Synthesis of polyimides containing pyridine or triazole moiety to improve their adhesion to sputter-deposited copper

Polyimides containing pyridine as well as triazole were synthesized. The incorporation of pyridine or triazole improves the adhesion between polyimides and sputter-deposited copper. 4,4'-oxydianiline (ODA) was separately mixed with 2,6-diaminopyridine (DAP) and 3,5-diamino-1,2,4-triazole (DATA), to form co-diamines. A series of polyimides was obtained in two steps by reacting co-diamines and 3,3',4,4'-pyromellitic dianhydride (PMDA). The structure of the polyimides was verified by Fourier Transform Infrared spectroscopy (FT-IR) and Nuclear Magnetic Resonance (NMR). Their thermal, mechanical and dielectric properties were investigated. The rigidity of both pyridine and triazole moieties influences the coefficient of thermal expansion, the tensile strength and the elastic modulus of the films. The adhesion strength of the sputter-deposited copper to polyimide films was proportional to the functional group content. At a molar ratio of DAP to ODA of I : 6, the 90 degrees-peel strength of copper/polyimide laminates reached a maximum of 990 J/m(2). At a molar ratio of DATA to ODA of I : 6, the 90 degrees-peel strength of copper/polyimide laminates reached a maximum of 696 J/m2. The corresponding polyimide films exhibited a good balance in thermal, mechanical and dielectric properties, as did the PMDA-ODA film. The locus of failure (LOF) examination by X-ray photoelectron spectroscopy (XPS) indicated that the LOF of laminates with low to moderate adhesion was mostly at the interface near the polyimide; the LOF of laminates with high adhesion was mostly in the polyimide. The N-1s core level spectra of the delaminated copper surface revealed a peak at 398.4 eV in copper/polyimide with DAP/ODA ratio of I : 6 and a pseak at 398 eV in copper/polyimide with DATA/ODA ratio of 1 : 6, perhaps due to the formation of N-Cu charge-transfer complex. This complex substantially promoted the adhesion between sputter-deposited copper and polyimides.