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 Ta 2 O 5 thin films by atomic layer deposition (ALD) were examined using Ta(N t Bu)(NEt 2 ) 3 (TBTDET) and Ta(N t Bu)(NEt 2 ) 2 Cp (TBDETCp) as Ta-precursors, where t Bu, Et, and Cp represent tert -butyl, ethyl, and cyclopentadienyl groups, respectively, along with water vapor as oxygen source. The grown Ta 2 O 5 films were amorphous with very smooth surface morphology for both the Ta-precursors. The saturated ALD growth rates of Ta 2 O 5 films were 0.77 Å 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 (NEt 2 ) 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 Ta 2 O 5 film using TBDETCp, which was possibly due to readsorption of dissociated ligands as small organic molecules in the growth of Ta 2 O 5 film by ALD. Despite the presence of high carbon concentration which might be an origin of large leakage current under electric fields, the Ta 2 O 5 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 Ta 2 O 5 films was 2.1–2.2 at 632.8 nm using both the Ta-precursors, and indirect optical band gap was estimated to be ∼4.1 eV for both the cases.