Room temperature bonding of smooth Au surface of electroformed Cu substrate in atmospheric air

Abstract

A Cu-based heat spreader with smooth Au bonding film was fabricated by electroforming for room temperature bonding with electronic device in atmospheric air. The Cu substrates were electroformed using Au/Cu (bottom to top) and Au/Ta/Cu seed layers deposited onto smooth thermally-oxidized Si wafers; next, they were exfoliated at the Au/SiO2 interface. The exfoliated Au surface was bonded with the surface of Au-metallized Si chip at room temperature in atmospheric air by the surface activated bonding method. The Cu substrate electroformed using Au/Cu layer was poorly bonded because Cu atoms diffused through the Au film formed CuO on the surface. In the case of the Au/Ta/Cu seed layer, however, the Cu substrate was strongly bonded because the diffusion of Cu was blocked by the Ta barrier layer. It is expected that this technique will contribute to direct bonding between semiconductor device and heat spreader at room temperature without the use of vacuum bonding equipment.