Size-Controllable Synthesis of Bimodal Cu Particles by Polyol Method and Their Application in Die Bonding for Power Devices

A simple and large-scale polyol method was utilized to successfully synthesize bimodal Cu particles in the air. The size, size distribution, and morphology of Cu particles were determined by the reaction parameters, such as Cu source and additives. The insoluble Cu source is suitable for the formation of bimodal Cu particles as they provide a slow and steady supply of fresh Cu nuclei, which modify the nucleation and growth of Cu particles. In addition, sodium sulfide (Na₂S) additive has the ability to control the size of Cu particles due to its reduction capability. As-synthesized bimodal Cu particles were used to fabricate sintered Cu joints for high-power electronic packaging. The results show that the shear strength of sintered Cu joints was determined by the size and size distribution of Cu particles. Sintered Cu joints with high shear strength above 20 MPa can be readily achieved at 300 °C with a slight applied pressure of 0.4 MPa. This strength is superior to joints utilizing nanoparticle Cu paste. The size-matching effect and the high oxidation resistance of the bimodal Cu particles were attributed to the excellent mechanical performance of the sintered Cu joints. Our results indicate that this affordable paste utilizing bimodal Cu particles of a suitably designed size and with a uniform distribution is highly promising as the die-bonding material for next-generation power devices.