Low cost electroless copper metallization of BCB for high-density wiring systems

A fully additive and selective metallization process for structured Cu metallization of BCB was evaluated. BCB was used as the polymeric dielectric due to its excellent electrical and mechanical properties. BCB surface pretreatment by RIE is necessary as a first step to achieve sufficient adhesion of electrolessly deposited Cu on BCB. In a second step, the BCB surface is coated by a precursor layer consisting of palladium acetate and a photoactivator. Precursor layer structuring is by standard i-line lithography followed by wet development with an organic solvent that dissolves unexposed precursor layer areas (negative working photolithography). The remaining pattern consists of Pd²⁺ in an organic matrix which is not yet catalytically active for electroless plating. An alkaline solution of KBH₄ in water activates the Pd²⁺ by reducing it to Pd⁰. Pd⁰ is highly active for electroless Cu deposition. The metallization process uses commercial electroless Cu baths. The electrical resistance of the electrolessly metallized vias is comparable to sputter metallization. The possibility of thin film multilayer metallization was evaluated by via metallization. The reliability of the deposited Cu was checked by air-to-air thermal cycling (-55°C/+125°C). The application of this process could be in the area of high density substrates such as the one used for MCM-D/L