Low-temperature solid-liquid interdiffusion bonding for heterogeneous integration

There has been a notable transition in the electronics industry towards the heterogeneous integration strategy to achieve a compact yet multifunctional devices. However, this approach present challenges when incorporating materials with differing characteristics, particularly regarding temperature-induced constraints for the assembly process. Low-temperature bonding techniques are one of the key technologies to address this problem and fully realize the potential of heterogeneous integration. Among the numerous bonding methods, this dissertation explores the low-temperature bonding processes based on the solid–liquid interdiffusion (SLID) technique utilizing Cu–Sn–In metallurgy. The eutectic behaviour of Sn–In allows a significant reduction of the bonding temperatures from a typical bonding temperature above 250 °C down to 150 °C. Microstructural characterization of the bonds formed at temperatures between 150 °C and 200 °C reveals a homogeneous interconnect featuring an intermetallic phase with a remelting temperature above 450 °C, which makes it thermally stable. Electrical measurements confirm the interconnects’ small resistance that is comparable to those obtained from well-known Cu–Sn bonding technique, underscoring the capability of this low-temperature process for vertical integration. Hermeticity assessment, conducted using seal-ring shaped bond, highlight the technique’s potential for microelectromechanical system (MEMS) or micro-optoelectromechanical system (MOEMS) packaging. Moreover, the bond exhibited tensile strength of a similar magnitude with the popular Cu–Cu thermocompression bonding method. Some challenges in the implementation of the low-temperature bonding process have been identified, including limitations for process integration, squeeze-out, and defect formations. These issues can be mitigated through optimization of the design and processing parameters. Despite these challenges, the results from this work highlight the prospect to fully demonstrate the capabilities of heterogeneous integration by addressing temperature-induced limitations.