| Stress-induced and electromigration voiding constitute important reliability concerns for interconnects in integrated circuits. Previous studies have shown that interconnect reliability is a strong function of processing. In this study, we focused on the effects of processing on the nucleation and initial growth stages of voiding. In order to understand the dynamics of electromigration voiding, an in-situ technique is necessary. For this purpose, a high voltage scanning electron microscope (HVSEM) has been employed to study electromigration voiding. The HVSEM (120 KeV) was operated in back-scattered electron imaging mode, enabling us to image metal lines through dielectric layers.; In the first part of this work we studied stress-induced and electromigration voiding in aluminum lines passivated with silicon nitride. Stress-induced voids nucleate in the interior of these lines. Nuclear Reaction Analysis data showed high concentrations of hydrogen in the aluminum, where plan-view TEM images revealed the presence of a fine dispersion of bubbles. These bubbles, which are thought to form from hydrogen that evolves from the silicon nitride layer during processing, serve as nucleation sites for stress-induced voids. The same interior void nucleation was observed during electromigration testing in nitride passivated samples.; In the second part of this work we studied the effect of line fabrication processing on void nucleation during electromigration testing in copper interconnects. In conventional (cloisonné) copper lines, voids nucleate at the line sidewall, as with conventional aluminum lines. In damascene copper lines, however, voids nucleate at the top surface of the copper. After nucleation, voids grow along the top interface, forming surface voids. This initial stage of void growth is explained by comparing interfacial energies in the system. We show that the source of flux divergence during electromigration in copper can be different from that in aluminum interconnects. Due to the introduction of chemical-mechanical polishing processes in the damascene structure fabrication, process-induced defects can be formed on the top surface of the copper lines. These defects serve as nucleation sites for electromigration voids, and are therefore detrimental to the reliability of interconnects. Controlling the top surface quality is thus very critical in damascene processing. |
