We discovered that the shapes of the (111) facet structures were closely related to the disintegration, the spheroidization and the stabilization of the native interfacial oxide layer in directly bonded Si wafer pairs. These (111) facet structures are generated from the anisotropic etching of (110) cross section of bonded (100) Si wafer pairs. After etching, the facet structures at the bonding interface look like a broken line as the interfacial oxide layers are disintegrated and spheroidized. When a uniform interfacial oxide layer is inserted between two silicon wafers, the oxide layer acts as an etch mask during anisotropic etching. Therefore, the (111) facet structure becomes wider and more pronounced. Also, we confirmed that most of the interfacial oxide existing at the bonding interface of well-aligned wafer pairs were disintegrated and spheroidized through a high temperature annealing process >900 degrees C. Finally, we inferred from our measurements, the bonding strength (surface energy) increased as the interfacial oxide is stabilized, disintegrated, and spheroidized.