Fabrication and Characterization of Nanoporous Gold (NPG) Interconnects for Wafer Level Packaging

A novel bumping and bonding technology using sponge-like gold depots with nanometer-scale skeleton construction has been developed and verified. The nanoporous gold (NPG) is formed up by selective etching the silver content from silver/gold alloys, which have been previously electrodeposited on lithographically patterned wafers. Due to the high resolution of the used photoresist systems, highest I/O densities and smallest bump sizes down to 1 μm are achieved. The desired alloy composition is adjustable in a wide range by adequate choice of the metal ion concentration in the electrolyte and by adaption of the deposition rate. The final porosity of the gold can be adjusted by sufficient choice of the etching conditions, and a final coarsening of the NPG texture can be achieved by thermal treatment prior to the bonding process. Thermal as well as mechanical tests were performed to get statements about the characteristics of the skeleton substructure and the reliability of the NPG interconnection. Exemplarily, some thermocompression (TC) bonding results are presented with focus on the NPG interconnect formation.