Mass Transport in Cu-Interlayer-Si Systems under Various Thermo-Electro-Mechanical Excursions

Abstract

This work falls in general area of the electro-thermo-mechanical driven mass transport in Cu-Si systems, which often finds relevance while accessing reliability issues pertaining to thin film interconnects in microelectronic devices. In such a system, the major driving forces are electric potential gradient (and hence electromigration), current crowding induced temperature gradient (and hence thermomigration) and coefficient of thermal expansion (CTE) mismatch induced stresses. Herein, a coupling between different driving forces, such as electromigration and thermomigration, may also occur, which can subdue or accelerate the mass transport in Cu. In addition, due to decrease in the thickness of interconnects to a few nanometers, the contribution of diffusion through the Cu-Si interface in overall mass transport cannot be neglected due to an increase in the interface area to volume ratio. Therefore, it can be inferred that electromigration, thermomigration and thermal stress induced failures of Cu-Si systems should be sensitive to the property of the interface, making it imperative to investigate the role of the interlayer placed in between Cu and Si on mass transport. Accordingly, this work focuses on studying the role of the coupling between the aforementioned major driving forces, especially electric potential gradient and temperature gradient, and the interlayer on the mass transport behavior in Cu-Si system. Firstly, the effect of current crowding induced temperature gradient on the electric current induced mass transport in Cu films was studied. This effect was studied using samples fabricated according to the standard Blech configuration, wherein long Cu thin film was deposited on Si substrate with a very thin W interlayer. In these tests, regular mass transport at the cathode, termed as forward mass transport, was observed along with an anomalous mass depletion at the anode, termed as backward mass transport, especially when currents of very high current density (>106 A/m2) was passed. The anomalous backward mass transport behavior is explained by illuminating the coupling between the temperature gradient induced mass transport (i.e., thermomigration) and the electric current induced mass transport (i.e., electromigration) at the anode. Herein, temperature gradient was estimated using finite element analysis, performed using COMSOL Multiphysics, using the full-length scale model. The kinetics of the anomalous backward mass transport at the anode was also studied by varying current density. The anomalous mass transport, which has origins in the establishment of very high temperature gradients at the anode, became more pronounced with increase in the higher current density. In addition to the temperature gradient, the temperature of the sample also increased with an increase in the current density, and since the kinetics of electromigration as well as thermomigration induced mass transport are diffusion controlled, an increase in the current density further exacerbates the net mass transport, irrespective of whether it is regular forward or anomalous backward mass transport. Subsequent to establishment of the existence of significant thermomigration-electromigration coupling in samples fabricated using Blech configuration, systemic experiments were performed to understand the role of the thermomigration-electromigration coupling induced mass transport on the so-called Blech length effect1. Herein, experiments were performed by passing current through a sample wherein a long Cu film on Si substrate with W interlayer was segmented into multiple stripes with length varying from 10 m to 200 m. Contrary to the Blech length effect, these samples showed enhancement in the regular mass transport at the 1 Blech length effect is understood as elimination of electromigration (i.e., material depletion at the cathode and material accumulation in form of whiskers or hillocks at anode) when the product of the current density and the sample length is smaller than a critical value. cathode with a decrease in the stripe length. We term this behavior as inverse Blech length effect. These results imply that thermomigration, besides electromigration, should also be considered while understanding the role of electric current on reliability of Cu-Si systems having bends, e.g., modern 3-D Cu interconnects fabricated using dual damascene process, etc., as the thermomigration-electromigration coupling violates the conventional wisdoms of mass depletion only at the cathode, existence of Blech length effect, etc. Finally, the role of interlayers, such as W, Ta, and Ti, in the mass transport in Cu in Cu-Si system due to the electromigration-thermomigration coupling and CTE mismatch induced thermal stresses was studied. A significant role of the interlayer was observed in the electromigration-thermomigration coupling induced mass transport, wherein a strongly bonded interface, such as Cu-Ti, did not show an inverse Blech length effect, whereas a weakly bonded interface, such as Cu-Ta, Cu-W and Cu-TiO2, showed the aforementioned inverse Blech length effect. The interface structure was characterized using transmission electron microscope, and the obtained information, along with the finite element analysis, was used to explain the observed results. Similarly, the experiments performed by cycling the temperature of the Cu-Si samples between -50 to 150 oC revealed a significant role of the interlayer on the extent as well as the nature of plastic deformation in Cu. These experiments were performed by depositing Cu islands on Si substrate with Ni, W or no interlayer and by measuring the extent of sliding of Cu film. In addition to sliding, a few Cu grains also protruded to accommodate the CTE mismatch induced stresses. In summary, the mass transport in Cu-Si system can be tuned by understanding the role of the sample geometry, coupling between driving forces and the interlayer.