Low Thermal Budget Cu/SiO₂Hybrid Bonding Using Highly <111>-oriented Nano-twinned Cu with Low Contact Resistivity and High Bonding Strength

Fine-pitch hybrid bonding with 1.25 μ m thick PECVD SiO2 as dielectric layer and highly < 111 > -oriented nano-twinned Cu as metal layer was fabricated in 12 -inch Si wafer. The dishing between the Cu and SiO2 layer could be precisely controlled within 3 nm while the surface roughness of Cu via can be obtained within 2 nm. Prior to the bonding, the wafers were treated by a 20-W ex-situ Ar plasma for 120 s. Thus, their hydrophilicity could be increased. The relationship between surface contact angle and plasma power was also investigated. We found that the 20-W Ar-plasma was the optimized parameter to effectively remove the oxide layer and obtain the lowest hydrophilicity of the nt-Cu and SiO2. The electrical properties of Kelvin structure and daisy chains with 2500 and 50 bumps were also studied using a 4-point probe method. Results showed that a contact resistivity of 10-9 Ω-cm2 was obtained. This is the lowest value found in literature as bonded at a temperature below 300°C. Die shear tests were subsequently executed. Results showed that the bonding strength exceeded 30 MPa, which was greater compared to those in some recent studies. Additionally, three-dimensional (3D) X-ray, dual beam focused ion beam (FIB-SEM) and transmission electron microscope (TEM) were also employed to examine lattice arrangement of the bonding interface. Results show excellent bonding quality and no CuOx signal found at the interface.