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Cu Pillar Planarization to Enhance Thermosonic Flipchip Bonding

Thermosonic flipchip bonding of Cu pillars has gained increasing attention for the low-temperature chip-to-chip (C2C) and chip-to-wafer (C2W) integration. By exploiting ultrasonic energy, which activates the interface and causes the deformation-induced vibration, Cu-to-Cu direct bonding is facilitated in significantly lower bonding forces, shorter process windows and lower thermal budget. However, for thermosonic Cu pillar bonding, the tolerance of bump height variation is highly stringent and the coplanarity of the surfaces is critical. In order to address the need to improve coplanarity during thermosonic bonding, a low-cost planarization process was applied to the bump surfaces. As a result, Cu pillars with a uniform thickness distribution through the wafer and a flattened surface were produced. The proposed planarization process led to an increase of up to 60% in the contact area between the bumps during thermosonic bonding. As a result, the thermosonic bond strength of the joints was significantly improved.