Machenisms Of Low Temperature Sintering Cu₆Sn₅ Nanoparticles And Manufaturing Of High Temperature Bearing Nanograin Interconnects

Along with the rapid development of high-performance electronic devices,the demand for developing interconnect materials which can serve at higher temperatures is becoming even eager.However,the processing temperature should not be too high.Therefore,this thesis comes up with a novel idea of utilizing Cu₆Sn₅ intermetallic compounds?IMC?nanoparticles as interconnect materials to manufacture joints totally made of IMC with nanocrystalline microstructure.Taking advantage of the size effect of Cu₆Sn₅ nanoparticles,low temperature sintering can be realized,and the melting point of the joint can recover the high melting point of bulk Cu₆Sn₅ to work at higher temperatures;the anisotropy of Cu₆Sn₅ is low,with negligible CTE mismatch with Cu substrate;the thermal and electrical conductivity of Cu₆Sn₅ is also excellent of interconnect.All those advantages urge us to understand the feasibility of using Cu₆Sn₅ as joining materials,and more importantly,the coarsening behavior,as well as the microstructure evolution behavior and mechanism.This way,we can develop novel interconnect materials for high-end electronic packaging and offer theoretical instruction for the atomic level behavior of alloy nanoparticles.By modifying and optimizing the coprecipitation method,we manage to manufacture Cu₆Sn₅ nanoparticles successfully without impurity and even realize size control.The existence of Sn ions can reduce the oxidation of Cu ions;Cu₆Sn₅ is the preferentially generated Cu-Sn IMC;with sodium citrate,SDS,or CTAB as surfactant,the product can be pure Cu₆Sn₅ without impurity,if the mole ratio of Cu and Sn ions in the precursor is 6:5;by reducing the reaction temperature,increasing the amount of the surfactant,and increasing the carbon chain length of the surfactant,the particle size of the product can be reduced.The average partile size can be reduced to be 6.4 nm if the reaction is carried out in ice bath with CTAB as surfactant.Low temperature sintered Cu₆Sn₅ nanopaste and joints possess excellent mechanical,electrical and thermal properties.The shearing strength of the joint can be higher than 17 MPa;the hardness and Young's Modulus of the joints are around 70 HV and 75 GPa respectively,being only 1/5 and 1/15 of bulk Cu₆Sn₅;the joints possess much better plasticity than the bulk Cu₆Sn₅.The microstructure study reveals that the joint is constructed by nanocrystalline Cu₆Sn₅,and atomic level reaction happens as Cu₆Sn₅ nanoparticles react with Cu substrate to generate Cu3 Sn.The electrical and thermal conductivity,high temperature serving capability,and temperature cycling resistance of nano-Cu₆Sn₅ joints are all better than conventional SAC305.Nano-Cu₆Sn₅ joints can serve well at temperature as high as 200 ?.The coarsening behavior and mechanisms between Cu₆Sn₅ nanoparticles,as well as between Cu₆Sn₅ nanoparticles and micro-sized Cu substrates are investigated with in situ TEM heating.The start sintering temperature of Cu₆Sn₅ nanoparticles is as low as 125 ?;incomplete phase transformation from ?'-phase to ?-phase happens when heated to higher than the phase transformation temperature186 ?;when particles are at contacted location,coarsening mechanisms like symmetrical and asymmetric coarsening,premelting are discovered;when particles are not contacted,mechanisms like Ostwald ripening and sublimation with orientation change are discovered.Oriented attachment phenomenon is also observed.Uniformation of the orientation before coarsening is a new mechanism discovered in the coarsening of Cu₆Sn₅ nanoparticles.While heating in TEM,we observe the reaction between Cu₆Sn₅ nanoparticles and monocrystalline and polycrystalline Cu substrate.Cu₆Sn₅ nanoparticles can react with Cu substrate and generate Cu3 Sn by the diffusion of Cu into Cu₆Sn₅ nanoparticles.Cu10Sn3 is generated at higher temperature.Atomic metallurgy interconnect can happen between and Cu₆Sn₅ nanoparticles and Cu substrate,which is the reason for the high shearing strength of the joint.Cooling behavior of Cu₆Sn₅ nanoparticles is observed with high resolution in situ TEM,which reveals the reason for the formation of nanocrystalline microstructure.The reversible and incomplete transformation between ?'-phase and ?-phase happens during heating and cooling.Densification happens during the whole cooling procedure.Before cooled to 186 ?,grains grow along time;after cooled to lower than 186 ?,because of the transformation from ?'-phase to ?-phase,the microstructure of sintered Cu₆Sn₅ nanoparticles is refined to smaller than 2.6 nm for the average grain size.This is the reason for the reduction of the hardness and the Young's Modulus,as well as the increase of the plasticity,which distinguishes Cu₆Sn₅ nanoparticles with other low temperature sintered nanoparticles for interconnect purpose.