Novel Cu-nanowire-based technology enabling fine pitch interconnects for 2.5D/3D Integration

This paper deals with the Cu-nanowire-based bonding technology for the fine-pitch interconnects. This Nano-material based bonding is a viable alternative to current state of the art Cu-SiO
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(oxide-oxide) hybrid bonding which is basically applicable for wafer-to-wafer bonding with fine-pitch interconnects for 3D integration. But as this technology exhibits cleaning challenges and surface preparation issues on die level, i.e. die-to-die (D2D) and die-to-wafer (D2W) assembly. Therefore, in order to overcome these challenges on die level, another substitute method for direct Cu/Cu bonding using Cu nanowires is investigated. Since Cu is the standard interconnect material, nanostructured Cu pads/bumps are desirable. In particular nanowires are interesting because an additional layer with Cu nanowires is selectively grown on top of regular Cu microbumps, thus enabling the bonding under room temperature. The initial mechanical connection at the interface is achieved by applying certain pressure during the D2D placement. Smaller pressures are required compared to TCB due to less dense structures. Hence this pressure allows nanowires to merge into each other and stay mechanically connected. Later annealing is done to attain the solid Cu/Cu joint. In this work we have investigated D2D bonding with top and bottom dies containing the Cu microbumps with nanowires. In this work, most importantly the technological parameters such as different nanowire thickness involving Ø 200nm and Ø 400 nm wires and different annealing temperatures i.e. 150 °C, 250 °C, and 350 °C were tested post bonding.