A hermetic chip to chip bonding at low temperature with Cu/In/Sn/Cu joint

A bonding joint between Cu metallization and evaporated Sn/In composite solder was produced at temperature lower than 200°C in air in this work. The isothermal solidification and subsequent interdiffusion of Cu and Sn/In took place along the bonding couples held at 180°C for 20 minutes. The interfacial reaction and the bonding quality is studied and evaluated. Scanning electron microscopy (SEM) exhibits the joint is uniform along the bonding interface and no crack or voids present, which has an interfacial tensile strength of 52 kg/cm². The overall bonding is examined by C-mode scanning acoustic microscope (C-SAM). Fine leak rate test shows the leak rate is about 5.8x10⁻⁹ atm·cc/s which indicates a hermetic sealing. Intermetallic compounds (IMCs) such as AuIn2, Cu6Sn5 and Cu11In9 have been detected by means of X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM) accompanied with energy dispersive X-ray (EDX). The chemical composition analysis also reveals that solder interlayers, Sn and In, have been completely converted into IMCs by reacting with Cu. All IMCs formed in the joints have re-melting temperature above 300°C according to Cu-In, Cu-Sn and Au-In phase diagrams. Therefore, the joint can sustain high service temperature due to the presence of IMCs. Such technique producing the joints with the good bond quality and high re-melting point has great potential in electronics and microelectronics packaging such as MEMS packaging and photonic packaging.