Investigation of the influence of microstructure on electromigration in damascene copper interconnects

Electromigration is a major reliability concern for interconnects in integrated circuits. A high-voltage scanning electron microscope (HVSEM) has been developed to conduct in-situ electromigration tests on passivated interconnects. High incident energy (120 keV) electrons penetrate through the passivation and elastically scatter from the buried metal layer. Images of the entire test line are recorded continuously throughout the test and analyzed digitally afterwards.; In this dissertation, the HVSEM has been employed to study various aspects of the electromigration process. In order to verify existing void nucleation theories, defects, in the form of argon bubbles, were selectively introduced into pure aluminum interconnects. Analysis of void nucleation times indicate that nucleation occurs more readily in areas with intentionally-added defects, but that nucleation in areas both with and without defects is delayed by the presence of a titanium underlayer.; Research on aluminum-based metallizations has led to many advances in the understanding of electromigration-induced damage. The ongoing transition to copper metallization necessitates a change to a damascene fabrication process. Electromigration behavior is expected to be different than in aluminum. We discuss how processing techniques and intrinsic differences between the metals influence diffusion mechanisms, void nucleation and motion. Observations from HVSEM testing of copper interconnects provide evidence of different behavior.; Electromigration-induced damage occurs where there is a divergence in the atomic flux along the dominant diffusion path, the copper/dielectric interface. In regions of electromigration-induced damage identified using HVSEM, grain orientations were characterized using x-ray microdiffraction to determine the influence of microstructure on electromigration damage. A one-dimensional model was developed to quantify the effective diffusivity along steps that comprise arbitrarily-oriented surfaces, assuming diffusion occurs preferentially along those steps. Model predictions correlate well with HVSEM observations in that metal accumulation occurs at transitions between faster-diffusing and slower-diffusing surfaces, while metal depletion occurs where there is a flux divergence of the opposite sign. Data indicates that a flux divergence of the correct sign is a necessary condition for accumulation or depletion, but cannot be used as the sole predictor of damage sites. Additional factors such as plasticity, anisotropy and availability of nucleation sites must also be considered.