Enhancing void reduction in direct copper bonding through electroless nickel coating on alumina substrates

The copper foil can be well-bonded to alumina substrate at temperatures above the Cu–O eutectic temperature by the direct copper bonding technique. During the bonding process, voids and defects are easily formed at the Cu/Al₂O₃ interface, giving rise to reliability issues. This study explored the effects of different oxygen content in the form of the thickness of the cupric oxide layer on copper foils and the thickness of the electroless-plated nickel on alumina substrates on the void content of the direct copper-`bonded substrate. Research findings indicated that insufficient oxygen in the copper foil hindered the complete bonding of the Cu/Al₂O₃ interface due to an inadequate eutectic liquid phase. A successful bonding occurred between the copper foil pre-oxidized at 800 °C and the alumina substrate after 30 s of electroless nickel-plating (with a nickel layer thickness of about 0.02 μm). During high-temperature bonding, the Ni layer reacted with oxygen to form NiO, which then reacted with the cuprous oxide on the copper foil surface, forming a Ni-doped CuAlO₂ phase. This phase effectively consumed oxygen during the bonding process, reducing the occurrence of voids. However, as the nickel coating thickness increased, the copper foil could not be bonded well to the alumina substrate because the thicker Ni reduced the oxygen content, preventing the formation of the Cu–O eutectic liquid phase.