Liquid-Solid Electromigration Behavior And Mechanism Of Micro Interconnect

Electromigration has become a serious reliability issue in electronic devices, with the demands of continuous miniaturization and high-performance. The solder bumps are downsized to the micron scale and the current density through them increases, causing the even more serious Joule heating effect and thus liquid-solid electromigration (L-S EM) has important theoretical and engineering application value. In the present work, synchrotron radiation real-time imaging technology is used to in situ observe the L-S EM behavior of Sn-52(wt.%)In, Sn-58(wt.%)Bi, Sn-37(wt.%)Pb and Sn-9(wt.%)Zn interconnects; an effective charge number (Z*) model is modified, and the calculated values are in good agreement with the experimental results; the physical mechanism of atomic diffusion is revealed and clarified. Technically, a novel current driven bonding (CDB) micro interconnect method is proposed to successfully fabricate highly oriented Cu6Sn5 whole intermetallic compound (IMC) interconnects with high melting temperature which is suitable for engineering applications.The main conclusions are drawn as follows:1. For the L-S EM behavior of Sn-58Bi interconnects, in the heating stage, Bi atoms directionally migrate from the cathode toward the anode, following a linear relationship with time; in the dwelling stage, Bi atoms back-diffuse from the anode to cathode, and then balanced three phases are obtained; in the cooling stage, Bi atoms directionally migrate from the cathode toward the anode again until the rich-Bi and rich-Sn phases separate completely. For Sn-37Pb interconnects, Pb atoms have a similar diffusion behavior with Bi atoms. The Z* of Bi atoms is calculated to be -5.50±0.2 at 140℃ based on the growth kinetics of the Bi-rich layer and steady diffusion models, respectively. The Z* of Pb atoms is calculated to be -3.20 at 185℃ based on the growth kinetics of the Pb-rich layer. The abnormal diffusion behavior of Bi and Pb atoms during EM will provide a reference for purifying Sn-based alloy with Bi and Pb impurities.2. For the L-S EM behavior Sn-52In interconnects, different from the Solid-Solid (S-S) EM behavior, the In atoms directionally migrate toward the cathode due to the back-stress induced by the preferential migration of the Sn atoms over the In atoms toward the anode, resulting in the segregation of the rich-Sn and rich-In phases at the anode and cathode respectively; during L-S EM, however, the In atoms directionally migrate toward the anode due to the effect of negative Z* of In rather than the back-stress, resulting in the "polarity effect", i.e., the IMCs growing continuously at the anode while becoming thinner at the cathode. Furthermore, the consumption rate of the cathode Cu during the L-S EM is three orders of magnitude higher than that in the case of the S-S EM.3. For the L-S EM behavior of Sn-9Zn interconnect, the reverse polarity effect occurs, evidenced by the IMC layer at the cathode growing continuously while that at the anode was restrained. The "reverse polarity effect" is induced by the positive Z* of Zn atoms, which is calculated to be +0.63. The abnormal directional migration of Zn atoms toward the cathode prevent the dissolution of cathode substrate, which is beneficial to improve the EM reliability of micro-bump solder interconnects. This work provides a new program design on the component design of anti-electromigrational micro solder joints.4. For the L-S EM behavior of (001) Cu/Sn/polycrystalline Cu interconnect, a structurally highly oriented Cu6Sns intermetallic compound (IMC) has been kept and accelerate grew. The growth rate was 4μm/min under current stressing, which is 20 times greater than that of the wetting reaction case. Technically, a novel current driven bonding (CDB) micro interconnect method is proposed. The highly oriented whole Cu6Sn5 IMC interconnects are firstly fabricated using CDB method, and no voids at IMC/substrate interfaces, high tensile strength, high service temperature and high elelctromigration reliability, which have potential value of applications in three dimensional integrated circuits (3D IC) packaging.