The microstructure morphologies and oxide distribution of copper bonded wafers were examined by means of transmission electron microscopy (TEM) and energy dispersion spectrometer (EDS). Cu wafers exhibit good bond properties when wafer contact occurs at 400°C/4000 mbar for 30 min, followed by an anneal at 400°C for 30 min in N2 ambient atmosphere. The distribution of different defects showed that the bonded layer became a homogeneous layer under these bonding conditions. The oxidation distribution in the bonded layer is uniform and sparse. Possible bonding mechanisms are discussed.