Anisotropic dielectric properties of polyimides consisting of various molar ratios of mete to para diamine with trifluoromethyl group

Abstract

A series of copolylmldes based on pyromellitic dianhydride (PMDA) with various molar fractions of 4,4′-(hexafluoro-isopropylidene)dianiline (4,4′-6F) and 3,3′-(hexafluoro-isopropylidene)dianiline (3,3′-6F) were synthesized by a two-step method. The five different composition copolyimides in the form of [(PMDA+3,3′-6F)m/ (PMDA+4,4′-6F)n] were mainly characterized using a dielectric analyzer (DEA) by single surface sensor (in-plane direction) and thin film parallel plate sensor (out-of-plane direction) measurements. DSC, TMA, and XRD were also used to study the structure property. The increasing of molar ratio of para diamine in the copolyimide system up to 35% affected glass transition temperature, coefficient of thermal expansion, in-plane dielectric constant and out-of-plane dielectric constant of copolyimides, correspondingly. The in-plane dielectric constant was higher than that of the out-of plane constant for our polyimide films. Anisotropy Δε of the dielectric constants was 0.14 for CPI(100/0)a, 0.19 for CPI(85/15)a, 0.11 for CPI(75/ 25)a, and 0.05 for CPI(65/35)a. The difference in curing history also exhibited an effect on solvent diffusion behavior in our polymer system. Polymers cured at a slower curing rate had smaller CTE than that cured at a faster curing rate, as confirmed by X-ray diffraction results. Polymers with smaller CTEs had larger dielectric constants at a slower curing rate, and vice versa. The experimental results suggested that CPI(65/35)a with smaller dielectric anisotropy could solve the crosstalk problem and provide equal electrical insulation in microelectronic devices. Therefore, a smaller and faster IC device could, it is hoped, be achieved, with smaller spacing between adjacent metal lines.