High durability of low-k value is a desired property for dielectrics serving under humid conditions, because absorbing a small amount of moisture by the material can considerably increase the k value so as to result in function deterioration. Aiming to develop a dielectric polymer with superior durability of low-k value and high thermal stability, a perfluorocyclobutyl (PFCB) biphenyl ether-based polyimide, PFCBBPPI, was synthesized. This polymer possesses a Tg of 310.3 °C and a 5% weight loss temperature of 510.5 °C. PFCBBBPPI exhibited an extremely low water uptake of 0.065 ± 0.018%, representing the best water resistance in polyimides. The increasing percentage in k value was below 2% for PFCBBPPI film exposed to moisture under various humidity conditions for 6 h. PFCBBPPI film equilibrated at 75% R.H. for 2 weeks still kept its k value below 2.50, remarkably outperforming the Kapton film. The remarkable water resistance and resulting high durability of low-k property displayed by PFCBBPPI are originated from the hydrophobic nature and small free volume fraction of the polymer, as confirmed by contact angle test and positron annihilation lifetime spectroscopy results. The outstanding moisture resistance and overall performance of PFCBBPPI make it a suitable candidate for dielectric applications under both dry and humid conditions.
Keywords: dielectric material; k value; perfluorocyclobutyl aryl ether; polyimide; water absorption.