unit.svg

Parylene N can be derivatived with respect to its main-chain phenyl ring and its aliphatic carbon bonds. The most common parylene is parylene C (one chlorine group per repeat unit, as shown above) followed by parylene D (two chlorine groups per repeat unit); both chlorine groups are on the main-chain phenyl ring. Due to its higher molecular weight parylene C has a higher threshold temperature, 90 °C, and therefore has a much higher deposition rate, while still possessing a high degree of conformality. It can be deposited at room temperature while still possessing a high degree of conformality and uniformity and a moderate deposition rate >1 nm/s in a batch process. As a moisture diffusion barrier, the efficacy of coatings scale 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. In that regard parylene D is a better diffusion barrier compared to parylene C; however, parylene D suffers from poor across-the-chamber uniformity and conformality at room temperature due to its high molecular weight (135 °C threshold temperature), as a result it is used much less than parylene C.

There are a couple of fluorinated parylenes commercially available, parylene AF-4 (generic name, aliphatic flourination 4 atoms) [parylene SF (AF-4, Kisco product), parylene HT (AF-4, SCS product)] and parylene VT-4 (generic name, fluorine atoms on the aromatic ring) [also Parylene CF (VT-4, Kisco product)]. Parylene AF-4 is very expensive due to its inefficient wet chemical synthesis of its precursor and its inefficient deposition due to its low polarizability. Polarizability ultimately determines how strongly the intermediate chemistry interacts with the surface and polarizability strongly correlates with molecular weight of the intermediate except in the case of the fluorinated chemistries. Parylene AF-4 is a PTFE analogue in the sense that its aliphatic chemistry has the repeat unit -CF2- and as a result has superior oxidative and UV stability. In contrast, parylene VT-4 (sometimes called just parylene F) has the aliphatic -CH2- chemistry and therefore has poor oxidative and UV stability. Parylene AF-4 has been used to protect outdoor LED displays and lighting from water, salt and pollutants successfully.

Dimer route parylene.svg

The standard Gorham process[15] regardless of the cyclophane starting chemistry is shown above for parylene AF-4. The octafluoro[2.2]paracyclophane is generally sublimed below <100 °C via different configurations. The cyclophane is transported to a pyrolysis zone where it is 'cracked' to the p-xylylene intermediate. This temperature is generally 700 °C, higher than the temperature (650 °C) used to crack the hydrocarbon 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 is able to occur under low pressure (1–100 mTorr) conditions. The threshold temperature of parylene AF-4 is very close to room temperature (30–35 °C), as a result, its deposition efficiency is poor.

Monomer route to parylene AF-4.svg

More recently an alternate route to parylene AF-4 was developed as shown above. 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 or vaporizer with a bubbler. Originally the precursor was just thermally cracked[16] to yield the same intermediate as that produced from the cyclophane; however, with the use of catalysts the 'cracking' temperature can be lowered resulting in less char in the pyrolysis zone and a higher quality polymer thin film.[17][18] By either method free radical bromine is given off as a by-product and is easily converted to hydrogen bromide, which has to be properly processed or equipment damage will occur.

[edit] Reactive parylenes

Parylene X repeat unit.svg

Most parylenes are passivation thin films or coatings. This means they protect the device or part from environmental stresses such as water, chemical attack, or applied field. This is an important property however many applications have the need to bond other materials to parylene, bond parylene to parylene, or even immobilize catalysts or enzymes to the parylene surface. Some of the reactive parylenes are parylene A (one amine per repeat unit, Kisco product), parylene AM (one methylene amine group per repeat unit, Kisco product), and parylene X (a reactive hydrocarbon cross-linkable version, not commercially available). Parylene AM is more reactive than A since it is a stronger base. When adjacent to the phenyl ring the amine group, -NH2-, is in resonance stabilization and therefore become more acidic and a result less reactive as a base. However, parylene A is much easier to synthesize and hence it costs less.

Among all the parylenes, parylene X is especially unique since it is: 1) cross-linkable (thermally or with UV-light) 2) Can generate the Cu-acetylide or Ag-acetylide metallorganic intermediates 3) Can undergo 'Click chemistry' 4) Can be used as an adhesive, parylene-to-parylene bonding without any by-products during processing 5) Is amorphous (non-crystalline) and is 6) A hydrocarbon polymer.

[edit] 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.[19]

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 than 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.

[edit] History

Parylene development started in 1947, when Michael Szwarc discovered the polymer as one of the thermal decomposition products of a common solvent p-xylene at a temperatures exceeding 1000 °C. Szwarc first postulated the monomer to be para-xylylene which he confirmed by reacting the vapors with iodine and observing the para-xylylene di-iodide as the only product. The reaction yield was only a few percent, and a more efficient route was found later 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 did not require a solvent and resulted in chemically resistant films free from pinholes.[20][21] Since the coating process takes place at ambient temperature in a mild vacuum, and because of parylene’s conformal properties, it has a wide variety of applications. Union Carbide commercialized a parylene coating system in 1965. Union Carbide went onto undertake research into the synthesis of numerous parylene precursors, including parylene AF-4, throughout the 1960s into the early 70's. 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. [22]

[edit] Characteristics and advantages

  • Hydrophobic, chemically resistant coating with good barrier properties for inorganic and organic media, strong acids, caustic solutions, gases and water vapor
  • Low leakage current and a low dielectric constant (average in-plane and out-of-plane: 2.67 parylene N and 2.5 parylene AF-4, SF, HT)[23]
  • A biostable, biocompatible coating; FDA approved for various applications
  • Dense pinhole free, with thickness above 1.4 nm[24]
  • Coating without temperature load of the substrates as coating takes place at ambient temperature in the vacuum
  • Highly corrosion resistant
  • Completely homogeneous surface
  • Oxidatively stable up to 350 °C (Parylene AF-4, SF, HT)
  • Low intrinsic thin film stress due to its room temperature deposition
  • Low coefficient of friction (AF-4, HT, SF)
  • Very low permeability to gases

[edit] Typical applications

Parylene films have been used in various applications, including [20]

  • Hydrophobic coating (moisture barriers, e.g. for biomedical hoses)
  • Barrier layers (e.g. for filter, diaphragms, valves)
  • Microwave electronics
  • Sensors in rough environment (e.g. automotive fuel/air sensors)
  • Electronics for space travel and military
  • Corrosion protection for metallic surfaces
  • Reinforcement of micro-structures
  • Protection of plastic, rubber, etc. from harmful environmental conditions
  • Reduction of friction, e.g., for guiding catheters, acupuncture needles and Microelectromechanical systems.

[edit] References

  1. ^ J.B. Fortin and T.-M. Lu (2000). J. Vac. Sci. Technol. A 18: 2459. 
  2. ^ P. K. Wu, G. -R. Yang, L. You, D. Mathur, A. Cocoziello, C. -I. Lang, J. A. Moore, T. -M. Lu and H. Bakru (1997). "Deposition of High Purity Parylene- F Using Low Pressure Low Temperature Chemical Vapor Deposition". J. Electron Mater 26 (8): 949–953. doi:10.1007/s11664-997-0280-8. 
  3. ^ James A. Schwarz, Cristian I. Contescu, Karol Putyera (2004). Dekker encyclopédia of nanoscience and nanotechnology, Volume 1. CRC Press. p. 263. ISBN 978-0-8247-5047-3. http://books.google.com/?id=RHcUpRj_wI8C&lpg=RA1-PA26&pg=PA263. 
  4. ^ J. J. Senkevich, B. W. Woods, J. J. McMahon, P.-I Wang (2007). "Thermomechanical Properties of Parylene X, A Room-Temperature Chemical Vapor Depositable Crosslinkable Polymer". Chem. Vapor Dep. 13 (1): 55–59. doi:10.1002/cvde.200606541. 
  5. ^ J.B. Fortin and T.-M. Lu (2001). "Ultraviolet radiation induced degradation of poly-para-xylylene (parylene) thin films". Thin Solid Films 397: 223–228. doi:10.1016/S0040-6090(01)01355-4. 
  6. ^ J.J. Senkevich, C.J. Mitchell, A. Vijayaraghavan, E.V. Barnat, J.F. McDonald, T.-M. Lu (2002). "The Unique Structure/Properties of Chemical Vapor Deposited Parylene E". J. Vac. Sci. & Tech. A 20 (4): 1445–9. doi:10.1116/1.1487870. 
  7. ^ J.F. Gaynor, J.J. Senkevich, S.B. Desu (1996). "A New Method for Fabricating High Performance Polymeric Thin Films by Chemical Vapor Polymerization". J. Mater. Research 11 (7): 1842–50. doi:10.1557/JMR.1996.0233. 
  8. ^ J.J. Senkevich, S.B. Desu "Near-Room Temperature Thermal CVD of Poly(chloro-p-xylylene)/SiO2 Nanocomposites (1999). "Near-Room-Temperature Thermal Chemical Vapor Deposition of Poly(chloro-p-xylylene)/SiO2 Nanocomposites". Chemistry of Materials 11 (7): 1814–21. doi:10.1021/cm990042q. 
  9. ^ J.J. Senkevich (1999). "CVD of NanoPorous Silica". Chem. Vap. Deposition 5 (6): 257–60. doi:10.1002/(SICI)1521-3862(199912)5:6<257::AID-CVDE257>3.0.CO;2-J. 
  10. ^ H. E. Winberg and F. S. Fawcett (1973), "Tricyclo[8.2.2.24,7hexadeca-4,6,10,12,13,15-hexaene"], Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv5p0883 
  11. ^ H. J. Reich, D. J. Cram (1969). "Macro rings. XXXVI. Ring expansion, racemization, and isomer interconversions in the [2.2]paracyclophane system through a diradical intermediate". Journal of the American Chemical SocietyEdition 91 (13): 3517–3526. doi:10.1021/ja01041a016. 
  12. ^ P. Kramer, A. K. Sharma, E. E. Hennecke, H. Yasuda (2003). "Polymerization of para-xylylene derivatives (parylene polymerization). I. Deposition kinetics for parylene N and parylene C". Journal of Polymer Science: Polymer Chemistry Edition 22 (2): 475–491. doi:10.1002/pol.1984.170220218. 
  13. ^ K. M. Vaeth and K. F. Jensen (1999). "Selective growth of poly(p-phenylene vinylene) prepared by chemical vapor deposition". Adv. Materials 11 (10): 814–820. doi:10.1002/(SICI)1521-4095(199907)11:10<814::AID-ADMA814>3.0.CO;2-Z. 
  14. ^ J.J. Senkevich, C.J. Wiegand, G.-R. Yang, T.-M. Lu (2004). "Selective Deposition of Ultra-thin Poly(p-xylylene) Films on Dielectrics versus Copper Surfaces". Chem. Vapor. Dep. 10 (5): 247–9. doi:10.1002/cvde.200304179. 
  15. ^ W.F. Gorham (1966). J. Polym. Sci. A 4 (2): 3027. 
  16. ^ Pebalk, A. V.; Kardash, I. E.; Kozlova, N. V.; Zaitseva, E. L.; Kozlov, Yu. A.; Pravednikov, A. N. (1980). Vysokomolekulyarnye Soedineniya, Seriya A 22 (5): 972–6. 
  17. ^ Lee, Chung J.; Wang, Hui; Foggiato, Giovanni Antonio, U.S. Patent 6,140,456, Oct 31, 2000.
  18. ^ Lee, Chung J., U.S. Patent 6,703,462, Mar 9, 2004.
  19. ^ C. J. Mitchell, G.-R. Yang, J.J. Senkevich (2006). "Adhesion aspects of gamma-methacryloxypropyltrimethoxysilane to poly(p-xylylene)". J. Adhesion Sci. Technol. 20 (14): 1637–1647. doi:10.1163/156856106778884217. 
  20. ^ a b Jeffrey B. Fortin, Toh-Ming Lu (2003). Chemical vapor deposition polymerization: the growth and properties of parylene thin films. Springer. pp. 4–7. ISBN 978-1-4020-7688-6. http://books.google.com/?id=9zhApzpRIOEC&pg=PA4. 
  21. ^ Mattox, D. M. The foundations of vacuum coating technology, Springer, 2003 ISBN 978-3-540-20410-7 Google books
  22. ^ http://www.scscoatings.com/about/history.aspx
  23. ^ J.J. Senkevich, S.B. Desu (1999). "Compositional studies of near-room temperature thermal CVD of poly(chloro-p-xylylene)/SiO2 nanocomposites". Chemistry of Materials 11 (5): 1814. doi:10.1007/s003390051076. 
  24. ^ J.J. Senkevich and P.-I. Wang (2009). "Molecular Layer Chemistry via Parylenes". Chem. Vapor Dep. 15 (4–6): 91. doi:10.1002/cvde.200804266. 

[edit] External links

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