Low-Resistance Interconnects by Bonding Porous Cu Bumps and Ultrathin Cu/Sn Pads

CuSn-Cu bonding is widely used in 2.5-D integration due to the ease of operation. However, it suffers from high bond resistance due to the high resistivity of Cu-Sn intermetallic compounds (IMCs) and the high contact resistance due to the Kirkendall voids at the Cu/IMC interfaces. We report a low-resistance CuSn-Cu interconnect fabricated by bonding a porous Cu bump and a Cu-Sn pad with a 200-nm Sn layer. The porous Cu bump, fabricated by treating normal Cu-Sn bumps with oxygen and formic acid, is bonded with the Cu-Sn pad through solid-state bonding at 100 °C under a 5-MPa pressure, followed by 250 °C annealing. The bonding pressure presses the porous Cu to distribute uniformly into the Sn layer, avoiding the Kirkendall voids by providing the Cu atoms locally instead of all from the Cu/Sn interfaces for Cu-Sn reactions. The IMC resistance is also reduced as the 200-nm Sn layer forms an ultrathin IMC layer. A large-scale bump array has been bonded successfully, and a single bond resistance of 6.5 m $\Omega $ and a specific contact resistivity (SCR) of $1.0\times 10^{-}9 ~\Omega \cdot $ cm2 have been obtained.