Direct Cu-Cu Thermo-Compression Bonding for 3D-Stacked IC Integration

IMEC’s 3D-Stacked IC approach consists of creating through wafer interconnects using front-end type processes thus enabling a maximal die-to-die interconnect density of up to 10,000 interconnects/mm². Whereas with state of the art processing techniques, we effectively demonstrated "Cu-nails" with a diameter of 5µm, a depth of 25µm and a minimal pitch of 10µm, this approach still has the potential to further scale. A further benefit of this approach is that it safeguards back-end interconnect capabilities as it enables to reroute interconnect lines through the wafer from any BEOL metal layer.

In the present paper we report on the Cu-Cu thermo-compression bonding of 5µm diameter Cu-nails to blanket, polished Cu. We assess the mechanical quality of the bond by measuring the die shear force after bonding and report shear stresses, normalized to the nominal Cu area of up to 100 MPa depending on the bonding conditions used. In this way we study the impact of bonding temperature, bond force or bond pressure, and bonding time on the mechanical strength of the bond.

The shear measurements are complemented by failure analysis after shear.  Profilometric measurements of the Cu-nail dies and blanket Cu substrates prior to bonding and after shear demonstrate the occurrence of different failure modes such as pad/nail interface failures and through nail shear.