Low-Temperature Ultrasonic Bonding of Cu/Sn Microbumps with Au Layer for High Density Interconnection Applications

Abstract

Flip-chip bonding has become an efficient method to realize fine-pitch interconnection in high density interconnection applications. Thermal-compression bonding of Cu/Sn microbumps can induce extra thermal stress because of high bonding temperature, long bonding time and high bonding force. Temperature, time and force are expected to be decreased to improve the thermal-mechanical reliability of the integration systems. In this work, low-temperature ultrasonic bonding of Cu/Sn microbumps with a thin layer of gold was studied. We also studied bonding of redistribution layers (RDLs) that consisted of electrodeposited copper and a thin layer of gold. The feasibility of the low-temperature ultrasonic bonding was demonstrated through the preliminary experimental results. Cu/Sn microbumps with Au layer were successfully bonded through a quick bonding process and a followed annealing process. However, in the case of bonding of the RDLs, the cross-section of some bonded RDLs showed that cracks existed at the interface of Au/Au layers, which resulted from the uneven surface. The electrodeposition process needs improving to get a flatter surface and the parameters of the bonding process still needs to be optimized.