| Through-Silicon Via?TSV?is the key technology for three-dimensional integrated circuits?3D-ICs?to achieve better electrical performance,higher packaging density and wider bandwidth.During the production process of TSV,Copper?Cu?is usually filled into the TSV by electroplating process,and the anneal process is adopted to stabilize the Cu microstructure.These process flows can help to control the microstructure formation of TSV-Cu,and the mechanical properties of TSV-Cu are closely affected by its microstructure.In the TSV structure,great mismatch of the coefficient of thermal expansion?CTE?between the Cu(17×10-6/?)and its surrounding silicon?Si?(2.8×10-6/?)is a critical issue to the reliability of TSV technology.Under manufacture and service conditions,the mismatch leads to great thermal mechanical stress generated in the TSV-Cu and shear stress developed at the TSV-Cu/Si interface.Thus the plastic and creep deformation of TSV-Cu could be induced by the thermal mechanical stress and leads to the protrusion.This kind of protrusion could break the surrounding structures during assemble process or in service.Besides,large shear stresses developed at interfaces can result in delamination or cracking of the interface,and lead to leakage and electric breakdown.Therefore,the protrusion characterization of TSV-Cu and the research of structural integrity for TSV-Cu/Si interface are significant meaningful for the reliability of 3D electronic integration.The relationship among the TSV process parameters?such as the current density,the plating additives concentration,and the ramp rate of annealing temperature?,the TSV-Cu microstructure,and protrusion was determined experimentally.Firstly,four sets of TSV-Cu samples are prepared by using four level sets of electroplating current density and additive concentration,and then the samples are annealed with three temperature ramp rates,10°C/min,1.2°C/min and 0.6°C/min,respectively.The effects of processing parameters on the evolution of TSV-Cu grain size during annealing were studied,and the relationship between the grain size and the amount of protrusion was fitted.The results show that the high current density and high concentration of additives help to make the more fine-grained TSV-Cu.During the annealing process,the temperature ramp rate affects the evolution of the grain size,and the grain growth is the most when the temperature ramp rate is 1.2°C/min.The protrusion strongly depends on the grain size both before and after annealing,and compared with the grain size before annealing,the effect of grain size after annealing on the protrusion is stronger.The TSV-Cu protrusion during annealing consists of plastic deformation,creep deformation and the promotion effect of TSV-Cu/Si interface cracking on protrusion.These factors are difficult to be characterized by the experimental study of TSV-Cu annealing.Therefore,in the following work,these three TSV-Cu protrusion mechanisms are studied.The plastic deformation mechanism of TSV-Cu during annealing was studied by finite element analysis,and the influence of the evolution of TSV-Cu yield stress on the plastic deformation was obtained.The creep deformation mechanism of TSV-Cu during annealing was studied.The diffusive creep mechanism of TSV-Cu during the annealing process was determined by SEM observation of microstructure.Based on the energy balance theory,the analytical expression of diffusion creep rate of TSV-Cu was deduced.Creep parameters were introduced into the finite element model,and the creep behavior of TSV-Cu was analyzed.The results show that the TSV-Cu which has smaller grain size also has higher yield strength,and protrudes less after annealing.But there is a gap between the simulated TSV-Cu protrusion and the experimental value.The diffusion creep of TSV-Cu is mainly caused by grain boundary diffusion and grain boundary sliding.The diffusive creep strain rate is positively correlated with the ambient temperature and the external load,but negatively correlated with the grain size.After taking the creep deformation into the finite element model of TSV protrusion,the simulation results of TSV-Cu protrusion are closer to the experimental values.In addition,the simulation results show that the microstructure of TSV-Cu affects its protrusion morphology.As the diffusive creep of Cu at the side wall increases with the increasing of Cu grain size at the side wall of TSV.The creep deformation of TSV-Cu near the interface is more serious,so that the TSV-Cu of the TSV side wall protrusion is much higher than that of other regions.As a result,the"donut"protrusion morphology of TSV-Cu is formed.The effect of TSV-Cu/Si interface integrity on TSV-Cu protrusion behavior was studied.The failure mode of TSV-Cu/Si interface was observed experimentally.The influencing factors of interface failure are analyzed.The predictive model of interface crack depth is presented.In the finite element model of the TSV structure,the failure mechanism of the interface was analyzed by introducing the cohesive element at the TSV interface,and the influence of the interfacial crack on the TSV-Cu protrusion behavior was studied.It was found that after annealing,most of the TSV interfacial cracks,and the cracks propagate along the Cu seed layer.The stress concentration in the seed layer of Cu is the fundamental reason of interfacial cracking.The predictive model of crack depth in TSV interface shows that the increase of interface roughness and the increase of the grain size of Cu seed layer will lead to a deeper interfacial crack.This is due to the increase of interface roughness will lead to more servere stress concentration in the Cu seed layer,which leads to a deeper interfacial crack.In addition,the morphology of grain boundary becomes simpler with the increase of Cu grain size,and the path of crack propagation along the grain boundary is simpler,as a result,leads to a deeper interfacial crack.Compared with the finite element model of TSV considering the plastic deformation and creep deformation,the simulation results of TSV-Cu protrusion are in good agreement with the measured values.The calculated results of TSV-Cu protrusion with the introduction of interfacial cracking factors are in good agreement with the measured values.In order to study the protrusion behavior of annealed TSV-Cu in assembly and service conditions.According to the temperature load condition of TSV in the assembly process,the thermal cycling profiles with 300?temperature difference was designed?25?-325??.In addition,according to the temperature load condition of TSV in service,the thermal cycling profiles with 125?temperature difference was designed?0?-125??.The relationship between the behavior of TSV-Cu protrusion and microstructure evolution in thermal cycling process is investigated by observing the microstructure and the protrusion behavior of TSV-Cu.The results show that there is no TSV-Cu protrusion detected during thermal cycling from 0?to125?,but both TSV-Cu protrusion and microstructure evolution are observed during thermal cycling from 25?to 325?.The Cu protrusion behavior during thermal cycling from 25?to 325?is analyzed with respect to its microstructure.It is found out that,during the first 5 thermal cycles,the grain size of Cu dominates Cu protrusion,and larger grain size of Cu before thermal cycling can cause greater Cu protrusion.With the increase of thermal cycle number,the dependence of Cu protrusion on its grain size declines but switches to its microstrain.Higher the microstrain within Cu results greater Cu protrusion increment in the following thermal cycles.With thermal cycle number increases to 25,Cu protrusion rate slows down due to strain hardening.After 30 thermal cycles,Cu protrusion is stabilized within the range of 1.92?m to 2.09?m. |
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