In-Situ X-Ray Microdiffraction Observation of Strain Evolution in CU Damascene Lines under the Influence of Electromigaration on a One Micron Scale

In contrast to the knowledge base that has been acquired for Al interconnects over the past 30 years, the investigations of electromigration in Cu damascene interconnects are still relatively new. One of the major issues is, as it has been for Al interconnects, whether the local microstructure of Cu has a direct influence on electromigation. In this study we utilized the newly developed facility for microdiffraction at the Advanced Light Source (ALS) at Lawrence Berkeley National Labs. The complete 3D strain tensor of any single grain along a conducting line ca be determined to an accuracy of 2e-4. The spatial resolution is better then 0.8microns. This technique was applied to Blech type Cu damascene interconnects in-situ at 250 C at current densities between 0.5 and 2 MA/cm sup 2. The segment lengths ranged from 10 to 100microns and the line widths from 0.8 to 2microns. The lines were passivated with 200nm of SiN sub x and showed a critical product (current density * segment length) of between 3000 and 5000 A/cm. Voiding was observed to occur at the Cu/SiN sub x interface, not only at the cathode end but also at specific locations along the entire length of the line. The correlation between the void sites, the local microstructure and the local strain/stress state will be demonstrated.