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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).
Date of Conference: 8-11 Aug. 2011