Pre-bond Surface Treatment of Titanium for Direct Cu-Cu Bonding Applications

Cu-Cu direct bonding plays a significant role in advancing the performance and density of three-dimensional integrated circuits. However, the surface of Cu exposed to air readily forms an oxide layer, resulting in negatively affecting subsequent bonding quality. To address the issue of copper surface oxidation, the 10 nm Ti passive layer deposited by Physical Vapor Deposition (PVD) has been proposed to protect the surface of Cu and decrease the bonding temperature. Although it can prevent rapid oxidation of Cu, the surface of Ti passive layer may partially oxidize in air to form TiO2, thereby hindering metal atomic diffusion and degrading bonding quality. Therefore, a method of pre-bonding surface treatment also has been proposed to remove the TiO2layer. First, the citric acid is used to remove the TiO2layer. Then, the Ar/N2plasma has been employed to hybrid treatment the Ti surface. Experiment results indicate that citric acid solutions at concentrations ranging from 1:40 to 5:40 can effectively remove TiO2from the Ti surface, while the Ar/N2plasma can convert Ti-O to Ti-N or Ti-O-N bonds and consequently enhancing the bonding quality. With the designed pre-treatment process, the well direct Cu-Cu bonding was finally achieved at the bonding temperature with 200 ℃, confirming the viability of this process.