High-Temperature Polymers for Advanced Microelectronics

Several series of aromatic polyethers and polybenzoxazoles with high thermal stability and low dielectric constant were prepared and characterized. The polyethers were synthesized by nucleophilic aromatic displacement of fluorine with phenoxides. In order to avoid strongly polar groups such as carbonyl and sulfone, trifluoromethyl groups were used to activate the fluorine for displacement. An additional benefit of the presence of the trifluoromethyl groups is the decreased dielectric constant, which fluorinated materials exhibit. This is attributed to two factors: the strong electronegativity of fluorine, resulting in very low polarizability of the C–F bonds, and the larger radius of a fluorine atom in comparison with a hydrogen atom, resulting in increased free volume. Trifluoromethyl substituted terphenyl and quadriphenyl poly(arylether)s prepared in this study exhibit decomposition temperatures far in excess of 500°C, even in air, dielectric constants below three, and mechanical properties comparable to engineering plastics such as polycarbonate and high-performance thermoplastics such as PEEK. Poly(benzoxazole)s were prepared with and without fluorine substituents. Since most of the poly(benzoxazole)s were insoluble, they were prepared via soluble poly(hydroxyamide) precursors, which were converted to the final polymers by thermal treatment. These materials had dielectric constants as low as 2.69 and also decomposition temperatures far above 500°C in air.