Synthesis and characterization of fluorinated benzoxazole polymers with high T_g and low dielectric constant

Next generation microelectronic packaging requirements are driving the need to produce increasingly lower dielectric constant materials while maintaining high thermal stability and ease of processing. Efforts have focused on the synthesis and analysis of new polymers with the goals of high thermal stability [degradation temperature (T_d) >400 °C, low glass-transition temperature (T_g) >350 °C], low water uptake (<1%), solubility in selected organic solvents, dielectric constant less than 2.5, and low thermal expansion coefficient. These stringent combined goals have been largely achieved with flexible aromatic benzoxazole polymers. Intramolecular hydrogen bonding between pendant hydroxyl groups and the double-bond nitrogen of the benzoxazole has been exploited to increase the polymer T_g, whereas the incorporation of perfluoroisopropyl units effectively decreases the dielectric constant. Out-of-plane impedance measurements on films of materials in this family (38-134 μm thick) have resulted in typical dielectric values of 2.1-2.5 at 1 MHz, depending on copolymer ratios and functionalizations. Results have been correlated with optical waveguide measurements of films 4-μm thick to determine film anisotropy and the high-frequency dielectric constant, and have been corroborated by in-plane interdigitated electrode dielectric measurements on samples 0.75 μm thick. Candidate materials exhibited extremely low water uptake (0.2%) even after submersion in boiling water for several days. Dynamic mechanical analysis of the polymers enabled the determination of the influence of intermolecular hydrogen bonding on the T_g and loss tangent magnitude. Finally, the coefficient of thermal expansion has been examined and correlated with copolymer constitution.