Bumpless interconnect through ultrafine Cu electrodes by means of surface-activated bonding (SAB) method

In this paper, we demonstrate the feasibility of ultrahigh-density bumpless interconnect by realizing the ultrafine pitch bonding of Cu electrodes at room temperature. The bumpless interconnect is a novel concept of bonding technology that enables a narrow bonding pitch of less than 10 μm by overcoming the thermal strain problem. In the bumpless structure, two thin layers including an insulator and metallic interconnections on the same surface are bonded at room temperature by the surface-activated bonding (SAB) method. In order to realize the bumpless interconnect, we invented a SAB flip-chip bonder that enabled the alignment accuracy of ±1 μm in the high vacuum condition. Moreover, the fabrication process of ultrafine Cu electrodes was developed by using the damascene process and reactive ion beam etching (RIE) process, and the bumpless electrodes of 3 μm in diameter, 10 μm in pitch, and 60 nm in height were formed. As a result, we succeeded in the interconnection of 100 000 bumpless electrodes with the interfacial resistance of less than 1 mΩ. An increase of the resistance was considerably small after thermal aging at 150°C for 1000 h.