Mechanical and microstructure characterization of high aspect ratio electroplated through-wafer copper interconnects

In this paper, we present the mechanical characterization of high aspect ratio through-wafer electroplated copper interconnects. Copper was deposited in very high aspect ratio (~15) and narrow DRIE etched through-vias (15 mum) in silicon substrate by a special electrodeposition technique. Since the basic understanding of the mechanical and material properties of electroplated copper is very critical for the development of next generation electronic devices and 3D wafer level packaging, these properties and the grain structure of electroplated copper were measured by appropriate characterization techniques. The elastic modulus and the hardness of electroplated copper were measured by nanoindentation continuous stiffness measurement, while the grain structure of electroplated copper was found out by atomic force 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. The Young's modulus and the hardness of electroplated copper interconnects measured at the top of interconnects (150 GPa and, 2.4 GPa, respectively) were found to be higher than the measured values at the cross-section of copper pillars (136 GPa and, 2.0 GPa respectively). It was found that the indentation modulus along the {111} plain is about 10-25% more than that along {100}. As a result of this difference, the copper pillars form texture and have preferred crystal orientation along the z-axis