Low Temperature Oxidation-Sintering Behaviors of Cu Fine Particles

Abstract

Cu fine-particle paste is a promising material to form a low-cost interconnect for flexible electronics devices. It has been reported that Cu particles can be sintered at low temperature (well below the half of the melting point) through two-step heat treatment processes of oxidation and reduction. However, the mechanism of the low temperature sintering is not clear yet. In this study, we investigated the oxidation sintering process of Cu fine particles by thermal gravimetric analysis (TGA) in the temperature range of 200-300℃, X-ray diffraction (XRD), and microstructural observation. It was found from TGA that the oxidation process was initially rate-controlled by surface reaction and then by Cu diffusion at grain boundaries of Cu₂O. Transmission electron microscopy observation revealed the formation of a core (Cu)-shell (Cu₂O) structure during the oxidation process. The adjacent Cu₂O shells were bonded to each other resulting in a cross-linked structure. The subsequent reduction process led to the formation of a porous structure by oxygen removal, but the cross-linked structure was maintained, which would make the low-temperature sintered Cu body as robust as solidified solder and sintered Ag paste.

Fig. 9　(a) Transmission Electron Microscope micrograph of the oxidized Cu fine particles at 300℃ for 10 min. Diffraction patterns taken from (b) Cu core and (c) Cu₂O shell.