Room-temperature Cu–Cu bonding was realized by applying ultrasonic vibration together with compression force to the bonding of a cone-shaped bump. The size of the bump was about 10 µm. The connection pitch was 20 µm. Mechanical characterization showed that the bonding strength increases with vibration amplitude and depends on the thickness of the counter electrode made of Cu. The thickness dependence of the bonding strength was found to be caused by an increase in the surface roughness of the counter electrode. It was shown that the bonding strength meets the requirement from application to products. Electrical characterization using a daisy-chain connection test demonstrated that more than 10,000 pins on a chip can be connected with a sufficiently low resistance.

The effect of coating the surface of a Cu microbump with a 1-hexanethiol self-assembled monolayer (SAM) on room-temperature solid-phase bonding was investigated. The microbump was cone-shaped while the counterelectrode was planar. Room-temperature bonding was performed by applying ultrasonic vibration. Chemical analysis showed that the SAM coating suppressed the surface oxidation of Cu. However, it was shown that the SAM coating also reduced the mechanical strength of the bonded interface, whereas little effect was observed in the electrical resistance of bump interconnects.

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