Low Temperature Cu-Cu Bonding Using an Intermediate Sacrificial Sn Layer

Thermocompression Cu-Cu bonding needs both high temperatures (
$300 ^{\circ }\text {C}\sim 400$
°C) for bonding and annealing and complicated chemical mechanical polishing (CMP) for surface pretreatment. This paper reports a new CMP- and annealing-free Cu-Cu bonding method that is initiated by a sacrificial Sn layer. This method includes three steps. (1) A Cu-Sn bump is reflowed at 250 °C with redox gases to change the Sn layer from solid to porous by sequential redox reaction. (2) Pre-bonding is performed at 200 °C to change the porous Sn to porous Cu-Sn inter-metallic compounds (IMCs) at solid-state. (3) Final bonding is performed at 250 °C with formic acid gas sweeping to reduce and volatize the Sn from the IMCs, changing the Cu-Sn bonding to Cu-Cu bonding. Using this method, Cu bumps with a diameter of
$5\,\mu \text{m}$
and a pitch of
$25~\mu \text{m}$
have been successfully bonded with high bonding strength and high yield.