Wafer level metallic bonding: Voiding mechanisms in copper layers

Promoted by the component miniaturization trend, three-dimensional integration appears as a promising option for implementation of the next generation of integrated circuits. In this context, copper is still an interesting material to be integrated to vertical interconnexion through direct metal-metal bonding processes. However, it was already reported that voiding phenomena occur in bonded copper layers for temperature beyond 300°C leading to serious reliability problems. This paper aims at explaining voiding mechanisms in the specific metal bonding configuration. Voiding characteristics are compared in different structures allows highlighting several thermal effects. It appears that the mechanical stress sustained by copper layers during post-bonding thermal processes is the main contribution in the voiding phenomenon. Creep mechanisms occurring in polycrystalline copper structure could be considered as the origin of this phenomenon. This study offers better understanding of reliability problems in structures involving encapsulated copper layers and can be used as guideline for metal bonding integration.