Study On Reverse Polarity Effect In Sn-9Zn Interconnect During Liquid-solid Electromigration

Three-dimensional integrated circuits are a current trend in microelectronics industry, and the diameter of solder joints can be reduced to almost10μm, which is known as microbumps. As a result, the current density through each solder joint will be up to104A/cm2. Under such a high current density, the low melting point solder bumps may melt locally and consequently cause early failure of the solder joints due to increasing Joule heating effect. Electromigration (EM) may occur in molten solder, which is recently identified as an operative-failure mechanism. Therefore it is critical to understand the fundamental aspects of liquid-solid EM (L-S EM). In the present work, the diffusion behavior of Zn atoms, cross-solder interaction between Cu and Ni atoms and microstructural evolution during L-S EM were investigated using Cu/Sn-9Zn/Cu and Cu/Sn-9Zn/Ni line-type interconnects to reveal the fundamentals of L-S EM behavior under a current density of5.0×103A/cm2at230℃.The following conclusions are drawn in the present work:(1) A reverse polarity effect was revealed in Cu/Sn-9Zn/Cu interconnects during L-S EM, i.e., the interfacial intermetallic compounds (IMCs) at the cathode grew continuously and were significantly thicker than those at the anode. This was resulted from the directional migration of Zn atoms from the anode towards the cathode, which was induced by the positive effective charge number (Z*) of Zn atoms but not the back-stress. Consequently, at the anode, the dissolution and massive spalling of the Cu-Zn IMCs occurred, and the depletion of Zn atoms resulted in the transformation of initial interfacial Cu5Zn8IMC into (Cu6Sn5+CuZn); at the cathode, the interfacial Cu5Zn8IMC gradually transformed into (Cu5Zn8+CuZn); in the solder, the Zn content reduced continuously from initial9wt.%to0.9wt.%. A growth model was proposed to explain the reverse polarity effect, and the average Z*of Zn atoms in Cu5Zn8was calculated to be+0.25.(2) Cu atoms could diffuse across the liquid solder to the opposite side more easily during the liquid-solid reaction in Cu/Sn-9Zn/Ni interconnects. At the initial stage, Cu5Zn8interfacial IMC formed at the Ni interface. With increasing reaction time, the dissolution of the Cu5Zn8IMC and the formation of Ni3Sn4-type IMC occurred at the Ni interface, resulting from the continuous diffusion of Zn atoms towards Cu side under the chemical potential gradient. For the diffusion of Ni atoms, even after liquid-solid reaction for8h, only a few Ni atoms arrived at the opposite Cu side, and the interfacial IMCs transformed from Cu5Zn8-type to Cu6Sn5-type.(3) The cross-solder interaction between Cu and Ni atoms was obviously influenced by L-S EM in Cu/Sn-9Zn/Ni interconnects. When Cu atoms were under downwind diffusion, L-S EM significantly enhanced the diffusion of Cu atoms to arrive at the opposite Ni side, resulting in the formation of interfacial Cu5Zn8at the Ni interface, and its thickness increased at the beginning but then decreased. When Cu atoms were under upwind diffusion, L-S EM effectively inhibited the diffusion of Cu atoms and only a small amount of Cu atoms could diffuse to the Ni side, resulting in the formation of (Ni,Cu)3(Sn,Zn)4. For the Ni atoms, L-S EM significantly enhanced the dissolution of Ni substrate and the diffusion of Ni atoms to arrive at the opposite Cu side when Ni atoms were under downwind diffusion, resulting in the formation of a large amount of (Ni,Cu)3(Sn,Zn)4phases at the Cu5Zn8/solder interface. When Ni atoms were under upwind diffusion, they were difficult to arrive at Cu side. Under the combined effect of the the chemical potential gradient and electronic wind, the Zn atoms with positive effedctive charge number would directionally diffuse towards Cu side under both directions of electrons flow. As a result, the interfacial Cu5Zn8formed at the Cu side, and its thickness continuously increased.