Comparison of the Time-Dependent Physical Processes in the Electromigration of Deep Submicron Copper and Aluminum Interc
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Comparison of the Time-Dependent Physical Processes in the Electromigration of Deep Submicron Copper and Aluminum Interconnects G. Zhang, C. M. Tan*, Z. H. Gan, K. Prasad, D. H. Zhang School of Electrical and Electronic Engineering Nanyang Technological University, Singapore 639798 *Contacting Author: [email protected]
ABSTRACT Electromigration (EM) is a major reliability issue in VLSI. Many physical processes are involved simultaneously in an EM process, namely atom migration due to electron wind force, thermal migration due to temperature gradient, stress migration due to stress gradient, and surface migration (just when free surface is available) due to surface tension. In this work, the intrinsic EM damages of Al and Cu, that is without the effect from the surrounding materials, was investigated and compared based on the finite element analysis (FEA). The FEA results show that, in the intrinsic Al EM damage, electron wind force induced flux divergence is always the main cause of void growth; however, in the intrinsic Cu EM damage, dominant flux divergence varies with time, with the final dominant flux divergence due to electron wind force. It is also found that current density and temperature gradient are the most important factors affecting flux divergence. These simulation predictions agree with experimental observation and theoretical analysis. INTRODUCTION Electromigration (EM) is a mass transport due to the collision between atoms and electrons causing momentum exchange. It is a complicated diffusion process controlled by multiple mechanisms, including electron wind force induced migration (EWM), thermal migration (TM), stress induced migration (SM) and surface migration (SFM). The associated driving forces therefore are the electron wind force [1], the thermal gradient induced force [1], the concentration gradient induced force [2], mechanical stress induced force [3] and surface tension that will affect the shape of a void in the metal line [4]. In addition, mass backflow due to EMinduced inhomogeneities [5] also plays an important role in EM performance, which affects EM rate considerably, and as a result a threshold current density exists in metal line with cluster length greater than the Blech length [6]. All of the above-mentioned factors lead to the complexity of the study of EM physics. In general, in the study of electromigration, EWM and SM (including EM-induced stress and thermal stress) due to stress gradient are widely studied as the two important processes involved in EM damage [3]. In contrast, the magnitude of TM flux is usually much smaller than EWM and SM, and is therefore ignored generally [7]. However, void development in metal interconnects is determined by flux divergence rather than the magnitude of the flux itself. An experimental done by Bastawros & Kim (1998) [8] showed a strong evidence that TM plays a significant role in EM-induced failure in conductor lines. SFM due to surface tension is usually separated from other mechanisms, and is thought as the root cause of void
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