Characterization of Nano-grained High Aspect Ratio Through-wafer Copper Interconnect Column

Through-wafer interconnect is one of the key technologies for fabricating next-generation compact 3D microelectronic devices. The microstructure and mechanical properties of high aspect ratio through-wafer electroplated copper interconnects are reported in this paper. Copper was deposited in very high aspect ratio (~15) and narrow DRIE etched through-vias (15 mum) in silicon substrate by electrodeposition. With the presence of nano-scale grains and higher density of nano-twins, the copper columns were found to have significant higher hardness and tensile strength than that of conventional coarse-grained copper, while retaining an electrical conductivity comparable to that of pure copper. The grain structure of electroplated copper was found out by atomic force microscope and transmission electron microscope. The induced strain, a result of mismatch in coefficient of thermal expansion, was studied by digital image speckle correlation analysis, when the copper interconnects were subjected to a temperature cycle from 25degC to 125degC.