Cu Bumping Characterization of Bond Pad and Underlying Structures and its Impact to Cratering

摘要

Copper wirebonding process has gained semiconductor industry's attention for power and discrete devices due to its cost reduction and device electrical advantages over the conventional Gold bonding process. Copper wire has several advantages over gold which includes the following properties: (1) Cu is lower in material cost than Au by 30-70% (2) Cu wire has higher conductivity property which allows higher current carrying capacity than Au. (3) Cu wire has stronger mechanical properties allowing a stronger Heat Affected Zone, stable looping and sag wire control. (4) Cu-Al has slower intermetallic growth than Au-Al, allowing better mechanical stability and higher reliability. Compared to Au-Al bonding, there is a need to understand the amount of IMC in Cu ball bonds. A well known feature of the Cu bonds is that Ball shears away from the metallization leaving deformed Al and no traces of Cu. The "lack of Cu remains" is a known behavior of the Cu wire compared to Au, but does not mean that the ball bond is inferior. Even there is high ball shear strength of Cu wirebonding, interaction thickness of IMC can be hardly found because of its small dimension. Since Cu-Al phases grow very slowly compared to Au-Al, the rate of intermetallic growth between Cu-Al interphase is 2 to 2.5 tie slwe tha tha of th Au/Al inerphase at constant temperature. It is difficult to observe interphase growth in a reasonable timeframe at normal HTS temperature like 150degC and 175degC.