Mechanism of Edge Bonding Void Formation in Hydrophilic Direct Wafer Bonding

The formation of edge bonding voids during hydrophilic direct wafer bonding is investigated. These defects are linked to the bonding wave propagation and to fluid dynamics between the wafers. Fluid mechanics modeling shows that a gas pressure drop occurs at the wafer edge described by a Joule-Thomson expansion. This adiabatic process results in a gas temperature change which can lead to the condensation of small water droplets close to the wafer edge. Therefore, controlling the gas atmosphere during the bonding process is needed to avoid such bonding defects. Several solutions are proposed to avoid the formation of edge bonding voids.