As the design rule becomes smaller, it becomes increasingly difficult to ensure the storage capacitance of DRAM. At present, ZrO2/Al2O3/ZrO2 (ZAZ) is being used for dielectric film of DRAM capacitor. When the design rule of device becomes < 20nm, the ZAZ film thickness should be too thin to crystalize it. As a result, the dielectric constant of a ZAZ thin film is lower than the expected value, particularly the upper ZrO2 layer [1]. In this regard, TiO2, which has a higher dielectric constant than that of ZrO2 and lower crystallization temperature, could be an alternative to the ZrO2 layer. However, the small band gap of TiO2 (3.4eV) makes it challenging to be used as a single dielectric layer in the structure. Therefore, in this study, the upper ZrO2 layer of the conventional ZAZ structure is replaced by a TiO2 layer, which may result in the lower equivalent oxide thickness (EOT) than that of the original structure without a significant increase in the leakage current.
In this study, the electrical and physical properties of ZrO2/Al2O3/TiO2 (ZAT) and ZAZ deposited on TiN substrate using ALD method were evaluated. A traveling-wave-type ALD reactor was used for in-situ ZAT and ZAZ and the oxygen source of O3 and the precursors of TEMAZr, TMA and TTIP were used to deposit ZrO2, Al2O3 and TiO2. Various ratios between upper and lower dielectric layers were evaluated in order to observe any changes in their properties. The capacitance density of ZAT dielectric was higher (Figure 1) even without the rapid thermal annealing (RTA) process, and there was no significant difference in the I-V properties. The capacitance of ZAT device was improved by 50% after the N2 ambient RTA performed at 600oC for 30 seconds, but the higher leakage current was observed. The EOT decreases due to the crystallization of TiO2 and the extent of crystallization was analyzed. A tetragonal phase was observed in ZrO2 after the annealing. With the use of thinner ZAT films, EOT was improved by ~0.5 nm at a given physical thickness of 7nm which is a significant merit over ZAZ.
References [1] D. Zhou, Appl. Phys. 108 (2010) 124104. [2] S. K. Kim, Appl. Phys. Lett., 85, 4112 (2004)