Dynamics of Resistance Evolution During Electromigration

The careful analysis of resistance evolution with time during electromigration tests provided valuable information about damascene architecture and intrinsic electromigration behaviour. We found first, that resistance evolution includes an abrupt event, the amplitude of which is usually corresponding to the failure criteria and second, that the height of this step (and sample resistance) is correlated with time to fail. We showed that the evolution of time to step was more likely to be due to the vertical growth of a void which length is making resistance step amplitude varying from sample to sample. Second we found a correlation between sample initial resistance and time to failure that is believed to be due to trench depth. However this effect is widely spread and correlation coefficient is as low as 0.3. These 2 phenomena combined together, increase the standard deviation affecting lifetime. As a matter of fact, resistance increase rate is expected to be directly proportional to void growth rate and copper diffusion coefficient whereas step height is function of the void shape and barrier thickness