New View of Low-Temperature Sintering Phenomenon of Nanometer-Size Particles Based on Molecular Dynamics Study
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New View of Low-Temperature Sintering Phenomenon of Nanometer-Size Particles Based on Molecular Dynamics Study Norie Matsubara1, Shinji Munetoh1 and Osamu Furukimi1 1 Department of Materials Science and Engineering, Kyushu University, 744 Motooka, Fukuoka, 819-0395, Japan. ABSTRACT In this study, we have investigated a behavior of particle with diameter several ten nanometers size at the time of heating on an atomic scale by numerical analysis using the molecular dynamics (MD) simulation. On solving the equation of motion, the Langevin equation was adopted. The Finnis-Sinclair potential, which can well reproduce the mechanical properties of a BCC-metal, was used as the interatomic force. We determined the relationship between the melting point (Tm) of the nano-sized particles and its diameter by MD simulations. We have also investigated the self-diffusion coefficient of each atom-forming at a temperature larger or less than Tm of the submicron-size metal particles . As a result, even in case of heating at a temperature larger than Tm, the mean self-diffusion coefficient at the center of a particle was 107 –10-6 cm2/sec. On the other hand, at the surface layer of the particle was two to three orders of magnitude larger than that at the center. Those particles were in a quasi-molten state. It is conceivable that the thickness of the surface layer can explain a phenomenon that sintering progresses as the heating temperature increases. INTRODUCTION There has been progress in the utilization of fine metal particles to join a semiconductor device and a circuit board [1]. Cements with fine metal particles show good bonding properties at low temperatures of 573 K or less and high heat resistance at temperatures of 473 K or more, and thus they are expected to be applied to next-generation SiC devices. When nanoparticles with diameters of several nanometers are used as cement, it is possible to use a phenomenon known as melting point depression to join materials. The bonding principle is that nanoparticles melt at temperatures substantially lower than their melting point because of their large specific surface area. The particles fuse to each other and coagulate while cooling to room temperature. However, it is difficult to handle nanoparticles of this size, and thus there has been no progress in utilizing such particles as cement. In contrast, sintering is the principle of bonding when using micron-sized particles due to the diffusion of atoms that comprise the micron-sized particles; therefore, heating at high temperatures close to the melting point (Tm) is required. However, it is difficult to use such particles as cement for semiconductor devices. For this reason, utilization of submicron-sized particles with diameters of several tens of nanometers that are sinterable at temperatures much lower than Tm have been considered, and there are many reports of experimental results [2-3]. To make such particles more widely applicable as a material to join semiconductor devices, it is important to elucidate the driving power behin
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