倒装芯片无铅凸点β-Sn晶粒取向与电迁移交互作用

Interaction Between β-Sn Grain Orientation and Electromigration Behavior in Flip-Chip Lead-Free Solder Bumps

采用原位电迁移实验研究了在150℃、1.0×10~4A/cm^2条件下倒装芯片Ni/Sn-3.0Ag-0.5Cu/Ni-P无铅凸点中β-Sn晶粒取向对金属间化合物(IMC)的聚集析出机制、阴极Ni芯片侧(UBM)溶解行为、电迁移失效机制以及电迁移驱动下β-Sn晶粒的旋转滑移机制的影响。原位观察发现,电迁移过程中(Ni,Cu)_3Sn_4类型IMC在凸点中仅沿着β-Sn晶粒的c轴方向析出,且倾向于在θ角(β-Sn晶粒的c轴与电子流动方向之间的夹角)较小的晶粒内析出;同时,阳极附近观察到β-Sn挤出现象,即凸点出现应力松弛。建立了阴极NiUBM溶解量与β-Sn晶粒取向的关系模型:β-Sn晶粒取向决定阴极NiUBM的溶解量,即当θ角很小时,NiUBM会出现明显溶解;当θ角增大时,NiUBM的溶解受到抑制,该模型与实验值基本吻合。电迁移导致β-Sn晶粒发生旋转滑移,认为是由于不同取向的相邻β-Sn晶粒中电迁移导致的空位通量不同,从而导致阳极晶界处于空位的过饱和,阴极晶界处于空位的未饱和状态,并促使空位沿着晶界出入于自由表面,最终在垂直方向上会产生空位梯度,由沿晶界的空位梯度对应的应力梯度产生的力矩使β-Sn晶粒发生旋转滑移。

With the increasing demands for miniaturization, the electromigration（EM）-induced failure by diffusion anisotropy in β-Sn is expected to be more serious than that induced by local current crowding effect, especially with the downsizing of solder bumps. In this work, the effects of Sn grain orientation on intermetallic compound（IMC） precipitation, dissolution of Ni under bump metallurgy（UBM） at the cathode, EM failure mechanism as well as the EM-induced β-Sn grain rotation in Ni/Sn-3.0 Ag-0.5 Cu/Ni-P flip-chip interconnects undergoing solid-solid EM under a current density of 1.0×10^4 A/cm^2 at 150 ℃were in situ studied.（Ni, Cu）_3 Sn_4-type IMCs precipitated in these β-Sn grains with a small angle q（between the c-axis of Sn grain and electron flow direction）, i.e., along the c-axis of β-Sn grains. Stress relaxation, squeezing β-Sn whiskers near the anode, was also observed during EM. A mathematical model on the relationship between the dissolution of NiUBM and β-Sn grain orientation was proposed： when the caxis of β-Sn grain is parallel to the electron flow direction, excessive dissolution of the cathode NiUBM occurred due to the large diffusivity of Ni along the c-axis; when the c-axis of β-Sn grain is perpendicular to the electron flow direction, no evident dissolution of cathode NiUBM occurred. The proposed model agreed well with the experimental results. EM-induced β-Sn grain rotation was attributed to the different vacancy fluxes caused by EM between adjacent grains of various grain orientation, when vacancies reached supersaturation and undersaturation at the interfaces of the anode and the cathode, respectively. Vacancy fluxes went through free surface along the interface, resulting in a normal vacancy concentration gradient. Accordingly, stress gradient produces a torque to rotate the β-Sn grain.