Grains, deformation substructures, and slip bands observed in thermosonic copper ball bonding

In integrated circuit manufacturing, bonding a copper wire of diameter 25–75 μm (1–3 mil) on an aluminum-metallized silicon substrate has been accomplished by the application of ultrasonic energy along with compressive force at 200 °C. Prior to the bonding, the wire tip was melted by electric sparking to form a spherical ball, which, when analyzed, displayed columnar grain growth. Microstructural study of the final bonding showed that the application of ultrasonic energy resulted in the subdivision of the grains of a few micrometers in size, giving rise to deformation substructures (or cells). In addition, slip bands were observed inside the cells. Microhardness studies showed that the ball bonds had a higher hardness (112 VHN) than that of the unannealed bare copper wire (102 VHN), suggesting that the applied ultrasonic energy strain hardened the copper ball by creating the deformation substructures and slip bands. Furthermore, the heat-affected zone above the copper ball clearly contained recrystallized grains along with deformation markings.