New insight in BEOL TDDB Cu diffusion mechanism: A constant current stress approach

The Time Dependent Dielectric Breakdown (TDDB) of Ultra-Low-κ (ULK) (κ=2.7) and porous ULK SiCOH (κ=2.55) was systematically investigated using both Constant Current Stress (CCS) and Constant Voltage Stress (CVS) method on our 32nm and 28nm technologies. The invariance of J²t unambiguously suggest that the NBlock-IMD interface Cu diffusion is the dominant failure mechanism. Also, the electric field independent TDDB thermal activation energy was experimentally identified. By comparing the distribution of t_BD and J²t_BD obtained by CVS, J²t_BD gives a better β value closer to intrinsic because die to die spatial variations are automatically compensated. The extracted β is also consistent with that from Poisson area scaling study. At high current stress conditions, J²t_BD start to drop, indicating another failure mechanism starts to dominate. In summary, the evaluation of J²t_BD provides a new insight in the low-κ TDDB breakdown mechanism.