Electromigration of Cu interconnects under AC and DC test conditions

Electromigration (EM) of a dual damascene, test vehicle was measured using direct current (DC), alternating current (AC) followed by DC, and three rectangular-wave DC stressing conditions at 598 K. In some of the experiments samples were allowed to cool to room temperature between the 598 K stress cycles.We find that that only net DC time at test has a significant effect on time to fail. AC stressing for one week at 2.5 MA/cm2 prior to DC stressing to EM failure had no effect on the EM performance. A similar result is obtained with various DC and thermal cycling conditions. Thermal history of the EM test samples, and in particular cooling to room temperature, has no effect on EM lifetime. All tests, regardless of thermal history and current cycling conditions, resulted in statistically similar times to fail and distributions of those times.We conclude that the standard test methodologies, using accelerated DC stress conditions at elevated temperatures, are adequate. Since only the net DC stress time had any measurable effect on EM lifetime, the use of accelerated testing provides a good predictor for lifetime expectations under operating conditions. We also conclude that neither low current density AC stressing nor DC cycling, and associated changes to grain structure that may result, provide any measurable benefit to EM lifetime.

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