Mechanical Strength Characterization of Direct Bond Interfaces for 3D-IC and MEMS Applications

Direct bond interconnect is gaining traction in a variety of applications including imaging and MEMS sensors as well as fine-pitch 3D-IC memory stacks, due to the room temperature bond with enhanced bond strength. The all-inorganic interface offers advantages of superior thermal performance as well. To analyze the quality of wafer-to-wafer bonding, the double cantilever beam (DCB) technique using razor-blade insertion is a widely practiced technique. In contrast, the techniques to evaluate the direct bond quality of die-to-wafer samples or MEMS cavity samples are not widely established. Although the same fundamental bond technique applies to these applications, there is a strong desire to measure the resulting bond quality from the specific processes for the particular application. In this study, we developed procedures to analyze the mechanical quality of direct bond in die-to-wafer samples and MEMS cavity samples. We designed a 3-point bend test combined with a subsequent finite element analysis (FEA) procedure to obtain die-to-wafer bond energy without the need of complicated sample preparation. A scratch test was useful for rapid comparison of samples with the same design, while it lacked the ability to provide an absolute value of interfacial bond strength or energy. For MEMS-type samples with cavities, we tested multiple shear and pull test configurations. The die shear test was found to be a convenient means to obtain reliable bond strength, with the caveat that the design of the top landing shear tool is critical. The 3-point bending and FEA method provided a rare experimental proof that die-to-wafer bond energy was lower than wafer-to-wafer bond energy, since the die-to-wafer process requires multiple additional steps that may affect the bond interface. The bond in our die-to-wafer samples and cavity samples was stronger than bulk Si after annealing at 250 - 300 °C, while the wafer-to-wafer bond was as strong after annealing at 150 °C.