Cu wire bond parameter optimization on various bond pad metallization and barrier layer material schemes

摘要

The use of copper wire for semiconductor package assembly has been gradually gaining acceptance throughout the industry over the last decade. Although copper has several advantages over gold for wire bonding applications, the manufacturing difficulties using copper wire have made high volume, fine pitch copper bonding slow to materialize. In recent years with the spike in gold prices, copper wire has become even more attractive, and this has driven many studies on the topic.Due to the propensity for copper to work harden upon deformation, which occurs during the ball bonding process as the capillary tip smashes the ball into the bond pad, a high amount of stress is transferred into the bond pad structure. This can result in catastrophic defects such as dielectric cracking or pad cra-tering. The current study aims to quantify the level of underlying bond pad damage with respect to various bond pad metallization and barrier layer schemes. A first bond parameter optimization was completed on each experimental group. The results indicate that barrier layer structure and composition have a significant impact on the presence of pad cratering. The experimental group containing only TiN as the barrier material showed a high occurrence of cratering, while groups with Ti and TiW barrier metals showed no cratering, even if a TiN layer was on top of the Ti. The bond pad metal thickness, on the other hand, does not appear to play a significant role in the prevention of bond pad cratering. Metal thickness values ranging from 0.825 to 2.025 μm were evaluated, and none had bond pad cratering other than the group with TiN as the barrier metal. In addition to the first bond parameter evaluations with various bond pad and barrier metal combinations, the initial free air ball (FAB) optimization is discussed.