Demand for the development of novel strategies for polymer metallization has been rapidly growing in the field of various flexible electronics applications, such as the liquid crystal displays, electronic paper, and solar cells. Polyimide film has been widely used to date for such applications as a low-k substrate, due to its chemical and thermal stability, and outstanding dielectric properties. Therefore, various processes for the metallization of polyimide films have been investigated for the development of flexible circuit elements in microelectronics applications. Additionally, adhesion strength between metal films and underlying polymer substrates becomes much more importent when the device dimensions are in a sub-micrometer scale. The conventional approach to the fabrication of metal patterns on polyimide substrates utilizes a subtractive-based strategy, i.e., lamination metal films on polyimide films followed by patterning through lithographic methods, but this approach requires stringent environmental control, costly equipment, and complex, multistep processes such as resist-coating, lithography, and etching.
In order to sustain the demand for generating multichip packaging systems for future flexible electronic devices, it would be exceedingly useful to develop an additive-based strategy with high-throughput capability that would allow site-selective, direct metallization of flexible low-k substrates. In this contribution, we present surface modification-based strategy for the direct fabrication of microscopic silver circuit patterns on polyimide surface. Deposition of silver patterns has been achieved by using ion-doped precursor films, through the process of chemical hydrolysis of polyimide by alkali treatment, doping of silver ions into the modified films followed by UV irradiation using photomasks. The high sensitivity of silver ions to UV light allows for direct area-selective photochemical reactions without the use of photocatalysts, providing granular nanostructures consisting of silver nanoparticles at metal/polymer interface. This granular structures ensure high adhesion between deposited metals and undelyimg polymer substrates. Subsequent electroless copper deposition provides copper circuit patterns on polyimide substrate, providing effective methodology for direct fabrication of circuit patterns on flexible polyimide substrate in fully additive based strategy. Specifically, the use of photomasks with square patterns (negative photomasks) provided flexible, transparent conductive films, which can be used in applications such as flexible displays and electronic papers.