The Reliability Of Ф5μm Cu/Sn High Density Microbump

As a novel chip interconnection technology,copper pillar bumps,which enjoy better electrical and thermal properties,can achieve ultra-fine pitch interconnection for high-density 3D integration.With minimum feature length shrinking from 45 nm to 20/14 nm,size of micro-bump is required to scale down from 40-50μm to several microns.Therefore,many issues concerning materials and reliability of microbumps arise,caused by the abnormal growth of intermetallic compounds（IMC）and Kirkendall voids.In this paper,Ф5μm Cu/Sn microbump with high uniformity was successfully fabricated by step electroplating.The evolution of IMC and Kirkendall voids was studied under isothermal aging at 150 ℃.Furthermore,the effects of bump diameter,grain size of solder layer and Ni barrier layer on the interfacial reaction were discussed for the high reliability application of microbump.The main conclusions are as follows:Bump diameter has a great impact on Cu/Sn interfacial reaction.The consumption rate of solder inФ5μm microbump is much faster and the complete phase transition time is shorter.The reaction constant of Cu₃Sn inФ5μm microbump is 2.92×10^-17 m²/s,which is 2.5 times than that inФ20μm.The size effect can be explained by diffusion mechanism.When the bump is smaller,the influence of surface diffusion is more significant,which fasten the diffusion rate.InФ5μm microbump,the formation of Kirkendall voids is closely related to Cu₃Sn phase.After long time aging,micro-cracks were observed at Cu/Cu₃Sn interface and porous Cu₃Sn started to form,which are induced by side wetting/diffusion of solder on the circumference of Cu pillar.Fast phase transition,formation of micro-cracks and porous Cu₃Sn will greatly weaken the mechanical and electrical properties of bump joints,resulting in serious reliability problems.By comparing the growth behaviors of IMC inФ5μm Cu/bright-Sn system and Cu/matte-Sn system with different grain size in solder layer,it is found that solder with large Sn grain will slow down the interfacial reaction.In Cu/matte-Sn system,there are finite columnar Sn grains in solder layer,grain size is larger,about 2.4μm,with no obvious preferential orientation,while the solder in Cu/bright-Sn system is with fine grains.Without current loading,the correlation between IMC growth and crystal anisotropy is weak,IMC growth is more affected by grain boundary.IMC growth model was also proposed,which illustrates that Cu atoms mainly take Sn grain boundary as diffusion channels,supplemented by bulk diffusion during the growth of IMC.Due to less grain boundary and large bulk diffusion distance,IMC growth in Cu/matte-Sn system will be slower,which has been confirmed by experimental results.In the subsequent aging,the reaction constant of Cu/matte-Sn system is 4.51×10^-1717 m²/s,about 1/2 of that in Cu/bright-Sn system.The effect of Ni barrier layer on interfacial reaction was also investigated in this paper.Results show that Ni barrier layer can inhibit the growth of IMC and the formation of voids,which will improve the reliability of microbump interconnection.Under aging treatment,the main component of IMC inФ5μm Cu/500nm Ni/Sn system is（Cu,Ni）₆Sn₅,no Cu₃Sn phase is observed.As to the growth rate of IMC,reaction constant of IMC in Cu/500nm Ni/Sn system is5.52×10^-1818 m²/s,about 1/16 of Cu/Sn system.Interestingly,with 500nm Nickel layer,Kirkendall voids doesn’t appear even after long time aging.We believe that the barrier effect of Ni on Cu diffusion,high stability of（Cu,Ni）₆Sn₅ and slow reaction rate of Ni/Sn all contribute to the positive role of nickel layer.