Ag-Ag direct bonding via a pressureless, low-temperature, and atmospheric stress migration bonding method for 3D integration packaging

3D packaging technology is a critical method in the realization of three-dimensional integrated circuits (3D ICs). In this work, we proposed a convenient Ag-Ag direct bonding method by using a two-step bonding process that allows bonding at a pressureless, low-temperature, and atmospheric condition. At first, a transient ultrasonic bonding was conducted for the prebonding to ensure a close-up of the Ag surfaces, and then a pressureless annealing process was applied to coalesce the bonding interface. An intact bonding was realized by using the two-step bonding process, which is due to a synergistic action of stress migration and Ag-O reaction. During the migration of the stress, the Ag surface became rough and generated hillocks, leading to the coalescence of the bonding interface even at a low-temperature annealing condition. At the same time, the oxygen act as an accelerator that greatly improved the self-diffusion coefficient of Ag. The proposed method can be applied to chip to wafer or chip to interposer bonding, which can significantly mitigate the requirements for the bonding instruments and conditions.