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The annealing effects of voids, amorphous layer, and bonding strength in the sequentially plasma-activated silicon/silicon bonded interface were investigated. The interfacial silanol groups and water were condensed and removed, respectively, below and above annealing at 150°C. About 400°C, the bonding strength was reduced because of the increased void density associated with the plasma-induced surface defects and the increased thickness of interfacial silicon oxide. The increase of the interfacial thickness layer after annealing was confirmed by high-resolution transmission electron microscope and detected as silicon oxide using the electron energy loss spectroscopy. The surface roughness and contact angle were measured to explain the influence of plasma processing parameters on the interfacial behavior after annealing. While the water contact angle increased with the increase in the O2 reactive ion etching (RIE) time, the surface roughness was initially decreased and then increased. The surface activation with 400-W O2 RIE plasma induced defect sites such as nanopores and craters. This study indicates that the O2 RIE plasma time and power have to be as low as possible to reduce surface roughness and defects but have to be high enough to properly activate the surface with enough surface energy to achieve high quality of Si/Si interface.