Effect of Ar/N₂ Two-Step Plasma Treatments on the Interfacial Characteristics of Low-Temperature Cu-Cu Direct Bonding

The effects of Ar/N₂ two-step plasma treatments on the interfacial adhesion energies of low-temperature Cu–Cu bonding interfaces were systematically investigated with four-point bending (4-PB) test. It was confirmed that the Cu surface roughness had increased, and a Cu nitride layer was formed by the Ar/N₂ two-step plasma treatment. X-ray photoelectron spectroscopy clearly showed that the Ar/N₂ two-step plasma treatment formed less Cu oxide due to the formation of a Cu nitride layer. As a result of the 4-PB test, as the N₂ RF power was increased, the interfacial adhesion energy decreased. An analysis of the delaminated surface after the 4-PB test confirmed that a Cu nitride layer was not formed, which was thought to be due to decomposition during the bonding process. As the N₂ RF power was increased, the roughness also increased, leading to poor Cu-Cu bonding. The decrease in Cu-N bonding resulted in the progression of Cu oxidation. Additionally, the interfacial adhesion energy decreased due to the formation of a disordered Cu layer on the Cu surface.