Chip level evaluation of wafer-to-wafer direct bonding strength with bending test

A wafer-to-wafer(W2W) direct bonding for 3D IC integration package has attracted a lot of attention due to the high-density I/O pads and small form factor by eliminating the interconnect layer between wafers. Instead of the interconnects, the activated surfaces of the wafers are directly bonded during the W2W bonding process. Therefore, the quality and reliability of the W2W bonding process are determined by the bonding strength between the wafers. Double cantilever beam (DCB) method is widely used to identify the W2W bonding strength by inserting a blade on the side of the bonded wafer. However, the DCB method is difficult to apply to evaluate the reliability of actual devices due to the following two limitations. First of all, a crack length measurement is required to characterize the bonding strength with the DCB method, but as the copper density of the chip increases, it becomes difficult to track the crack propagation with the IR camera. In addition, since the DCB method only characterizes the bonding strength in the side region of the bonded wafer, it is impossible to measure the bonding strength in the center region. Therefore, to overcome the limitations of the DCB method, a chip level evaluation method of the W2W bonding strength with bending test is formulated. The bonded wafer is fabricated to the chip level where the initial tip exists through dicing process and the crack propagation from the tip is induced through 3-points bending test. The finite element analysis (FEA) based on J-integral approach and VCCT approach is performed to study the mechanics near crack tip, estimate the bonding strength, and mimic crack propagation behaviors during the bending test. It is demonstrated that results from the developed numerical models are well matching with the experimental results. With the developed numerical models, a series of evaluations are conducted to study the effect of die length and surface treatment conditions. In case of the test specimen extracted from the side region, it is confirmed that the bonding strength measured from bending test is consistent with that measured by the DCB method. In addition, the bonding strength variations are found between the center region and the side region of the bonded wafer according to the W2W bonding process condition.