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Formic acid vapor treated low temperature Cu-Cu direct bonding method was developed. The in situ dry process of reduction using formic acid vapor was applied to realize Cu-Cu direct bonding at low temperature and in low vacuum. In order to evaluate the effect of formic acid vapor on Cu film surface, the CMP-Cu film surface is investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM) 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. Grain boundary was found on the surface after treatment due to surface etching by formic acid. Surface roughness only increases a little with formic acid vapor treatment. CMP-Cu/EB-Cu film direct bonding and CMP-Cu film/Cu bumpless electrodes direct bonding were conducted in N2 atmosphere with formic acid vapor treatment at 150°C~200°C, respectively. The bonding strength of CMP-Cu/EB-Cu film direct bonding is about 9.8MPa when samples are treated by formic acid vapor at 200°C for 20min. CMP-Cu film/Cu bumpless electrodes bonding also was done, and the bonding strength is about 24.1MPa. Finally, the bonding interface is observed by transmission electron microscope (TEM) and scanning electron microscope (SEM).