High-Density Direct Au-Au Interconnects Using Thermo-Compression Bonding for Heterogeneous Integration

This work demonstrates solderless, passivation-free direct Au-Au pillar bonding using Thermocompression bonding (TCB) method. The Au pillar interconnects were electroplated to make an array of 21 daisy chains using a custom-designed electroplating setup. The TCB process parameters, including bonding temperature, contact time, and applied force, were optimized to achieve robust bonding. Direct Au-Au interconnects exhibited an average shear strength of 128 MPa, which is about 350% more than the minimum specified bond strength, by MIL-STD-883G. All the daisy chains showed electrical connectivity. The SEM image for sheared devices revealed that the failure interface occurred away from the bonding interface, indicating a strong bond. The Au-Au pillar TCB offers several advantages, including high-density interconnects without forming intermetallic compounds (IMCs), as the pillars are neither capped with solder nor metals. Further, Au-Au pillar TCB bonding can be performed at ambient conditions without any protective capping as it doesn't oxidize, unlike copper (Cu), reducing the fabrication steps and cost. The developed interconnects can be utilized for heterogeneous integration of CMOS and MEMS-based chip lets, enabling the creation of more complex and compact devices.