Cu-Cu bonding by Ag nanostructure at low temperature of 180 °C

In order to lower Cu-Cu bonding temperature and shorten bonding time applied for 3D integration, nanostructure has been introduced on bonding Cu surface. However, few studies have been reported on Nano Particles (NPs) formation by film deposition process such as pulsed laser deposition (PLD), which would be compatible with CMOS process. In this work Ag nanostructure containing strings of NPs was formed by optimized PLD process, which is mostly used for nanofilm deposition. Then NPs morphology was observed by scanning electron microscope (SEM) and transmission electron microscopy (TEM). Ag nanostructure was consisted of strings of loose mesh structures filled with NPs with size range from several nanometers to tens of nanometers. They were compressible and of high movability, which was significantly benefit for lowering bonding temperature. With these Ag nanostructure on Cu pads, chips were pre-bonding at the low temperature of 180°C for 5 min and afterwards all the chips were simultaneously annealed at the temperature of 200 C for 25 min. After die shear test, average shear strength of 15.8 MPa was obtained and fracture surface was inspected by SEM. After TEM observation of bonding interface, continuous Cu-Ag-Cu interfaces with almost no void were observed. It confirmed low temperature Cu-Cu bonding with Ag nanostructure by PLD was a reliable and time-saving process, which might be a promising technology for 3D integration.