Effects Of Thin Ni Layer On Interfacial Reaction In Sn/Cu System

Copper pillar bump,as a representative interconnect technology in advanced packaging filed,has excellent electrical and mechanical properties and has been more and more widely used.The miniaturization of the copper pillar bumps leads to the acceleration of interfacial reaction and the rapid increase of the number of Kirkendall voids,which seriously affects the reliability of interconnect structures.In order to slow down the consumption rate of the Sn layer and the growth rate of the intermetallic compound?IMC?,a Ni barrier layer is introduced between the Sn layer and the Cu substrate to suppress mutual diffusion of atoms.In this paper,Cu/?Ni?/Sn samples with different thicknesses of thin Ni layers are prepared by electroplating method.The effects of thin Ni layer thickness on Cu/Sn interface reaction before and after exhaustion of Sn solder layer are investigated,including morphology change of interfacial IMC,Kirkendall voids growth,IMC growth kinetics analysis and IMC growth mechanism exploration.Finally,the growth of IMC and Kirkendall voids for Cu/?Ni?/Sn bump structure is investigated,and the blocking effect of thin Ni layer on the interface reaction of Cu/?Ni?/Sn bump structure was verified.Before the completely consumption of Sn layer,the IMC of Cu/Sn planar structure grows fastest,the diffusion coefficient is 5.610×10^-17 m²/s.The IMC of Cu/700 nm Ni/Sn planar structure grows the slowest and the diffusion coefficient is 0.396×10^-17 m²/s,which is about 1/10 of the diffusion coefficient of Cu/Sn plane structure.The Kirkendal voids are mainly concentrated in the?Cu,Ni?₃Sn/Cu interface and the?Cu,Ni?₃Sn layer,and as the thickness of the Ni layer increases,after aging for same hours,the number of Kirkendall hole gradually decreases.When the thickness of the Ni layer exceeds 700 nm,the Kirkendall voids disappear.After the completely consumption of Sn layer,the IMC morphology changes greatly.Among Ni-containing samples,the surface IMC loses the initial grain characteristics and the IMC region is planarized.Additionally,contrary to expectations,after the Sn layer is exhausted,the Ni layer loses the blocking effect,which promotes the growth of?Cu,Ni?₃Sn instead,and the number of Kirkendall voids increases.In the Cu/?Ni?/Sn bump structure,the 100 nm Ni layer significantly inhibited the growth of IMC.After aging for 4 h,The?Cu,Ni?₆Sn₅ thickness is about 1/2 of Cu₆Sn₅ thickness in Cu/Sn bump,and a small amount of Kirkendall voids are generated at the?Cu,Ni?₆Sn₅/Cu interface.The 100 nm Ni layer is quickly consumed and loses the barrier effect.Furthermore,the blocking effect of the 700 nm Ni layer is obvious,the IMC is?Ni,Cu?₃Sn₄.There is no generation of Cu₃Sn and Kirkendall voids.When the bump diameter is 5?m or less,the proportion of surface diffusion of Cu and Sn atoms is larger and larger.Therefore,surface diffusion needs to be considered when calculating the diffusion coefficient.