I. Introduction
Nanoparticles have unique and intriguing properties which are completely different from bulk materials due to their large specific surfaces [1]–[3] . Recently, the low-temperature-sintering property of a metallic nanoparticle attracts much attention because it enables the nanoparticle to be applied to the conductive adhesive as an alternative lead-free solder to a conventional lead-rich solder [4]–[6]. The sintering temperature of metallic nanoparticles is much lower than that of bulk metals, but once the particles are sintered, the physical properties such as electrical conductivity, thermal conductivity and melting point ideally become the same as the bulk metals. Accordingly, the high-temperature reliability and the repeatable soldering process are achieved by using the nanoparticle solder. In metallic nanoparticles for soldering material, Ag nanoparticles have been intensively investigated because they show a good sintering property, relatively high stability and excellent conductivity [7]. Cu nanoparticles are also attractive as a solder material which has not only high conductivity but also better electromigration resistance property and lower cost compared with Ag [8]. The sintering property and joint performance are strongly influenced by the morphology of the nanoparticles. For example, the enhancement of mechanical property of junction, that is, the improvement of a bonding strength by mixing the nanoparticles with sub-micron or micron particles was reported [9], [10]. Hence, the control of the particle size or shape is greatly important for the practical application of nanoparticles. In terms of synthesis techniques of metallic nanoparticles, a lot of liquid phase synthesis methods have been previously reported [11]– [17]. Among them, we have intensively studied an electroless deposition of Cu nanoparticles in an aqueous solution where the reaction condition was optimized based on the electrochemical consideration [18], [19]. The spherical Cu nanoparticles having a diameter of about 30 nm was successfully synthesized by the reduction of Cu(II) ion from CuO as a precursor with hydrazine as a reductant and gelatin as a dispersant. However, the detailed study about the size distribution control of Cu nanoparticles has not been reported. In the liquid phase synthesis method, there are numerous parameters which affect the morphology of product; for example, the type and quantity of solvent, precursor, reductant and dispersant, and reaction condition including temperature and pH.