A new combined process of formic acid pretreatment for low temperature Cu direct bonding was proposed. Cu film and Cu microelectrodes samples were treated by formic acid and successfully bonded together at 200 • C. Cu surface oxide was reduced at 200 • C using formic acid treatment without/with Pt catalyst. Particularly, the treatment with heated Pt catalyst is more effective for Cu surface reduction than that without Pt catalyst or with room temperature Pt catalyst. After formic acid treatment under various conditions, Cu film and Cu micro-electrodes samples were bonded together at 200 • C in N 2 . Among them, the bonded samples treated by formic acid using heated Pt catalyst showed the strongest bonding strength. Very few oxide and carbon was remained at the bonding interface. It also proves that the treatment can reduce surface oxide resulting in strong bonding at low temperature.

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Copper direct bonding is required for electronics devices, especially power devices, and copper direct bonding using formic acid is expected to lower the bonding temperature. In this research, we analyzed the reduction reaction of copper oxide using formic acid with a Pt catalyst by electron spin resonance analysis and thermal gravimetry analysis. It was found that formic acid was decomposed and radicals were generated under 200 °C. The amount of radicals generated was increased by adding the Pt catalyst. Because of these radicals, both copper(I) oxide and copper(II) oxide start to be decomposed below 200 °C, and the reduction of copper oxide is accelerated by reactants such as H2 and CO from the decomposition of formic acid above 200 °C. The Pt catalyst also accelerates the reaction of copper oxide reduction. Herewith, it is considered that the copper surface can be controlled more precisely by using formic acid to induce direct bonding.

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Low-temperature Cu/Cu direct bonding technology using formic acid vapor in situ treatment was developed. Effect of formic acid vapor treatment conditions on Cu surface and bonding was studied. Cu surface oxide was reduced using formic acid vapor in situ treatment at 150 and 200°C, respectively. With higher temperature and longer treatment time, surface reduction is more effective. Grain boundary etching was found on chemical mechanical polished Cu film after initial treatment by formic acid. However, Cu surface roughness is minimally influenced by long-time formic acid vapor treatment. Cu film/Cu film direct bonding was realized in N 2 atmosphere with formic acid vapor in situ treatment under temperature below 200°C. For lower treatment temperature, longer treatment time is required to achieve good bonding. The bonding strength is about 9.0 MPa when Cu surface is treated at 200°C for 10 min.Index Terms-Cu/Cu direct bonding, formic acid vapor, low temperature.