| Traditionally, Al-based metallurgy has been used for on-chip interconnect applications, while more recently, performance considerations have led to the development of Cu interconnects. The thermal expansion mismatch between the metal and the Si substrate can give rise to large stresses in the metal, which in turn affects the reliability of these interconnects.; An x-ray stress measurement system optimized for use with thin films and interconnects has been set up. The method utilizes low-index reflections for higher diffracted intensities, and is designed to meet the extremely tight alignment demands of low-angle x-ray stress experiments. Thermal stress in Al(1.0 wt.% Cu) interconnects with line-widths ranging from 6.0 to 0.2 {dollar}mu{dollar}m have been characterized using this technique. Results show a biaxial stress state for the wide lines evolving to a triaxial stress state and almost purely elastic behavior for sub micron wide lines. The residual principal stresses after cool down are observed to increase with decreasing line width. Finite element method (FEM) modeling of the thermal stress was able to only reasonably predict the effect of line aspect ratio on the three principal stress components. It was found that the agreement in the magnitude of the stress can be improved by assuming a higher metal yield stress for the model, as suggested by the experimental data. But, however, the model is unable to predict the experimentally observed continuous increase in residual stress when line aspect ratio increases above one. These observations imply that the yield stress of the Al(Cu) is dependent on the line dimensions, and may be extremely high for deep submicron lines.; Texture and grain misorientation in Al(Cu) interconnects were characterized using Electron back-scatter diffraction (EBSD). Results indicate that a post-passivation anneal can increase the concentration of low-energy grain boundaries. No clear correlation with line width could be observed.; Stress and microstructure of blanket Cu films and 0.2 {dollar}mu{dollar}m wide interconnects have been characterized. Low temperature stress behavior of passivated Cu films is similar to bulk Cu. High temperature behavior, however, deviates from bulk behavior. The discrepancy might be due to the role of the Cu/passivation and Cu/barrier interfaces on grain boundary diffusion. The lower stress levels observed for Cu interconnects as compared to Al(Cu) is expected to be due to its lower thermal expansivity and suggest that Cu films may be less prone to stress-induced reliability problems than Al(Cu). |
