The Energy Evolution And Dynamic Wetting Mechanisms Of Cu/Ag-Cu-Ti/SiC Interfacial Systems In Atomic Scale

Silicon Carbide is a developing wide bandgap semiconductor,copper materials are used for making lead frames because of better electrical and thermal conductivities.The electronic packaging now meets the problem of raising the bonding quality of Cu/SiC MOSFET.Diffusion welding via the interlayer of Ag-Cu-Ti can manage to achieve Cu/SiC soldered joint,however,antecedent experiments for the Ag-Cu-Ti solder are needed.During those experiments,the high research costs,complex preparation works and characterization processes of Ag-Cu-Ti should be dealt with.Simulation methods taking place of experiments,are used for analyzing mechanisms and properties,e.g.interfacial ones.That is one way to expand the performance database for Ag-Cu-Ti alloys,and it can be the guidance of diffusion and wetting investigations.In this study,we mainly use molecular dynamics methods.Different atomistic models are firstly built,then interatomic potentials are prepared which can estimate properties for ternary alloys,in the end,atomic relaxations and dissolutions among solder/Cu,solder/SiC,as well as liquid/solid dynamic spreading under multi-physical fields are simulated systematically.First,the investigation for different Ag-Cu-Ti solders in the ground state is made through density functional theory（DFT）.The most stably existed one among all these models is selected out by evaluations of electronic properties,as well as single point energies.The surface and interfacial energies of solders are deeply analyzed.Here,separate work values can represent the bonding strength of metal/ceramic interfaces.Having taken a series of Ag-based alloys with（100）,（110）,and（111）planes into consideration,Ag-Cu alloys are built via virtual crystal approximation,Ti atoms substitute the 1^st～7^thlayer of Ag-Cu atoms where the surface energy range is0.995～1.439J/m².The higher value solder has,the stronger interface bonding energy of solder/base system is.Interfacial energies are separately calculated for top,center,and hollow site models,among which interfacial energy of hollow site is the highest.When high interfacial energy is needed,C-terminated structure is more preferred than Si-terminated.Among atoms in Ag-Cu-Ti,Ti is prone to combine with Si and C atoms.All those elements including Ag,Cu,Ti,Si,and C are necessary for systems of the solder/base.Establishing a complex force field with all binary interactions in order to support analog calculations.Force field is the empirical format of potential functions,which affects the reliability of molecular dynamics（MD）.Interatomic potentials for single and binary exist in large quantities,while only few multiple potentials are available.Using single-parameter fitting is the solution in our thesis.Energy-volume relationships are optimized via data from training sets.An appropriate Morse interatomic potential function is made,and then verified by parts of experiment results,after calculations for interfacial energies by molecular mechanics methods.After making quantum computing and molecular mechanics simulations,we build models of(Ag₇₂Cu₂₈)₉₈Ti₂,(Ag₇₂Cu₂₈)₉₆Ti₄,(Ag₇₂Cu₂₈)₉₄Ti₆,and(Ag₇₂Cu₂₈)₉₂Ti₈solders.In MD simulations,the results of melting temperatures,1022K,1037K,1086K,and 1146K,are separately for four solders.During the structural evolution of melting kinetics,Cu-Ti metastable phases are found to prevent the activation of Ti atoms on the second sequence.The self-diffusion coefficient of Cu,Ag,and Ti decreases in turn.Surface characteristics are analyzed after forming Ag-Cu-Ti/Cu interface diffusion layers,where the thicknesses first increase then reach equilibrium.The ability of atom diffusion gets enhanced at a higher temperature.In MD results,no direct correlations are found between the sizes of Cu particle grains,the amounts of grain boundaries,and the finally reached thickness values.The further step is to investigate the bonding process of Ag-Cu-Ti and Cu/SiC.Here,the behavior of molten solder/solid interface formation is mainly focused on.Utilizing the scale laws of kinetic wetting demonstrate the rate of spreading,with solders on the surface of Cu or SiC substrates.That is showing instructions of wetting degree in the diffusion brazing.There have appeared the better degrees as solders are spreading on C-terminated surfaces,rather than C-terminated surfaces.We finally filter out a most suitable Ag-Cu-Ti alloy for both Cu and SiC substrates,which is with the component of(Ag₇₂Cu₂₈)₉₄Ti₆.The terminal study centers on the micro-mechanism,the law of influence of interfacial wettabilites under various physical fields.The effect of holding temperature is,when raised to a higher point,the diffusivity is elevated,which drives the process of dissolutive wetting.When simulating the ultrasonic field’s introduction,it is found that high-frequency and high-amplitude vibrations can intensify the phenomenon of precursor films.After establishing of polycrystal models,grains in Cu bases are refined,we found that more Cu atoms originally distributed in the boundary of Cu grains are inclined to be activated easily.That has also accelerated the process of Ag-Cu-Ti/Cu dynamic wetting.