Investigation of Cu-Cu Direct Bonding Process Utilized by High Porosity and Nanocrystal Structure

In this report, we investigated properties of nano-porous Cu (NP-Cu) structure formed by one-step electroplating. In addition, we evaluated its characteristics during bonding with NP-Cu bumps. By adjusting the electroplating conditions, we confirmed NP-Cu films of different porosity was formed, and highly oriented (111). XPS results showed that NP-Cu contained the additive component and surface condition was different from that of electroplated Cu. In addition, NP-Cu suppressed surface oxidation by the additive. The shear strength increased with NP-Cu compared to Cu-Cu bonding, which suggested that the fineness of crystal grains and suppression of surface oxidation by the additive promoted the sintering of the NP-Cu surface. NP-Cu bumps were formed by electroplating NP-Cu on Cu pillars, and investigated the behavior of the NP-Cu bumps. It was confirmed that the NP-Cu bumps can be compressed more easily than a normal Cu pillar, and plateau region that compressed at constant pressure was observed. Furthermore, it was confirmed that the porous area gradually compressed and became denser, when compression pressure was increased from investigating the compression behavior of the NP-Cu bump. In the case of the NP-Cu bumps bonding, it was observed that only NP-Cu was compressed at about 27% before and after being bonded at 10 MPa of pressure for 1 minute of compression in a nitrogen atmosphere at 300°C. These results suggest that the NP-Cu bump can be used for direct bonding without planarization processes such as CMP or plasma process which is necessary for bonding at Cu-Cu direct bonding.