Time dependent dielectric breakdown at ultra low frequencies in low-k dielectrics

Ultra-low frequency time dependent dielectric breakdown (TDDB) in a low-k interlevel dielectric was investigated was studied to examine the possibility of relaxation effects. It can be argued that during the “off” cycle, the dielectric may regain some strength, which would result in a longer lifetime as compared to a constant voltage corresponding to the “on” time. The pulse frequencies were very low based on a 24 hour period with varying duty cycles and compared to a constant voltage (100% duty cycle). Regardless of the duty cycle, a characteristic time to failure (t
₆₃
assuming a Weibull distributor) of about 155 hours for one wafer, and about 80 hours for a second wafer when normalized to the “on” time. This suggests that relaxation is a very small, or nonexistent factor in TDDB. The current model to predict TDDB failure time in industry does not account for any relaxation that may or may not occur, and our findings support the continued use of this apprach.