The process of the low-temperature-direct-bonding and smart-cut technologies is optimized, and the SOI material is fabricated at 550℃ under 2.1×10-2 Pa. The bonding strength of this structure is 153.7 kg/cm2, the total thickness variation and the defect density of the top monocrystalline silicon film are 8.5 nm and 90 cm-2, respectively. This method can produce a good insulator layer between active layers in the fabrication of the three-dimensional integrated circuits (3D ICs), avoiding the unfavorable effects of the high-temperature process on the device structure, material quality and performance of the active layers. At the same time, the high quality monocrystalline silicon layer can be available for producing subsequent active layers in the fabrication of the 3D ICs.