Low-temperature Cu–Cu direct bonding enabled by a new method of plasma activation

The Cu–Cu bonding used in hybrid bonding requires high bonding strength and an oxide-free bonding interface during low-temperature annealing below 300 °C. However, the hydrophilic bonding currently used in hybrid bonding forms hydroxy groups at the bonding interface of the interlayer film to create a bond. This process oxidizes the Cu surface and forms an oxide layer with localized CuO and CuOH with high electrical resistance at low-temperature annealing below 300 °C. To suppress the formation of this oxide layer and achieve direct Cu–Cu bonding at low-temperature annealing below 300 °C, we formed a bonding surface suitable for Cu–Cu bonding using plasma-assisted activation with a low-inductance antenna that can be processed with low plasma damage. With this new plasma-assisted activation, high bonding energy and good bonding can be achieved without the oxide film on the Cu–Cu bonding surface at a significantly lower temperature than the post-annealing temperature currently used for hydrophilic bonding. These features enable the Cu–Cu bonding used in hybrid bonding to be performed at low temperatures and reduce the mismatch in thermal expansion coefficients, which is a technical issue in direct bonding.