Parylene electrical properties

Electrica.1 Properties. The bulk electrical properties of the parylenes make them excellent candidates for use in electronic constmction. The dielectric constants and dielectric losses are low and unaffected by absorption of atmospheric water. The dielectric strength is quoted for specimens of 25 p.m thickness because substantially thicker specimens cannot be prepared by VDP. If the value appears to be high in comparison with other materials, however, it should be noted that the usual thickness for such a measurement is 3.18 mm. Dielectric strength declines with the square root of increasing... 

Fig. 6. Variation of electrical properties of Parylenes N and C with temperature.

It also has superior electrical properties, for example extremely high dielectric strength and very high resistivity. For this work, parylene C was used because of its excellent combination of physical and electrical properties. Table 15.1 lists its important properties. 

The dry nature of CVD process enables copolymerization more easily than solution based techniques. This approach has been utilized to tailor the thermal and electrical properties of parylene thin films for ULSI applications. One of the first reports of vacuum copolymerization of xylylene was done with maleic anhydride as the comonomer. Subsequently, 9-vinylanthracene, 4-vinylbiphenyl and perfluoro-octyl methacrylate were successfully copolymerized with parylenes, and reported in... 

The other commercially available members of this gronp are Parylene C and Parylene D. Parylene C is poly-mono-chloro-para-xylylene. Parylene C offers significantly lower permeability to moisture and gases, such as nitrogen and oxygen, while retaining excellent electrical properties. Parylene D is poly-dichloro-para-xylene. Properties of Parylene N, C, and D are shown in Table P.3. 

Parylene films have excellent electrical properties with high volume and surface resistivitities whilst their dielectric constants and dissipation factors are quite low, although not as good as PTFE. The two main types of parylene are Parylene N and Parylene C. Parylene N is the unsubstituted polymer, whilst Parylene C is chlorine substituted. The best overall electrical properties are exhibited by Parylene N but Parylene C is superior with respect to D.C. dielectric breakdown voltage for films under 5 fxm in thickness. The principal electrical properties of parylenes are shown in Table 10.14. 

Parylene is a completely different class of coating. The p-xylylene monomer is stable as a gas at low pressure but polymerizes spontaneously on any surface on which it condenses. The polymer formed has excellent moisture, chemical, mechanical, and electrical properties, but expensive application equipment is required. 

Table 11 gives the properties for various commercial parylenes (pi ra-xylylene). These materials have excellent moisture and chemical resistance as well as good electrical and mechanical properties. They are noted for their abiHty to penetrate small spaces by vapor deposition (see Xylylenepolymers). 

Although Parylene-N possesses an outstanding combination of physical, electrical, and chemical properties, the benzylic C—H bonds present are potential sites for thermal and oxidative degradation. It is well known that replacing a C— bond with a C—F bond not only enhances the thermal stability of the resulting polymer, but also reduces the dielectric constant. Because incorporation of fluorine is known to impart thermal and oxidative stability, it became of interest to prepare poly(a,a,a, a -tetrafluoro- p -xylylene), Parylene-F Joesten reported that the decomposition temperature of poly(tetrafluoro-j9-xylylene) is ca. 530°C. Thus, it seemed that the fluorinated analog would satisfy many of the exacting requirements for utility as an on-chip dielectric medium. 

The physical properties of parylene films are remarkable Parylene is a very good insnlator with an electrical breakdown strength of up to 10 MV/cm, superior chemical stability, and high optical clarity. 

By attaching one chlorine atom to the xylylene ring (Parylene C), the permeability to moisture and other gases, can be significantly reduced. Polymers of dichloro-p-xylylene have better electrical and thermal properties as ordinary PPXs. The introduction of fluorine atoms still improves the thermal resistance. 

Parylene is widely used as a protective coating in the field of electronics, aerospace and medical applications. Parylene is formed on surfaces from the gas phase, in contrast to most other coating techniques that use Uquid precursor materials. The resulting film is thin and conformal, has no pinholes at sufficient thickness, and is chemically resistant. The coatings impart several properties concomitantly, including electrical insulation, moisture and chemical isolation, mechanical protection, enhanced lubricity. The poor adhesion of the surface rejects dust and soil. 

Parylene C, the second commercially available member of the Parylene series, is produced from the same raw material (dimer) as Parylene N, modified only by the substitution of a chlorine atom for one of the aromatic hydrogens. Parylene C has a useful combination of electrical and physical properties, plus a very low permeability to moisture and corrosive gases. 

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