Influence of bonding process parameters on chip cratering and phase formation of Cu ball bonds on AlSiCu during storage at 200 °c

Wire bonding remains the predominant interconnection technology in microelectronic packaging. Over the last 3 years a significant trend away from Au and towards Cu wire bonding has become apparent. This has been due to general efforts to lower manufacturing costs and price increases for raw materials like Au. Although much research has been carried out into wire bonding over recent decades, most has focused on Au ball/wedge bonding. The results of this research have shown that bonding parameters, bonding quality and reliability are closely interconnected. However, the different material properties of Cu compared to Au, such as affinity to oxidation and hardness, mean that these insights cannot be directly transferred to Cu bonding processes. Thus, further research is necessary. This paper discusses a study of bonding interface formation under various bonding parameters. Cu wire was bonded on AlSiCu0.5 metallization and a bonding parameter optimization was carried out to identify useful parameter combinations. On the basis of this optimization, different samples were assembled using parameter combinations of low, medium and high US-power and bonding force. An interface analysis was subsequently carried out using shear testing and HNO3 etching. Intermetallic phase growth was analyzed on cross sections of devices annealed at 200 °C for 168 h and 1000 h. Contacts bonded with low bonding force and high US-power tended towards cratering during shear testing. Bonding force proved to have a significant effect on intermetallic phase formation whereas US-power was found to exert only a minor influence. The intermetallic phase formation of annealed samples was analyzed using EDX and interpreted on the basis of phase formation kinetics. Three main intermetallic phases were identified