Enhancement of peel strength between Cu and polyimide film by introduction of 4,4′-bipyridine monomolecular layer

A simple 4,4′-bipyridine (4-BP) organic molecule was utilized to improve the peel strength between a polyimide (PI) film and sputter-deposited copper (Cu). The 4-BP layer was formed by spin coating on the PI surface treated with oxygen plasma. One of the two nitrogens at each end of the 4-BP molecule is fixed by hydrogen bonding with the carboxyl groups of the PI surface. Due to the low binding force between the 4-BP molecules, 4-BP readily forms a monomolecular layer on the surface. The nitrogen at the opposite end of the bonded 4-BP molecule forms a coordination bond with Cu, thereby improving the interfacial adhesion. 4-BP could also improve the surface roughness of the Cu layer by promoting the nucleation of deposited Cu. It was shown that the peel strength could be improved by up to 104.5 % by applying 4-BP to the interface between the PI film and Cu compared to the case without 4-BP. As a result, the peel strength between the PI film and the 12 μm thick Cu layer was as high as 0.994 N/mm, which is the highest value among those previously reported for PI substrate-based flexible Cu clad laminates using deposited Cu.