Strain measurements in aluminum interconnects by x-ray microdiffraction

Reliability of metal interconnects in integrated circuits is a major concern to the microelectronics industry. Understanding the impact of stress and strain in metal lines is crucial for improving their reliability. Because of thermal expansion mismatches between the interconnect, substrate, and passivation material, high tensile stresses are developed during processing. In addition, stress gradients develop due to electromigration because of the high current densities passing through the lines. X-ray diffraction is an ideal technique for directly measuring these stresses. However, most of the earlier measurements were performed using millimeter size x-ray beams, allowing only macroscopic determinations of stress. Spatially resolved measurements of stresses in interconnects were not possible.; A synchrotron-based white beam x-ray microdiffraction technique was developed and applied for localized strain mapping on polycrystalline thin film interconnects. The system was shown to achieve micron-scale spatial resolution and strain sensitivities on the order of 2 × 10−4.; Two methods for performing in-situ calibration of the detector angles utilizing the (001) silicon substrate were presented: the energy method and the interplanar angle method. Various (hkl) reflections were measured from the (001) silicon substrate and the displacement of the x-ray beam was determined. Although discrepancies arose between these methods, both proved satisfactory to correct the Al (hkl) d-spacing measurements.; Thermal and electromigration-induced hydrostatic stress distributions in a 2.6 μm wide passivated A1 line were investigated. The Al line exhibited thermal stress behavior consistent with confinement by the silicon/silicon dioxide interface. The electromigration-induced stress evolution indicated the presence of bamboo grains acting as blocking boundaries.