Carbazole-grafted Polyimide with Enhanced Adhesion to Smooth Copper

Polyimide (PI) has become a crucial material for interlayer dielectrics in advanced packaging thanks to its admirable comprehensive properties. PI and copper wires collaborate to construct the redistribution layer (RDL), the key part of fan-out wafer-level packaging (FOWLP), allowing for higher I/O density. However, the weak adhesion and the mismatch of coefficient of thermal expansion (CTE) between PI and copper lead to reliability issues. Herein, a novel polyimide (named GPI) containing benzimidazole ring in backbone and carbazole ring in side chain was designed and synthesized. Since the nitrogen-containing heterocycles can form coordination with copper, the prepared polyimide was supposed to have stronger interactions with copper. Besides, the grafted side groups tuned the flexibility of chains, and further improved the bond between polyimide and copper. With the grafting of carbazole ring into polyimide, the peel strength of GPI-3 increased to 0.9424N/3mm in 90° peel test, showing satisfactory adhesion. Furthermore, the fracture mode was proved to be cohesive failure by analyzing the peeled surface, indicating a strong interfacial bonding between GPI-3 and copper. Meanwhile, this new polyimide possesses outstanding mechanical (the tensile strength of GPI-3 is 176.6 MPa and the Young’s modulus is 3.97 GPa), thermal properties (T5% of GPI-3 is 464.83 °C, CTE is 22.43 ppm/K, close to the CTE of copper). Therefore, the prepared GPI has application prospects in FOWLP and provides a reference for the design of PI materials with enhanced adhesion to smooth copper.