Novel soluble polyimides containing pyridine and fluorinated units: preparation, characterization, and optical and dielectric properties

Abstract

To acquire low dielectric constant polyimide films with good mechanical and thermal properties and low coefficient of thermal expansion (CTE) applied in microelectronic fields, as a feasible tactic, three novel diamines containing pyridine and –C(CF₃)₂– groups were firstly designed and synthesized to employ polymerization with 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropanedianhydride (6FDA) via a two-stage process with a heating imidization method. Three diamine monomers included one unsubstituted pyridine ring, and another two methyl-substituent groups on two pyridine rings at the 6- and 4- position. The structure–property relationships between the different pyridine rings of the fluorinated PI films, including dielectric constant, thermal stability, mechanical strength, optical transparency, and solubility, were systematically investigated. The fluorinated PI films exhibit low dielectric constant in the range of 2.36–2.52 at 1 MHz, while they still display excellent mechanical properties with tensile strengths as high as 114 MPa. Meanwhile, the PI films show good thermal stability with glass transition temperatures (T_g) in the range of 262–275 °C, low coefficients of thermal expansion (CTEs) ranging from 64 to 68 ppm °C⁻¹ and 5% weight loss temperatures (T_5%) located between 468 °C and 499 °C. Further, PI films possess outstanding solubility for easy fabrication. Therefore, these types of fluorinated PI films provide a potential application as alternative dielectric layers in future microelectronic technology.