Study on Ar(5%H2) Plasma Pretreatment for Cu/Sn/Cu Solid-State-Diffusion Bonding in 3D Interconnection

In this work, Ar mixed 5% H2 plasma was applied to improve the surface properties of electroplated Cu and electroplated Sn for Cu/Sn/Cu solid-state-diffusion (SSD) bonding in 3D interconnection. The surfaces of electroplated Cu and electroplated Sn were easily covered with a thin oxide layer which would reduce the surface activity and degenerate the bonding performance. With an optimal pretreatment time of 120s, the contact angle of electroplated Cu decreased from 29°C to below 10°C. However, the optimal pretreatment time was only 60s for contact angle of electroplated Sn with a reduction from 36°C to below 5°C. The average roughness of electroplated Cu and electroplated Sn was also reduced to 6.8nm and 78.0nm respectively. And the electroplated Cu and electroplated Sn could keep a more stable surface activity after storage in clean room for 30min and 60min. Subsequently, a sandwich-structure Cu/Sn/Cu interconnection with a pitch of 20µm was realized at 200°C in wafer level using solid-state-diffusion (SSD) bonding. The as-bonded interface consisted of Cu/Cu3Sn/Cu6Sn5/Cu3Sn/Cu, without pure Sn remained and Sn overflow. The interface transformed to Cu/Cu3Sn/Cu as exhausting Cu6Sn5 after an annealing process. Higher shear strength and lower daisy-chain resistant were also obtained. And the bonding properties also degenerated less after high temperature storage (HTS) test. It is concluded that the optimal Ar(5%H2) plasma pretreatment could effectively activate the surface of bump and improve Cu/Sn/Cu SSD bonding for future's 3D interconnection.