Cu–Cu Bonded Microbump Interconnects With a 10-μm Pitch for 3-D-Stacked Chiplets

This article reports the fabrication and test results of fine-pitch microbump interconnects for stacked chiplets fabricated using a new Cu–Cu bonding method that is developed from Cu–Sn transient-liquid-phase (TLP) bonding. A redox treatment method is devised to change conventional Cu–Sn bumps to porous Cu–Sn bumps that can initiate TLP bonding by forming porous Cu–Sn intermetallic compounds (IMCs). The porous Cu–Sn bonding is changed to dense Cu–Cu bonding by removing the Sn composition using redox reactions and compressing the porous bumps using bonding pressures. The method inherits the merits of low resistivity and high reliability from Cu bonding and low temperature, low pressure, and free of chemical-mechanical planarization (CMP) from TLP bonding. The formation mechanism and the bonding model of the porous bumps are proposed, and the process details are given. The bonding of $1000\times 800$ microbump array with a $10~\mu $ m pitch has been demonstrated for chiplet interconnects. The results show that the bonded microbumps have high yield, low resistance, and high reliability.