Low temperature Cu-Cu direct bonding using formic acid vapor pretreatment

This paper describes a novel low temperature Cu-Cu direct bonding method using formic acid vapor pretreatment. The in situ dry process of reduction using formic acid vapor is developed to realize Cu-Cu direct bonding at low temperature and in low vacuum. In order to analyze the Cu film surface reduction by formic acid vapor, the CMP Cu film surface is investigated by X-ray photoelectron spectroscopy (XPS) before treatment and after treatment. It shows that the Cu peak becomes strong and O, C peaks become weak after formic acid vapor treatment at 200°C. It is successful to bond Cu film chips when formic acid vapor treatment conditions are 150°C for 10min, 175°C for 5min and 200°C for 1min, respectively. The bonding strength is about 9.8MPa when sample is treated by formic acid vapor at 200°C for 20min. The bonding interface is observed by transmission electron microscopy (TEM).