Transferring Fine-Pitch Cu Nanoparticle Bumps for Low-Temperature Cu-Cu Bonding in Chip-Scale Integration

Cu-Cu bonding using nanomaterials as an intermediate has attracted increasing attention for its eased requirements for bonding temperature and surface roughness. In this work, we developed a novel low-temperature Cu-Cu bonding technique using transferable Cu nanoparticle bumps. These nanoparticle bumps were first fabricated on donor substrate through pulsed laser deposition (PLD) and then transferred to the target chip by low-temperature pre-sintering ( 160 (degrees) C). The conventional dry transfer method using Si donor substrate was proved less feasible since the Cu-O-Si bond formed between Cu nanoparticles and SiO2 natural oxide layer on Si surface could cause transfer failure. An alternative wet transfer approach using Al as a sacrificial layer was proposed, wherein Al was dissolved by KOH solution and nanoparticle bumps were subsequently transferred. The self-release characteristic of wet transfer ensured a higher transfer yield. Transferred nanoparticle bumps on target chip maintained sintering activity and could realize reliable die shear strength (37.4 MPa) with target substrate at 200 (degrees) C, 15 MPa, and 5 min. The electrical resistance of sintered Cu joints showed negligible change before and after transfer. Joint strength decreased to 27.3 MPa due to oxidation after a thermal shock test (TST) (-65 to 150( degrees) C) for 500 cycles. The transfer strategy could enable a more flexible application of nanomaterials for all-Cu interconnection in chip-scale integration.