Hybrid surface pre-treatments for enhancing copper-to-copper direct bonding

• A hybrid surface pretreatment combining plasmas and pulsed flash was developed. • Surface activation, oxidation and stresses can be controlled by the pretreatment. • N 2 plasma activates Cu surface and forms Cu 4 N layer inhibiting oxidation. • Pulsed flash causes compressive stress enhancing diffusion and bonding. • H 2 -containing plasma further activates Cu surface but causes excessive oxidation. Three-dimensional integrated circuits (3D IC) require low-temperature, high-reliability Cu–Cu direct bonding to support fine-pitch vertical interconnects and heterogeneous integration. This study developed a hybrid surface pretreatment method combining plasmas and pulsed high-energy flash irradiations to modify Cu surface conditions and enhance bonding performance. Sputtered Cu films were treated using various plasma gases (N 2 and those with different H 2 contents up to 30%), followed by flash exposure, and their effects on surface energy, stress evolution, and joint strength were systematically evaluated. N 2 plasma activated Cu surface and gave rise to the formation of a stable Cu 4 N layer that effectively inhibited oxidation. Subsequent flash exposure caused compressive stress, which accelerated Cu atom diffusion and advanced the bonding. Hydrogen-containing plasmas further activated Cu surface, promoted the hydrophilicity and improved joint strength. However, the extremely activated surface thus formed is susceptible to excessive oxidation in the following flash light exposure. It can be proposed that for achieving robust joint strength the oxide thickness on the Cu surface should be controlled less than 7 nm. These findings highlight that reliable bonding can be achieved by controlling the combined effects of surface activation, oxidation and stresses introduced during pretreatment.