Comparison of Parylene C, N, and F : Similarities, differences and their application areas

At Diamond MT, we offer parylene coatings of different polymeric varieties (N, C, and F) as listed in the following Table. The basic parylene molecule is the Parylene N (poly-para-xylylene) monomer. Modification of the Parylene N monomer by a functional group such as Chlorine and Fluorine leads to Parylene C (poly(2-chloro-para-xylylene)) and Parylene F, respectively. The derivatization of new varieties can be done by the addition of functional groups to Paryelene N main-chain phenyl ring and its aliphatic carbon bonds.

Derivatization results in a set of new material properties: crystallinity, melting temperature, resistivity, mechanical and electrical properties. New derivatives with enhanced properties can be used under different service conditions.

Table: Parylene Types and Properties *Materials data: MatWeb

Property	Unit	Parylene N	Parylene C	Parylene F
Molecular structure
Chemical Formula		C16H16	C16H14Cl2	C16H8F8
Optical		Transparent	Transparent	Transparent
Biocompatibility		Yes	Yes	Yes
Electrical properties
Dielectric constant (@ 1 MHz)		2.66	2.95	2.17
Dielectric constant (@ 1 kHz)		2.66	3.10
Dielectric constant (@ 60 Hz)		2.66	3.15
Dissipation Factor (@ 1 MHz)		0.0006	0.013	0.008
Dissipation Factor (@ 1 kHz)		0.0002	0.019	0.0013
Dissipation Factor (@ 60 Hz)		0.0002	0.02	0.0002
Volume Resistivity	ohm-cm	1.40e+17	8.80e+16	2.0E+17
THERMAL  PROPERTIES
Melting point	°C	420	290	≤ 500
Durable Heat Resistance	°C	80	100	350
Thermal conductivity	Cal/sec	3	2
MECHANICAL/PHYSICAL PROPERTIES
Tensile Strength	psi	6500	10000	7800
Yield Strength	psi	6300	8000	7600
Water Absorption		0.01%/24 hour	0.06%/24 hour	0.01%/24 hour
Oxygen Transmission  (@Temperature 25.0 °C)	cc-mm/m²-24hr-atm	15.4	2.80	34.7
Advantages		Constant dielectric coefficient at all frequencies High dielectric strength Less wear (low friction coefficient.)	Low gas permeability High Chemical Resistance	Sub-micron coverage [1] High thermal resistance UV-resistive High-density

Parylenes are pure organic polymers that contain trace amount of impurities or zero percent chloride, sodium,  potassium ions [2]. Parylene thin films offer complete coverage of surface even between closely spaced features that is useful in microelectronic applications. Due to their multiple advantages they have a wide application area in various industries.

Parylenes are transparent in the visible region of the solar spectrum. They exhibit a high absorption in the UV- region, λ˂ 350 nm. This high absorption results in their photodegradation process through chain scission and formation of macroradicalar species [3]. Accordingly, the photodiscoloration of Parylene C was reported to be higher compared to that of Parylene N. For all cases, parylene conformal coatings are advised to be used indoors without direct exposure to sun. Parylene F on the other hand is less susceptible to photodegradation. A problem interfering with wider scale adaptation of Parylene F is difficulty synthesizing dimer.  Low availability of F dimer inhibits it commercial viability, although researchers actively seek alternatives to dimer synthesis.

One of the advantages of parylene conformal coatings is that they are stress free, this aids in protecting the underlying application from generated stresses due to the coating process. They are light and mechanically strong with high tensile and yield strengths (table).

As discussed earlier Parylene N is the most basic form of parylenes, its advantages are its availability, constant dielectric coefficient at all frequencies, low friction and high dielectric strength. Parylene N is has a lifetime of 10 years at 60°C before failure, at 80°C it fails in a year, and can only withstand 24 hours at 120°C in air. Under vacuum parylenes can withstand longer at higher temperatures. [2]. The decomposition of Parylene N takes place through diffusion of oxygen into the molecule and reacting with C in the polymer at the decomposition temperature. The reported temperatures for decomposition under air, N₂ and vacuum are 175, 350, and 425 °C, respectively [4]. Parylenes C and D follow similar Electrical properties of Parylene N were reported to be more favorable than that of Parylene C [2].  Parylene C’s volume resistivity drops to 10^-15 compared to 10^-16 of Parylene C at 160 °C’s. Dielectric constant of Parylene C increases with temperature while Parylene N has a relatively stable dielectric coefficient between 20 -200 °C’s [2].  As the thickness of Parylene C and Parylene N increases from 0.5 μm to 4-5 μm their breakdown voltage becomes closer and Parylene N shows a higher breakdown voltage at higher thicknesses. Parylene N was shown to be useful as a dielectric layer for use in microelectronic systems. In an advanced study, it was used as a conformal dielectric layer in 50 μm through silicon vias (TSV) [5].

Parylene C on the other hand is the most widely used type. It finds applications in microelectronics, microfluidic devices, medical implants (stents, needles, etc.), PCBs due to its diverse properties such as fast deposition rates, common CVD type deposition method. It complies with the FDA regulations with the biomedical use (ISO-10993 Biological Evaluations for Medical Applications).

Paryleen F is deposited using the Gorham method, with the cyclic dimer octofluoro-[2.2|paracyclophane].    Low availability and price of F dimer inhibits it commercial viability, although researchers actively seek alternatives to dimer synthesis.

Parylene N, C and F offer a low dielectric constant (k≈2-3) which makes them well suited for use as intra- and interlayer dielectrics electronic devices [6]. The dielectric constant is relatively high for Parylene C due to the polar bonds C-Cl. Among them, Parylene F exhibits a very high melting temperature (≤ 500 °C) compared to parylene N (420 °C) and C (290 °C). And, the highest thermal durability is observed for ParyleneF. The decomposition of fluorinated parylene takes place by the decoposition of -CF2- functional groups into -CF- functional groups both in air and nitrogen at the decomposition temperature and no reaction products with oxygen was observed [4]. It has the lowest dielectric constant (2.17 @ 1 MHz) among the three which makes Parylene F more attractive. It can be used as a dielectric layer in the semiconductor chips where high temperature exposure is required. Also, they were shown to be patternable in the micro-scale using Ar/O₂ and N₂/O₂ discharges with straight sidewalls under right processing conditions [6]. The drawback of fluorinated parylene is the difficulty in obtaining the commercial dimer which makes it an expensive alternative to other parylenes. Also, it has the best conformal coating property through entering the smallest crevices.

In conclusion, each parylene type offers different advantages under various types of service conditions (optical, thermal, mechanical, electrical, biomedical). The selection criteria must be based on the aimed end use of the product.

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References:

[1]         W. R. Dolbier and W. F. Beach, “Parylene-AF4: a polymer with exceptional dielectric and thermal properties,” J. Fluor. Chem., vol. 122, no. 1, pp. 97–104, Jul. 2003.

[2]         J. J. Licari, Coating Materials for Electronic Applications: Polymers, Processing, Reliability, Testing. William Andrew, 2003.

[3]         M. Bera, A. Rivaton, C. Gandon, and J. L. Gardette, “Comparison of the photodegradation of parylene C and parylene N,” Eur. Polym. J., vol. 36, no. 9, pp. 1765–1777, Sep. 2000.

[4]         P. K. Wu, G.-R. Yang, J. F. McDonald, and T.-M. Lu, “Surface reaction and stability of parylene N and F thin films at elevated temperatures,” J. Electron. Mater., vol. 24, no. 1, pp. 53–58, Jan. 1995.

[5]         B. Majeed, N. P. Pham, D. S. Tezcan, and E. Beyne, “Parylene N as a dielectric material for through silicon vias,” in 2008 58th Electronic Components and Technology Conference, 2008, pp. 1556–1561.

[6]         T. E. F. M. Standaert et al., “High-density plasma patterning of low dielectric constant polymers: A comparison between polytetrafluoroethylene, parylene-N, and poly(arylene ether),” J. Vac. Sci. Technol. A, vol. 19, no. 2, pp. 435–446, Mar. 2001.