Comparison of the atomic layer deposition of tantalum oxide thin films using Ta(N^tBu)(NEt₂)_3, Ta(N^tBu)(NEt₂)₂Cp, and H₂O

The growth characteristics of Ta₂O₅ thin films by atomic layer deposition (ALD) were examined using Ta(NtBu)(NEt₂)₃ (TBTDET) and Ta(NtBu)(NEt₂)₂Cp (TBDETCp) as Ta-precursors, where ^tBu, Et, and Cp represent tert-butyl, ethyl, and cyclopentadienyl groups, respectively, along with water vapor as oxygen source. The grown Ta₂O₅ films were amorphous with very smooth surface morphology for both the Ta-precursors. The saturated ALD growth rates of Ta₂O₅ films were 0.77 Ä cycle^-1 at 250 °C and 0.67 Å cycle^-1 at 300 °C using TBTDET and TBDETCp precursors, respectively. The thermal decomposition of the amido ligand (NEt₂) limited the ALD process temperature below 275 °C for TBTDET precursor. However, the ALD temperature window could be extended up to 325 °C due to a strong Ta-Cp bond for the TBDETCp precursor. Because of the improved thermal stability of TBDETCp precursor, excellent nonuniformity of ∼2% in 200 mm wafer could be achieved with a step coverage of ∼90% in a deep hole structure (aspect ratio 5:1) which is promising for 3-dimensional architecture to form high density memories. Nonetheless, a rather high concentration (∼7 at. %) of carbon impurities was incorporated into the Ta₂O₅ film using TBDETCp, which was possibly due to readsorption of dissociated ligands as small organic molecules in the growth of Ta₂O₅ film by ALD. Despite the presence of high carbon concentration which might be an origin of large leakage current under electric fields, the Ta2O5 film using TBDETCp showed a promising resistive switching performance with an endurance cycle as high as ∼17 500 for resistance switching random access memory application. The optical refractive index of the deposited Ta₂O₅ films was 2.1-2.2 at 632.8 nm using both the Taprecursors, and indirect optical band gap was estimated to be ∼4.1 eV for both the cases.