Formation and Characterization of a Well-Packed Particulate Structure Consisted of Cu Nanoparticles By Capillary Force for Low Resistivity at Low Temperature
Authors: Yokoyama Shun , Motomiya Kenichi , Takahashi Hideyuki , Tohji Kazuyuki 
Published: February 2020

Cu nanoparticles (NPs) have been studied extensively to develop nano ink for printable electronics on flexible substrates because of its low cost, high conductivity and low sintering temperature. Although there are many synthesis methods with low resistivity which normally require high temperature, high pressure, toxic regent and special equipment, it was difficult to achieve the practical use of Cu NPs because the synthesis methods are expensive. We have developed a facile and green synthesis method of Cu NPs by using water, citric acid, ascorbic acid as solvent, complex/capping agent and reducing agent, respectively. The particle size of the citric acid modified Cu NPs can be controlled from 30 nm to 10 µm by varying pH and concentrations of Cu ion and ascorbic acid. For low resistivity, a high packing density of the Cu NPs is required to increase the interparticulate junction area before heat treatment because the well-packed particulate structure leads to a low temperature densification reaction of metal NPs. We have succeeded in increasing the interparticulate junction area by combining with the Cu NPs and the micro-sized particles (MPs). The bimodal size distribution leads to the high packing density because small particles are placed in voids of packing large particles. However, the resistivity of the Cu particles (32 µΩ・cm) after heat treatment at 200oC is far from that of bulk Cu (1.7 µΩ・cm) because the Cu MPs prevent low temperature sintering of the mixture. Therefore, a well-packed particulate structure consisted only of Cu NPs is require for low resistivity at low temperature. In this study, capillary forces generated from solvents evaporation were utilized to obtain the high packing density of the Cu NPs for low resistivity at low temperature because capillary forces have been applied to obtain closely packed various NPs. For characterizations of Cu films prepared from the Cu NPs, XRD, FT-IR, SEM and XPS measurement were carried out. Besides, the electrical resistivity of Cu films after heat treatment was measured.

Since the citric acid modified Cu NPs can well disperse in alcohols and boiling points of alcohols are relatively low as compared other organic solvents, alcohols are suitable to form stable Cu ink and use capillary forces. Cu films were prepared from drop casting of the Cu alcohol ink and doctor blade process using Cu NPs paste consisted of the Cu NPs and α-terpineol as a traditional method. The weight of these Cu films is controlled to be approximately the same. After dry treatment of the Cu films, the thickness of the Cu film (8 µm) prepared from the Cu alcohol ink is much thinner than that (30 µm) of prepared from the Cu paste. The high packing density of the Cu NPs arises from capillary forces during alcohol evaporation. After heat treatment at 200oC under Ar+H2 (98%+2%), the resistivities of the Cu films obtained from the Cu ink and the Cu paste are 7 µΩ・cm and 80 µΩ・cm, respectively. It was found that the well-packed particulate structure consisted only of Cu NPs by capillary forces facilitates a densification by heat treatment, resulting in the low resistivity at low temperature. In our presentation, detailed results about the Cu films prepared from the Cu ink will be introduced. This work was supported by JSPS KAKENHI Grant-in-Aid for Young Scientists (B) 15K16155.