Electromigration in AlCu lines: Comparison of Dual Damascene and metal reactive ion etching

Abstract

The electromigration behavior and microstructural features of AlCu Dual Damascene lines are compared to those of AlCu metal reactively ion etched (RIE) lines. Test structures consist of 0.18-, 0.35- and 1.33-μm-wide lines terminated by W diffusion barriers, and are tested at 250°C. A remarkable finding for the 0.18-μm-wide Damascene samples is a threshold-length product of nearly 40000 A/cm. Different failure mechanisms are revealed for Dual Damascene and metal RIE structures by observing the resistance shift vs. time as well as the lifetime vs. current density behaviors. It is found that the Damascene structures exhibit a long resistance incubation period followed by a rapid increase in resistance, while the RIE structures show a short resistance incubation period followed by a gradual increase in resistance. The current density exponent is found to be close to 2 for the Damascene process and close to 1 for the RIE process. The Damascene samples show a significant lifetime improvement over the RIE samples for low levels of resistance change, while the relative lifetime improvement decreases as the maximum allowed resistance shift increases. In order to understand the electromigration performance of each metallization system, various physical analysis techniques are implemented. The average grain size, determined from transmission electron microscopy (TEM), is found to be significantly larger for Damascene lines due to a higher AlCu deposition temperature. Both TEM and scanning electron microscopy (SEM) analyses indicate that TiAl3 intermetallic formation occurs in both Dual Damascene and RIE lines, but is much more prevalent in the RIE case. Electron backscatter analysis reveals a weak Al crystallographic texture in sub-micron Dual Damascene samples and a strong (111) fiber texture in RIE samples. Optical and SEM inspections illustrate different failure signatures for 0.18 and 0.35 μm Dual Damascene and RIE lines. © 2001 Elsevier Science B.V.