Intrinsic low dielectric constant and low dielectric loss polyimides: the effect of molecular structure

Polyimide with relative low dielectric constant and high insulation have been a great dielectric material for interlayer dielectric (ILD) materials in redistribution layers (RDLs). However, with the advent of the 5G era and the ever decreasing chip size, signal crosstalk, delay and loss are getting worse, the even-lower dielectric constant and dielectric loss are great in demand. Herein, copolyimides were synthesized by a two-step process from hexafluorodianhydride (6FDA), 2,2'- bis(trifluoromethyl)benzidine and 2,2-bis[4-(4- aminophenoxy)phenyl]hexafluoropropane. Due to the low electron polarizability of fluorine, the dielectric constant of polyimide can be effectively reduced. Intrinsic dielectric constant k values could be less than 2.9. Meanwhile, by adjusting the ratio of the two diamines, the chain density can be destroyed, thus the hysteresis resistance of the dipole orientation can be weakened, and the dielectric loss can be as low as 0.00595. As- prepared polyimides also demonstrate good hydrophobicity for the presence of CF3. In addition, the addition of biphenyl structure successfully enhances thermal stability (T₅>500 °C) and mechanical properties. Tensile strength can reach 100 Mpa and elongation at break is more than 5%. So the as-synthesized polyimides prove good comprehensive properties with an easy strategy for ILD materials in the high frequency communication of 5G.

I. Introduction

Polyimide is a class of aromatic heterocyclic polymers containing an imide ring in a repeating unit of a molecular chain. The polymers and their composites have been proved with excellent comprehensive performance, including heat resistance, radiation resistance, aging resistance, solvent resistance, dimensional stability, mechanical properties, insulation properties, dielectric properties, and etc [1]. Thus, polyimides are widely used in aerospace, engineering plastics, insulating coatings and especially in the field of microelectronics, such as interlayer dielectric material (ILD) [2] in the redistribution layers (RDLs).