Temporary extrusion failures in accelerated lifetime tests of copper interconnects

A novel electromigration failure mode was detected in 0.13-μm technology, copper dual damascene interconnects. Extrusions formed between the test lead and neighboring monitor lines, resulting in short-circuit failure. However, many of these extrusions were short lived, shrinking within a period of a minute to an hour. These phenomena indicated that a temporary "soft failure" existed in accelerated copper electromigration tests in addition to the traditional permanent failure or "hard failure." These soft failures would be missed unless short sampling intervals (less than a minute) and continuous monitoring of the leakage current between test metal lines and neighboring circuits were carried out. Consistent with our experimental results, physical modeling suggested that capillary forces were able to rupture a long and narrow extrusion and the electric field across the extrusion could accelerate this process.