Nanoalloying And Anti-electrochemical Migration Mechanisms Of Ag-Cu Supersaturated Solid Solution Nanoparticles

Die attach materials for the third generation wide bandgap semiconductor devices represented by SiC,are challenged by the harsh working conditions of high temperature(>300?)and large temperature ranges(-100?~600?).The conventional low temperature solders are not able to meet the high-temperature operating requirements.Ag nanoparticles(NPs)and Cu NPs have attracted wide attention.However,their applications are limited when considering the problems of easy-migration and high porosity of Ag,and the easy-oxidation of Cu.Therefore,it is necessary to find a die attach material which can realize low temperature joining,high temperature service,low cost and excellent comprehensive performance,which become an urgent problem to be solved for the application of the third generation semiconductor devices.In this thesis,a new type of Ag-Cu NPs and nanopaste with eutectic composition ratio(72/28)and supersaturated solid solution structure are successsflly fabricated,which have achieved the objectives of low temperature joining and high temperature service for SiC power device through nano-effect.Thermal-induced phase separation as that single-phase supersaturated solid solutions NPs transfer into the homogeneous biphase eutectic-like alloy structure,which has good mechanical,electrical and thermal properties,especially good anti-oxidation and anti-electrochemical migration(ECM)performance,providing a new strategy for the third generation power semiconductor devices.The effects of dispersant type and reaction temperature on the morphology,size,structure and stability of Ag-Cu nanoparticles were studied.Because the citric acid has a short carbon chain,Ag and Cu atoms are randomly arranged to form a kind of Ag-Cu NPs with supersaturated solid solution structure which break through the solid solution limit in the phase diagram of bulk Ag-Cu.Cu atoms are dissolved in the lattice of Ag,so that it is not easily oxidized because of the interaction of Cu and Ag atom bonds,presenting strong oxidation resistance and chemical stability.The morphology and phase transition of Ag-Cu supersaturated solid solution NPs were studied by the combination means of in-situ transmission electron microscopy(TEM)heating and high temperature X-ray diffraction(HT-XRD).It was found that the Ag-Cu NPs retained the Ag-based solid solution state as the temperature increasing from room temperature to 235?.In the temperature range of 235?~265?,there was a rapid two-phase separation process.the Ag-based supersaturated solid solution start to change into Ag-rich phases as Cu element in the solid solution precipitated gradually in the form of Cu-rich phases in the heating;eventually homogenous eutectic-like solid alloys form with a large solid solubility in the two phase.A new type of Ag-Cu nanopaste with single organic additive,was developed and its sintering performance was studied.Results show that with the increase of the sintering temperature,holding time and applied pressure,the sintered microstructure develops toward the direction of densification,and the mechanical,thermal and electrical properties are gradually improved simultaneously.The optimal comprehensive properties were achived when sintered at 350?.Specifically,the porosity is 1.03%,the shear strength is 117 MPa,the hardness is 2.02±0.14 GPa,the Young's modulus is 136.05±8.36 GPa,the density is 6.23g/cm3,the thermal conductivity is 95 W/m·K,the resistivity is 5.9??·cm,and the thermal expansion coefficients in the temperature range of 30?~300? are 16.08×10-6?-1,21.20×10-6?-1,26.97×10-6?-1,26.98×10-6?-1 and 26.98×10-6?-1,respectively.The comprehensive performance of Ag-Cu supersaturated solid solution nanopaste is better than that of Ag and Cu nanopaste.The sintered Ag-Cu supersaturated solid solution nanopaste present a coral-like eutectic microstructure composed of Ag-rich phase and Cu-rich phase.A large number of spots-like Cu-rich phase dispersed in the grain boundary and Ag-based solid solution,and played a role of dispersion strengthening and fine grain strengthening,so that Ag-Cu nanopaste sintered joints have a high shear strength through large plastic deformation.Moreover,the scattering of the lattice distortion caused by the Cu-rich phase is much lower than that of the dissolution caused by the solid solution atoms in the Ag-rich phase.In addition,the dislocation density is reduced as the increasing of sintering temperature and holding time,and the scattering degree of the dislocation to electrons reduced,so that the sintered Ag-Cu nanopaste maintained a high level of conductive properties.The ECM process of Ag-Cu nanopaste sintered electrodes was studied through the means of water drop test.It is found that the ECM failure time of Ag-Cu nanopaste sintered electrodes is 5 times that of the Ag nanopaste sintered electrode.On one hand,the electrode potential of Cu on the anode is lower than Ag,so Cu is preferentially dissolved in the anode to form the insoluble precipitates of Cu(OH)2 and CuO,which adhere to the surface of the anode and hinder the dissolution of Ag and migration of Ag+.On the other hand,the bongding energy between Cu and Cu atoms in Cu-rich phase are higher than those of between Ag and Ag atoms,therefore it need to overcome higher barrier to the break metal bond between Cu and Cu atoms,resulting in low diffusion and migration rate of Ag atoms and vacancies.The electrical and thermal properties of single SiC-MOSFET device and full-bridge power drive modules attached by Ag-Cu nanopaste was studied.It shows that the SiC power module maintained a good conduction output characteristics and high voltage insulation turn-off performance when tested at 22?,100?,150?,and 200?,providing that Ag-Cu nanopaste can ensure the stability and reliability of interconnection of SiC devices at high temperature.