Formic Acid Treatment with Pt Catalyst for Cu Direct Bonding at Low Temperature

The paper describes a new process of Cu-Cu direct bonding at a temperature lower than 200ºC in atmospheric pressure. The method consists of reduction process of Cu oxide using formic acid treatment and subsequent generation of Cu nano-particles by means of Pt catalyst. The main idea of the proposed method is that due to Pt catalytic effect, combined to the conventional formic acid treatment, a part of the formic acid is decomposed to generate hydrogen radicals to reduce the oxide of the Cu surface, CuO and produce Cu formats very effectively. As the result, nano-particles of Cu are precipitated from the formats on the Cu surface and a tight bonded interface can be formed at a temperature below a 200ºC. Cu surface was heated in water vapor to enhance the oxidation process and compared regarding to the precipitation of the Cu nano-particles.

The mechanism of precipitation of Cu nano-particles and Cu direct bonding at low temperature using formic acid treatment was investigated. It was assumed that the top of the oxide layer of Cu surface, probably CuO, is reduced effectively by H radicals which are generated from formic acid by means of Pt catalyst. The surface layer of Cu after the reduction, probably Cu2O, is reduced subsequently by formic acid, leading to precipitation of Cu nano-particles according the conditions of the heat treatment and the formic acid treatment. It was shown also that the precipitation of Cu nano-particles may contribute the direct bonding of Cu surfaces at low temperature.