β-Ga₂O₃ as a ultra-wide bandgap semiconductor material with a great development potential has a bandgap width of 4.8 eV, and a theoretical breakdown electric field of 8 MV/cm. Its Baliga advantage value of β-Ga₂O₃ is also several times greater than that of silicon carbide (SiC) or gallium nitride (GaN). However, the low thermal conductivity of β-Ga₂O₃ seriously affected the device's heat dissipation and the application in the high-power field. Wafer bonding is an indispensable key technology in semiconductor devices, which enables the formation of new structures, efficient fabrication, device integration, and cost reduction. The silicon substrate with the advantages of low cost and high thermal conductivity is an important substrate material for heterogeneous integrations. SiO₂ layer is easily formed on the surface of silicon, and the surface structure of SiO₂ consists of a Si-O-Si three-dimensional network structure together with an unsaturated Si-O structure, and the existence of this structure is conducive to reducing the probability of bubble generation, thus improving the quality of the bonding bonding interface, so it is easier to realize the bonding of silicon and β-Ga₂O₃ by using SiO₂ as an intermediate layer. This paper was to analyze the effect of O₂ plasma activation on the wafer surface, including changes in contact angle, roughness, and surface chemical composition. In addition, the effect of annealing temperature on the strength and thermal stability of the bond structure and the structure and composition of the β-Ga₂O₃/SiO₂ bond interface was also discussed.

SiO₂/Si substrate with the thickness of 550 μm and β-Ga₂O₃ single crystals sunstrate with the thickness of 650 μm were prepared. The two wafers were cleaned with alcohol and acetone via ultrasonic cleaning , and isopropanol for 10 min, then rinsed with deionized water and blown dry with N₂. Also, β-Ga₂O₃ and SiO₂/Si substrates were processed by oxygen plasma in a reactive ion etching (RIE) equipment (PlasmaPro 100 Cobra, Oxford Instrument Co., UK) at 50 mtorr and 300 W. The two activated wafers to be bonded were initially aligned and bonded, and pressure was applied to the wafer surface with tweezers to fully expel the gas from the bonding interface and make the bonding closer. Finally, the bonded wafers were placed on a pressurized platform for pressurized resting to complete the pre-bonding process. The pre-bonded wafers were placed in the annealing furnace for low-temperature annealing to enhance the bonding strength. A testing machine was used to determine the bond strength of the bonded pairs at different annealing temperatures. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to determine the bonded interface images and elemental distribution.

The contact angle and hydroxyl density with treatment time were investigated via adjusting the treatment time of O₂ plasma. The hydrophilicity of SiO₂ and β-Ga₂O₃ wafers greatly enhances, and the hydroxyl density on the wafer surface improves as the O₂ plasma activation time increases. O-H in β-Ga₂O₃ is produced by the breakage of oxygen atoms (O_w) in water that combines with surface Ga atoms (Ga_s) to form Ga_s-O_wH_w, and H combines with surface oxygen atoms (O_s) to form O_s-H_w. The effect of annealing temperature on the surface quality and bonding strength was also investigated. The results indicate that the bonding strength increases with the increase of annealing temperature. When the annealing temperature increases to 500 ℃, the bond strength increases to 7.31 MPa. However, this process leads to the cracking of the β-Ga₂O₃ substrate. Some cracks in β-Ga₂O₃ occur due to the thermal stress resulting from the different thermal expansion coefficients between SiO₂ and β-Ga₂O₃. β-Ga₂O₃ and SiO₂ surfaces are atomically bonded with no interfacial voids or cracks. The bonding interface's elements O, Ga, and Si are fully diffused during annealing.

In this study, a high-strength bonding of β-Ga₂O₃ and SiO₂ substrates was achieved at low temperatures via O₂ plasma activation. The FTIR analysis showed an increase in the hydroxyl density of the β-Ga₂O₃ and SiO₂/Si surfaces with the increase in plasma treatment time. The XPS and FTIR analysis demonstrated the changes in the hydrophilic functional groups on the surfaces before and after the plasma treatments. The bond strength gradually increased with the annealing temperature and reached 7.31 MPa, but unbonding and cracking also occurred at the edges of the bonded regions. The SEM and TEM images of the bonding interface demonstrated the existence of tight bonding between β-Ga₂O₃ and SiO₂ with full diffusion of elements Ga, O, and Si. In addititon, a model for direct bonding of oxygen plasma-activated β-Ga₂O₃/SiO₂/Si was also proposed.