Fabrication Of The Cu@Ag Core-shell Nanoparticle Paste And Its Corresponding Low Temperature Sintering Mechanism

The fast development of SiC power devices has triggered the extensive attention on medium and high temperature interconnection materia ls between the substrate and die,but the high bonding temperature needed in traditional bonding process could damage devices,especially those sensitive to heat.When the size of materials goes down to nanoscale,their sintering temperature would be great ly lowered down,which would make it possible for the low temperature bonding.After heated,sintered nanomaterials lose their small size effect and could endure a much higher temperature without any change,which means it could function well at a relatively high temperature.However,the reported Ag nanoparticle paste suffered from high costs and iron/electrical migration,and the Cu nanoparticle paste got oxidized easily and required a rather strict bonding process.Thus,it is necessary to develop a new nanopaste that is of low cost,high reliability,and easy processing at the same time.The thesis firstly developed a simple solution method to synthesize Cu nanoparticles and Cu@Ag core-shell nanoparticles and figured out how to control the morphology of as-synthesized Cu nanoparticles and Cu@Ag core-shell nanoparticles.In situ sintering Cu nanoparticles and Cu-Ag bimetallic nanoparticles individually were conducted to investigate the mechanism of the joining of Cu-Ag bimetallic nanoparticles at a low temperature.The nanopaste in this work was obtained by mixing Cu nanoparticles or Cu@Ag core-shell nanoparticles with a certain amount of the organic dispensing agent,and then the Cu/nanopaste/Cu sandwich structure was prepared by a simple printing method,after which the bonding strength and sectional micro structure of the sandwich structure under different bonding process were studied.Research results showed that the mean diameter of as-synthesized Cu nanoparticles was 57.5 nm and they got oxidized easily,while as-prepared Cu@Ag core-shell nanoparticles could inhibited oxidation due to the cladding of tiny-sized Ag nanoparticles(the mean diameter was less than 10 nm)on the surfaces of Cu nanoparticles,whose morphology could also be well controlled.By the comparison of in situ sintering Cu nanoparticles and Cu@Ag core-shell nanoparticles,it was found the sintering beginning temperature of Cu@Ag core-shell nanoparticles was about 70? lower than that of Cu nanoparticles,and tiny-sized Ag nanoparticles promoted the joining of Cu@Ag core-shell nanoparticles at a relatively low temperature while Cu nanoparticles showed little change at the same or a higher temperature.The shear strength of Cu/Cu@Ag core-shell nanoparticle paste/Cu sandwich structure,the value of which could achieve 26.5 MPa at 250?,was much higher than that of Cu/Cu nanoparticle paste/Cu sandwich structure under the same bonding process,during which process the dispersibility of nanoparticle would have a great influence on the shear strength of the bonded sandwich structure.