Poly tetrafluoro-/?-xylylene

Fuqua and co-workers17 tried to develop a much shorter route from a,a,a, a -tetrafluoro-p-xylene to poly(tetrafluoro-/ -xylylene) but were unsuccessful in generating a polymer because they conducted their pyrolyses at 820-925°C/3-5 Torr, and under those conditions instead of losing H2 to form a,a,a, a -tetrafluoro-j9-xylylene, a,a,a, a -tetrafluoro-/ -xylene lost HF and underwent rearrangement to form P,p,j9-trifluorostyrene [Eq. (1)]. 

Scheme 3. Different routes to poly(tetrafluoro-/)-xylylene).

Although Parylene-N possesses an outstanding combination of physical, electrical, and chemical properties, the benzylic C—bonds present are potential sites for thermal and oxidative degradation. It is well known that replacing a C—H bond with a C—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-/ -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 mediimi. 

As noted above, the synthesis of dimer involved a complicated synthetic procedure and produced very low yields. Alternative routes for deposting poly-(tetrafluoro-p-xylylene) thin film were studied the precursor method and a new... 

To simplify the synthetic effort required to deposit such films, attempts were made to deposit films by pyrolyzing tetrafluoro-p-xylene (F4C8H6). Under similar reaction conditions, a polymer film was deposited that was different from poly(tetrafluoro-p-xylylene) as the FTIR spectrum indicates that it contains more hydrogen and less fluorine. Presumably HF is preferentially eliminated rather than H2. 

Attempts were made not only to find an alternative way to replace dimer and to deposit high-quality poly(tetrafluoro-p-xylylene) film, but also to eliminate the dibromide as the precursor because of the difficulty of synthesis. Therefore, the deposition of poly(tetrafluoro-p-xylylene) film by using hexafluoro-p-xylene as the precursor instead of dibromotetrafluoro-p-xylene was tried. However, no polymer film was deposited on the wafer. Effort was expanded and other metal reagents such as nickel or copper were used to react with l,4-bis(trifluoromethyl)-benzene to generate a,a,a, a -tetrafluoro-p-xylylene to deposit poly(tetrafluoro-p-xylylene) film. However, the result showed that no film was deposited, which was not unexpected, because a C—X bond that is weaker than C—F bonding might be necessary to initiate the formation of the desired intermediate. 

Accidently, using hexafluoro-p-xylene with the contaminated copper wire obtained from the precursor method experiments, a polymer film was deposited on the silicon substrates. Obviously, some dibromotetrafluoro-p-xylene from the precursor method that adhered to, or reacted with, the metal could somehow initiate this VDP process. However, a complete explanation of these results is not yet available. As an extension of this discovery, commercially available 1,4-bis(trifluoromethyl)benzene in conjunction with a catalyst/initiator has proved to be a potential alternative by which to deposit poly(tetrafluoro-p-xylylene) film successfully.23... 

The copyrolysis of 1 wt% dibromotetrafluoro-p-xylylene with commercially available hexafluoro-p-xylene (Aldrich) with metals was examined and it was found that it was indeed possible to prepare films that were spectroscopically indistinguishable from those deposited from dimer. The PA-F films obtained are of excellent quality, having dielectric constants of2.2-2.3 at 1 MHz and dissociation temperatures up to 530°C in N2. A uniformity of better than 10% can be routinely achieved with a 0.5-gm-thick film on a 5-in. silicon wafer with no measurable impurities as determined by XPS. During a typical deposition, the precursor was maintained at 50°C, the reaction zone (a ceramic tube packed with Cu or Ni) was kept at 375-550°C, and the substrate was cooled to -10 to -20°C. The deposited film had an atomic composition, C F 0 = 66 33 1 3 as determined by XPS. Except for 0, no impurities were detected. Within instrumental error, the film is stoichiometric. Poly(tetrafluoro-p-xylylene) has a theoretical composition ofC F = 2 1. Figure 18.2 illustrates the XPS ofthe binding energy... 

Figure 18.2. XPS of poly(tetrafluoro-/7-xylylene) from the new method.

Poly-p-xylylene (Parylene N)431 Polychloro-p-xylyiene (Parylene C)43 Poly 7,7,8,8-tetrafluoro-p-xylylene54 ... 

Poly(a,a,a, a -tetrafluoro-p-xylylene) Poly(m-xylylene adipamide)... 

Polyethylene isophthalate) Poly(a,a,a ,a -tetrafluoro-/ -xylylene) Poly(N-vinyl carbazole) Poly(/>phenylene-2,5-thiodiazole) Poly(ethylene terephthalate)... 

The conventional fabrication process for poly(tetrafluoro-p-xylylene) (Par-ylene F) is difficult, involving many process steps, and is more expensive than that for Parylene N. Typically, this process first involves the formation of a dimer, l,l,2,2,9,9,10,10-octafluoro[2.2]paracyclophane. The dimers are cracked at 720-730°C to get the monomer TFPX. 

Also, low molecular weight fluorine-containing polymers form from peifluoroaromatic compounds through a loss of aromaticity when they react with bis(fluoroxy)difluoromethane. More interesting is the formation of poly(a,a,a, a -tetrafluoro-/7-xylylene) by a polymerization technique that closely resembles the preparation of poly(p-xylylene) by vacuum pyrolysis of a dimer ... 

Poly(tetrafluoro-/7-xylylene) can be used as a gate dielectric for pentacene-based organic field-effect transistors [41]. This polymer can be deposited by chemical vapor polymerization even at room temperature. The dielectric constant shows an extraordinary stability at sr of 2.3. 

Erouel M, Tardy J, Andrd E, Garden JL. Poly(tetrafluoro-p-xylylene) a low dielectric constant polymer gate insulator for organic transistors. Sens Lett 2011 9(6) 2275-8. 

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