Transmission Electron Microscope Observations of Si/Si Interface Bonded at Room Temperature by Ar Beam Surface Activation

Silicon wafers are strongly bonded at room temperature by Ar beam surface etching in ultrahigh vacuum. The bonding interface is investigated using a transmission electron microscope (TEM). Residual strain originating from surface roughness exists in the interface region. It is observed only in specimens bonded at room temperature, because it relaxes even by low-temperature annealing at 400°C. An amorphous-like intermediate layer is observed in high-resolution images of the interface. The layer is quite unstable and different from the oxide layer which is often observed at the Si/Si interface prepared by the conventional hydrophilic wafer bonding method. Both annealing and electron beam irradiation during TEM observation cause recrystallization of the layer. The layer contains Ar, but the O concentration is less than the detection limit of analytical TEM. The layer thickness changes according to the kinetic energy of Ar beam. These points demonstrate that Ar atom implantation during the surface etching introduces surface damage, forming the amorphous-like layer.