Fusion Bonding of Copper and Silicon at -70°C by Electrochemistry

Abstract

Wafer bonding processing typically employs thermal energy to fuse two surfaces by stimulating atomic interdiffusion at high temperatures. However, we found that fusion bonding of copper and silicon can occur at an extremely low temperature in cryo-electrochemical processing cooled by dry-ice (-20°C) or even by liquid nitrogen (-70°C). The results demonstrate that electrical energy can replace the thermal energy that must be used in semiconductor processes. The bonding phenomenon occurred repeatedly, even the copper surface was not favorable for spontaneous bonding. Notably, the bonding strength of Cu/Si was very high. Even after forcibly inserting a razor at the bonding interface, a copper layer was split from the Cu host substrate to transfer onto silicon. Secondary-ion mass spectrometry (SIMS) analysis revealed that the bonding was caused by nanoscale interdiffusion between surface copper and silicon atoms. We propose a possible mechanism in which holes are driven into the bonding interface of Cu/Si under bias, positively charge the Cu atoms and form cations (that is, surface activation). The electric field continuously drives Cu cations to bond with the dangling bonds on the mating silicon to form Si-Cu bonds. The late-forming Cu cations can pass over the bonding interface and quickly diffuse into the silicon interstitials. This study of fusion bonding at -70ºC by electrochemistry-assisted interdiffusion rather than by thermal energy has profound implications for the bonding mechanism.