Novel Low Dielectric Constant Thin Film Materials by Chemical Vapor Deposition

A modified CVD reactor was designed with a deposition chamber capable of accommodating 8" wafers, with the capacity to remotely pyrolyze two different precursors. The design was based on a previous working reactor, with the most notable improvements being a showerhead design for more even delivery of gaseous precursor and a separate heating control of the substrate holder and deposition chamber walls. The performance of the reactor was analyzed by testing the pressure gradients within and the thickness uniformity of films deposited on 8" wafers. The reactor exhibited a linear pressure gradient within, and the thickness uniformity was excellent, with a slight increase in thickness towards inlet of the showerhead. The thickness difference between the maximum and minimum thickness on an 8" wafer was 14%. Films of polyparaxylylene (PPXN), polychloroparaxylylene (PPXC), SiO₂, and PPXC/SiO₂ were deposited, with deposition rates and indices of refraction comparable to those obtained on the old reactor design. A full factorial study was performed to determine the effect of the substrate temperature, the sublimation temperature, and the pyrolysis temperature on the deposition rates of PPXC. It was determined that the substrate temperature has the greatest effect, with about 50% contribution, and deposition rates increased with decreasing substrate temperature. The sublimation temperature contributed 25%, with increasing sublimation rates leading to higher deposition rates. The pyrolysis contributes very little, with about 2%, and the variance ratio did not fall within a 90% confidence level.

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 °C 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 °C 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 5 °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 °C 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 °C for PPXC and 195 °C for vinyl-T8 and a pyrolysis temperature of 200 °C. 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 °C, 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.