Origin of voids at the interface of wafer bonded sapphire on sapphire

摘要

Two sapphire (Al{sub}2O{sub}3, r-cut) wafers were bonded at room temperature. The bonding energy increased during heating up to 1100 °C. The interface was investigated by plan-view and cross-section TEM. The bonding strength after heating to 1100 °C is similar to the bulk bonding strength. A high density of voids at the bonded interface was observed. The voids are elongated along the interface in a pancake shape. The surfaces are low index planes. The volume inside the voids is higher than expected from a model, where a hydrophobic initial bonding is assumed. The strength of initial bonding and the free volume at the interface can be understood, if the sapphire surfaces are hydrophilic and covered by several monolayers of water during initial bonding. During annealing the water diffuses out along the bonded interface and a free volume remains.