13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Numerical Study of Allowable Current Density for Electromigration Damage of Multilevel Interconnection in Integrated Circuit Kazuhiko Sasagawa1,*, Kazuhiro Fujisaki1, Takahiro Yanagi1 1 Department of Intelligent Machines and System Engineering, Hirosaki University, Hirosaki, 036-8561, Japan * Corresponding author: sasagawa@cc.hirosaki-u.ac.jp Abstract The high current density occurring in integrated circuits induces electromigration (EM) of the metal lines used for electric wirings. A void is formed by EM in the line material and the growth of void leads to the line failure. Recently, multilevel interconnection is widely used in electronics devices and MEMS by connecting upper and lower metal lines through vias. The reservoir structure is often constructed in the multilevel interconnection. It is known that there is threshold current density of EM damage in multilevel interconnection with vias. It is important to evaluate the threshold for determination of allowable electric current in the interconnection. In this study, a numerical simulation technique for analyzing the atomic density distribution generated by EM in the line is applied to evaluate the EM risks of metal line in several kinds of the multilevel structures. The thresholds of current density leading to EM change were calculated through the simulations. We confirmed that the atomic density distribution in the line was essential to increase the threshold and to prevent EM damage in the line. And we also showed the simulation technique was useful in the design of safety structure of electric wirings in integrated circuits. Keywords Integrated Circuit, Reliability, Electromigration, Multilevel Interconnection, Allowable Current Density 1. Introduction The high current density occurring in integrated circuits induces electromigration (EM) of the metal lines used for electric wirings. EM is a phenomenon that metallic atoms are transported by electron wind and that void, due to depletion of metallic atoms, is formed in the metal line. As the voids are growing the current density in the metal line increases and then the excessive Joule heating leads to metal line failure. Recently, multilevel interconnection is widely used in electronic devices and MEMS by connecting upper and lower metal lines through vias. The structure of interconnect tree and reservoir structure are constructed in the multilevel interconnection. Reservoir structures have an overhang from via connection, and it is usually located at vias on both anode and cathode sides as shown in Fig. 1. Reservoir structure gives delay of EM failure in multilevel interconnection by increasing margin of critical void length. The effect is caused by metallic atoms supplied from the overhanging parts as atom’s reservoir to the metal line. The transportation of atoms is induced by tensile stress at the connection part on cathode side via as a result of EM. It is known that there is threshold current density jth of EM damage in multilevel interconnection with vias. It is important to evaluate the threshold for determination of allowable electric current in the interconnection. Some research groups have developed evaluation method of jth [1]. The threshold current density is also evaluated by numerical simulation. The building-up process of atomic density distribution in the line is simulated. And the simulation is based on a governing parameter for EM damage in polycrystalline line AFD* gen [2]. The parameter is applicable to two-dimensional line shape. Sasagawa et al. have evaluated jth of several kinds of interconnect tree structure [3]. In this study, a numerical simulation technique for analyzing the atomic density distribution generated by EM in the line is applied to evaluate the EM risks of metal line in several kinds of the reservoir structures. The threshold jth of several kinds of straight via-connected line with reservoir are evaluated by the numerical simulation. From the evaluation results, reservoir effects on the threshold current density are discussed.
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