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

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 times slower than that of the Au/Al interphase at constant temperature. It is difficult to observe interphase growth in a reasonable timeframe at normal HTS temperature like 150degC and 175degC. The Cu/Al IMC needs high thermal activation energy for propagation, which makes Cu a reliable material for bonding. Over its known advantages to gold, copper wire has material characteristics which need careful process and equipment characterization. The two major Cu characteristics over Au are: (1) Cu wire oxidizes readily compared to Au (2) Cu is relatively harder than Au. Since Copper wire oxidizes easily during sparking, there is a relative difficulty to maintain a consistent Free Air Ball (FAB) for Cu - - than Au due to oxidation. Equipment and parametric modifications are needed to prepare the machine for Cu bonding. Since Cu uses higher ultrasonic and bonding parameters than Au, assessing the spark generator capability is necessary. Oxidized Free Air Ball makes bonding susceptible to bonding failures and prone to crater since oxidization makes the Free Air ball harder. The use of forming gas, higher H₂ ratio and flow rate controls to prevent free air ball oxidation must be comprehended. Modification of the forming gas nozzle is also necessary to properly supply enough flow of forming gas during sparking. Since Cu FAB oxidizes very easily, equipment parametric adjustment needs to be characterized to allow a minimal exposure time of the Free Air Ball prior to bonding to prevent hardening of the ball. Compared to Au bonding process where the FAB is initially formed prior to the descent of the bondhead to bonding area, in Cu bonding process, free Air Ball formation is done simultaneously during the downward descent of the bondhead to the bonding pad. This allows shorter exposure of the Cu Free Air ball to bonding. Cratering has high susceptibility to occur in Copper bonding than Gold bonding due to the relative hardness of the Cu wire. The process requires higher bonding parameters associated with bonding Copper. To prevent cratering and bond pad damage, the need to thoroughly understand the bond pad material structure and underlying property is critical to check the impact of the bonding process to the bond pad material. In this study, experiments were done to determine the optimum bond pad property for Cu bonding. The amount of aluminum remaining under the Cu ball can be an indication of the level of stress associated with bonding Cu. Experiments using varying levels of Aluminum thickness, barrier metal thickness and structures were evaluated. The study recommends maintaining a consistent form of barrier metal structure to withstand cratering.