Study of a practicable wire bonding method for applying copper wire bond to large-scale integrated circuits

Since 1992, copper wire bonding technology has been developed in semiconductor manufacturing as an alternative interconnection material instead of gold wire due to many advantages in mechanical characteristics and cost efficiency. It has been widely applied to mass production of discrete components recently. However, copper wire bonding technology is still on the way of experimental development for large-scale integrated circuits with big challenges caused by interaction of high hardness of copper and special bond pad design of large-scale ICs. A new wire bonding approach named COG (Copper ball on gold bump) bonding is being developed to solve the challenges using current wire bonder. The whole study of the COG bonding encompassed machine hardware modification, copper wire selection, wire bonding process window development, reliability assessment and micro-structure analysis at two-metal interfaces. Wire bonding process window development for the COG bonding mainly focused on two critical stages, copper free air ball (FAB) formation and bonding parameter optimization for both gold bumps and copper bonded balls using Design of Experiment (DOE). Wire pull test, ball shear test and cratering test were involved in outputs measurement per JEDEC criteria during the process window development. Packaging assembly integrity and reliability assessment with the COG bonding were fully investigated by performing electrical characterization and package internal delamination detection on the experimental samples, which have undergone two reliability tests respectively. Furthermore, micro-structure analysis focused on IMC change at two interfaces of Au-Al and Cu-Al couples through the reliability tests. The study result finally indicates that COG bonding is a practicable approach to achieve copper wire application to large-scale integrated circuits with risk-free.