Nano-Porous Gold Interconnect

Abstract

Pure metal contacts offer some advantage as high thermal and electrical conductivity, ductile deformation behavior and reduction of mechanical peak stresses. They are used in flip chip assemblies as gold bumps, in die attach as sintered silver powder or as gold layer on very smooth surfaces for wafer bonding. Analyzing the demands in most suitable material properties we were looking for a compressible metal contact, which could compensate for all implanarities, with a highly reactive surface to reduce bonding temperature. It should be a noble metal, but with good adhesion to polymers. We have developed a nanoporous metal layer processed by electroplating a silver-gold alloy with 20 to 30% gold. High plating rates could be achieved exceeding those of standard gold electrolytes. Subsequent de-alloying by etching off the silver provides a nano-porous gold layer as an open-porous sponge with 70 to 80% porosity. Pore and ligament size were measured from different samples between 20 to 100 nm. Aging experiments showed the coarsening of pore size. We have successfully plated nano-sponge layer of 10 μm height on top of gold bumps on wafer level. But we don't see a limitation yet to further increase the thickness of gold nano-sponge. Chips were bonded by thermocompression resulting in a reduction of bump height without changing the bump diameter. At lower bond force and lower bond temperature the collapse of the porous structure occurred in the bond interface mainly, leaving the porosity in most of the bump volume unchanged. This leads to a new interconnect structure of high porosity. For very high porosity stiffness should be reduced with the square of the volume ratio. We therefore expect improved reliability due to the reduction in stiffness. Due to the compressible deformation behavior of the nano-sponge it could improve the yield during wafer-to-wafer bonding and stacking as the particle will be absorbed into the sponge. This allows bonding of wafers without the need of planarization the dielectric layer, e.g. due to steps over conductor lines. Therefore we will use the nano-sponge in stacking and 3D integration. Beside compression bonding and sintering we use the nano-sponge also as a thermal interface for adhesives. Filler particles are pressed into the sponge structure leading to a larger contact area between particle and substrate surface, thereby the adhesive matrix can penetrate into the open porous film. The locking interface should also provide very high adhesion strength.