Improved leakage current properties of ZrO₂/(Ta/Nb)O_x-Al₂O₃/ZrO₂ nanolaminate insulating stacks for dynamic random access memory capacitors

The influence of amorphous high-k interlayers, such as Al₂O₃, (Ta/Nb)O_x (TN), and (Ta/Nb)O_x-Al₂O₃ (TNA), on the leakage current (J) and dielectric constant (k) for metal-insulator-metal capacitors with ZrO₂/high-k/ZrO₂ nanolaminate insulating films and TiN electrodes was investigated. The insulating films were prepared by atomic layer deposition followed by post-deposition annealing at 600 °C. The capacitance equivalent thickness (CET) of the capacitors increased in the order ZrO₂/(Ta/Nb)O_x/ZrO₂ (ZTNZ) < ZrO₂/(Ta/Nb)O_x-Al₂O₃/ZrO₂ (ZTNAZ) < ZrO₂/Al₂O₃/ZrO₂ (ZAZ), owing to the k values for Al₂O₃ (~6), TNA (~9), and TN (~11). The J values at 0.6 V for capacitors with a CET of 1.1 nm increased in the order ZTNAZ < ZAZ ZTNZ. The effect of a high-k interlayer on the J characteristics appeared above a thickness of 0.4 nm in the case of Al₂O₃ and TNA, while a 0.8-nm-thick TN maintained high J values. Based on these results, there are three important factors as a high-k interlayer to reduce J value, such as a band gap larger than that for TN (4.4 eV), a thickness of ≥0.4 nm, and an amorphous structure. Therefore, to achieve the low J and CET, TNA is a promising candidate material for a high-k interlayer for future dynamic random access memory.