Copper nanowires bonding for three dimensional interconnections

Abstract

Over the past few decades, the semiconductor industry has been advancing by increasing transistor density on a microchip through continuous device scaling based on the technology roadmap driven by Moore’s Law. However, new physical phenomena at scaled dimensions and fundamental limitations in material properties are pushing the limits of existing planar devices. Moreover, interconnect delay, bandwidth and power dissipation are increasingly dominating the integrated circuits’ (IC) performance. Hence, a paradigm shift from the present IC architecture is needed to overcome the saturation in chip performance. Three dimensional (3D) integration has emerged as one of the promising techniques to overcome Moore’s Law, by replacing long horizontal wires with short vertical interconnects. This has resulted in smaller form factor, a reduction in RC (resistance-capacitance) delay and allows integration of heterogeneous systems with diverse functionality catering to “More than Moore” technology. However, 3D packaging which involves stacking of devices through the use of solder bumps or solder capped copper (Cu) pillar bumps, is facing challenges due to scaling down and reliability issues. In this work, fabrication of Cu nanowires (NWs) arrays coupled with thermocompression bonding is proposed as an alternative to existing 3D interconnection approaches. To explore the feasibility of adopting this alternative 3D interconnection, main emphasis is placed on the process development of the Cu NWs arrays on Si substrate and characterization of these bonded NWs arrays. The diameter of Cu NWs can be controlled by varying the anodization voltage, electrolyte type or pore widening duration during the fabrication of the porous alumina template. Growth of Cu NWs with preferred texture can be obtained by manipulating the electrodeposition conditions such as voltage and temperature. The use of different metal interlayers on the substrate influences the NWs density and subsequent processes. Characterization of the Cu NW revealed smooth and continuous morphology with preferred crystal growth orientation in [111] direction. The second part of the research focuses on the bonding of Cu NWs arrays. At low bond temperature of 200oC, NW-Film and NW-NW bonding exhibit higher average shear strength than that of Film-Film bonding. However, scanning electron microscope (SEM), transmission electron microscope (TEM) and electron energy-loss spectroscopy (EELS) characterizations revealed that the Cu NWs have transformed into Cu oxide nanotubes. Further investigations show that the transformation was a result of Nanoscale Kirkendall Effect due to the low vacuum environment during bonding. Different annealing treatments on the NWs arrays also revealed that multiple voids form initially in the Cu NWs, with further growth through surface diffusion of Cu atoms along the voids’ surface which eventually leads to a hollow core. To overcome Cu oxide nanotube formation, another set of experiments was carried out using a commercial bonder with a high vacuum environment.…