Novel Low Dielectric Constant Thin Film Materials by Chemical Vapor Deposition
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A low dielectric constant polymer, poly(tetrafluoro-p-xylylene) (VT-4), was synthesized by chemical vapor deposition from 4,5,7,8,12,13,15,16-octafluoro-[2.2]-paracyclophane (DVT-4). The main motivation was to find a cheaper alternative to poly( alpha, alpha , alpha ', alpha '-tetrafluoro-p-xylylene) (AF-4) with similar properties. The dielectric constant of VT-4 was measured as 2.42 at 1 MHz, and the in-plane and out-of-plane indices of refraction were 1.61 and 1.47 at 630 nm. The large negative birefringence suggests a low out-of-plane dielectric constant, which is desired for interlayer dielectrics. The VT-4 polymer was found to be stable at 460 oC by thermogravimetric analysis (TGA).
Polymer/Siloxane nanocomposites were studied as an alternate path to a polymer/silica composite. This study showed that incorporation of a four-ringed liquid siloxane precursor into the parylene PPXC is not feasible. A solid precursor cube-like molecule, vinyl-T8, was incorporated with ease. Pyrolysis of vinyl-T8 at different temperatures revealed complex behavior, with the formation of polymerized vinyl-T8 (through free radical addition at the vinyl groups) as well as silica-like structures forming above 500 oC as a result of the breaking up of the cage structure of vinyl-T8. Codepositions of PPXC and vinyl-T8 were then examined as a possible path towards a polymer/silica nanocomposite. At deposition temperatures below 5o C, precipitation of excess vinyl-T8 into cubic micron-sized crystals occurred. As this was undesirable, studies were continued at higher deposition temperatures. A Taguchi orthogonal array was set up to study the effect of the sublimation temperatures of the two precursors as well as the pyrolysis temperature and the substrate temperature on the deposition rate, the index of refraction and the weight loss after a 500 oC anneal. The deposition rate depended mostly on the sublimation temperature of the PPXC and the substrate temperature. The lowest index of refraction (and thus the lowest dielectric constant) was obtained with the lowest sublimation temperatures of 134 oC for PPXC and 195o C for vinyl-T8 and a pyrolysis temperature of 200 oC. Each of the factors was found to have an effect on the index of refraction, with the sublimation temperature of vinyl-T8 having the most influence. The films degraded at 500 oC, indicating that post-deposition annealing of the films did not lead to a conversion of the vinyl-T8 to a SiO2 -like structure (which would be stable at that temperature). X-ray diffraction spectra of the films revealed peaks which were not present for any of the vinyl-T8 films or characteristic of PPXC. Therefore, some type of interaction between the two components occurred and affected the morphology, most likely the formation of a block copolymer. Thus, though polymer/silica films were not attained, the resulting composites had comparable properties with higher deposition rates and a cleaner process.
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