Development of Low-Temperature Sintering Cu Nanoparticles
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Development of Low-Temperature Sintering Cu Nanoparticles Toshitaka Ishizaki, Ryota Watanabe, Kunio Akedo and Toshikazu Satoh Toyota Central R&D Labs., Inc., 41-1, Yokomichi, Nagakute, Aichi 480-1192, Japan ABSTRACT Cu nanoparticles capped with fatty acids and amines were developed as low-temperature sintering materials. The fatty acids and amines used were decanoic acid + decyl amine (C10) and oleic acid + oleyl amine (C18), respectively. The synthesized Cu nanoparticles were analyzed using X-ray diffraction, transmission electron microscopy, and thermogravimetric and differential thermal analysis. Because both of the capping layers could be decomposed at temperatures lower than 300°C even under an inert atmosphere, bonding and sintering experiments could be carried out in the absence of oxygen to prevent the oxidation of the Cu nanoparticles. The sintered structures were observed using scanning electron microscopy. The shear strengths of Cu plates bonded using the C18 Cu nanoparticles were larger than those of plates bonded using the C10 Cu nanoparticles. At 300°C, the strength was higher than 30 MPa, and of the same order as ordinary high-temperature solders, even though the processing temperature was low. The resistivity of a film sintered using the C18 Cu nanoparticles was 12 μΩcm at 300°C, which was lower than the values reported in previous studies. INTRODUCTION It is well known that the sintering temperature of metallic nanoparticles is much lower than the melting temperature of the corresponding bulk materials because of the higher surface energy of the nanoparticles compared with the bulk materials [1]. Recently, there has been increasing interest to apply low-temperature sintering of metallic nanoparticles to bond semiconductor chips [2-4] and for inkjet printing of fine wiring on plastic films in the electronics packaging technology field [5-7]. Many studies have investigated the development of Ag nanoparticles for these applications, because Ag has the highest electronic and thermal conductivity of any metal. Ag nanoparticles have the advantage that small particles can be synthesized easily because of their stability; however, Ag is expensive, and has low resistance against ion migration in the presence of moisture [8]. Interest in the development of Cu nanoparticles is therefore growing rapidly because the electronic and thermal conductivities of Cu and Ag are very similar, but Cu is much cheaper and has higher resistance to ion migration. A practical method for the synthesis of Cu nanoparticles, however, is yet to be achieved, because Cu nanoparticles are very sensitive to oxidation and aggregate easily as the particle size decreases. Many researchers have synthesized Cu nanoparticles using polymers to prevent them from oxidizing and aggregating [9-14]. However, polymers are hardly decomposed after thermal process, especially in inert or reductive atmospheres. Because Cu is very sensitive to oxidization, the thermal process must be carried out without oxygen. Any remaining polymers inhibit the sinteri
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