Low temperature solid-state diffusion bonding of fine pitch Cu/Sn micro-bumps assisted with formic acid vapor for 3D integration

Abstract

Low-temperature solid-state diffusion (SSD) bonding of 5 μm diameter Cu/Sn micro-bumps was achieved with the assistance of formic acid vapor. Efforts were made to overcome surface oxidation of copper and uneven tin morphology, which are two major challenges in SSD bonding. Formic acid vapor was used as pre-treatment gas before bonding and protection gas during bonding. The results demonstrated that formic acid vapor is highly effective in removing surface oxidation on copper and preventing secondary oxidation, thereby facilitating a strong bond. Temperatures of 160 °C and 200 °C in 120 s were identified as ideal for pre-treatment. In SSD thermal compression bonding, 30 MPa TCB pressure was found to be necessary to overcome the uneven tin morphology. Other bonding parameters were also optimized, achieving a die shear strength of up to 59 MPa while reducing bonding temperature and time to 150 °C and 10 min. As bump scale shrinks, the interface analysis revealed a unique “teeth-like” structure in the bonding interface, contributing to improved shear strength due to intermetallic compound (IMC) growth and a favorable stress distribution. The assistance of formic acid vapor and the optimization of bonding parameters enhances the likelihood of future applications of solid-state bonding in industry, which could be an alternative choice for fine-pitch micro-bump bonding application.