Room temperature copolymerization to improve the thermal and dielectric properties of polyxylylene thin films by chemica
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Polyxylylene thin films grown by chemical vapor deposition (CVD) have long been utilized for uniform, pinhole-free conformal coatings. Homopolymer films are highly crystalline and have a glass transition temperature around room temperature. We show room temperature copolymerization with previously untested comonomers during the CVD process. Samples were studied with wavelength dispersive analysis, FTIR, scanning variable angle ellipsometry, and x-ray diffraction. Copolymerizing chloro-p-xylylene with perfluoro-octyl methacrylate results in dielectric constants at optical frequencies as low as 2.19, compared to 2.68 for the homopolymer. Copolymerizing p-xylylene with 4-vinylbiphenyl resulted in films whose onset of weight loss in TGA measurements was 450 °C, compared to 270 °C for the homopolymer.
I. INTRODUCTION
Vacuum polymerization of cyclo-di-p-xylylene is a unique polymerization reaction that requires no catalyst and creates no reaction by-products, resulting in extremely pure material with exceptional conformity. Here, we report on a copolymerization method which leads to increased thermal stability, reduction in refractive index, and lowered crystallinity. This may increase the suitability of this method for producing intermetallic dielectrics for the electronics industry or low-loss planar waveguides. The first report1 of room-temperature copolymerization during CVD of xylene monomers was with maleic anhydride as the comonomer. The successful copolymerization of 9-vinylanthracene, 4-vinylbiphenyl, and perfluoro-octyl methacrylate (PFOMA) with paraxylenes has been reported earlier and is discussed in detail here.2 Several vinylic monomers have been reported which were copolymerized at temperatures below 0 °C, but resulted in poor morphology due to the inability of the polymer to achieve its most stable configuration during the growth process.13 The low vapor pressures of the comonomers listed above allow their use at room temperature. Parylene films grow by a free-radical mechanism, as shown in Fig. 1. Initiation requires three or more molecules to be close enough together to form the more stable diradical, which is only likely to occur after the monomer has adsorbed on the substrate.4'5 a) Author
to whom correspondence should be addressed. J. Mater. Res., Vol. 9, No. 12, Dec 1994
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Cydo-para-xylylene sublimation chamber I 135C
Convonomer sublimation chamber Various temperatures CH = CH CH = C H
Cyclo-dipara-xylylene
Paracydophane (created in furnace)
Poly-para-xylylene
FIG. 1. Vacuum deposition equipment and reaction path.
Additional monomer adds to the diradical, propagating the polymer outward from the surface. The rate of initiation is proportional to the third power of monomer concentration, and the propagation rate is proportional to the monomer concentration, leading to high molecular weights. This polymerization mechanism, unique to cyclo-di-paraxylylenes, is largely responsible for the utility of these films; the conformity and purity of
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