Uniqueness in activation energy and charge-to-breakdown of highly asymmetrical DRAM Al2O3 cell capacitors

Al₂O₃ cell capacitors for dynamic random access memory (DRAM) applications were tested using constant voltage, time-dependent dielectric breakdown (TDDB) tests. The capacitors had area-enhancing, hemispherical grain (HSG) polysilicon as bottom electrodes (BEs). These electrodes acted as points of high electric field, and eased charge injection into the Al₂O₃. As a result, the capacitors had highly asymmetric current-voltage (I-V) characteristics. Time-to-fails (TTFs) were polarity-dependent, and, thus, much worse for HSG injection. However, activation energy (E_a) and charge-to-breakdown (QBD) obtained from conducting stress under opposite polarities were a unique function of the electric field only. The results point to a common, polarity-independent mechanism responsible for final breakdown, and the possibility that only the kinetics of degradation is electrode controlled. Good correlation with the thermochemical E model suggests that the breakdown mechanism in Al₂O₃ might be similar to SiO₂.