Electroless Plating for Seed Layer Deposition and Direct Metallization of Glass for Interposer Fabrication

As a silicon substitute for interposer substrate, much attention has been turning to glass recently. Glass presents a number of advantages over traditionally used silicon such as low cost, small footprint, transparency, electrical insulation and potential for roll to roll or large area processing. However, it also bears shortcomings such as low thermal conductivity and difficulty in via formation. As economy is a major incentive for developing glass packaging technology, high cost fabrication techniques used for silicon, like sputtering, hinder the potential for application. To contribute to glass packaging development, a method for copper plating directly on glass substrate was proposed where an electroless copper seed layer is deposited then electro copper plating and via filling, followed by thermal diffusion to increase the adhesion (Scheme 1). A porous thin metal oxide film of a few ten nanometers fused to the substrate was used to support the catalyst for and anchor the seed layer to glass. The solution processed metal oxide films can be deposited by economic solution processing, can be applied to large area, do not impair substrate transparency, and maintain a smooth interface between copper and glass. Compared to alternative direct plating techniques that involve roughening the glass surface, impedance characteristics conducive to high frequency devices can also be expected. Two catalyst systems were investigated; adsorption of palladium onto the metal oxide film and inclusion of copper in the metal oxide, both of which sufficiently initiated autocatalytic reactions. Although seed layers showed high adhesion, adhesion significantly deteriorated after electroplating. As a solution to adhesion failure, thermal infusion of plated copper into the glass substrate was implemented resulting in 90° peel strengths of 0.3-0.4 kN/m for borosilicate glass and 0.5-0.6 kN/m for soda-lime glass. To allow thermal treatment under oxygen containing atmosphere, a high temperature resistant silica based oxidation prevention treatment was developed. With the anti-oxidation treatment, copper plating could be treated at up to 500°C in air without discoloration. Formation and mechanism of the passivation layer was analyzed by GD-OES and discussed. In circumstances where patterning is performed, this metallization method may prove compatible with subtractive and semi-additive methods, however the initially coated metal oxide anchor precursor films were found to be photodefinable. Based on photopatterning of the metal complex films that served as the metal oxide precursors, fully additive plating methods were also investigated. Photochemistry of the metal complexes and direct pattern plating will also be discussed.