Comparison of the Atomic Layer Deposition of Tantalum Oxide Thin Films Using Ta(NBu)(NEt), Ta(NBu)(NEt)Cp, and HO.

The growth characteristics of TaO thin films by atomic layer deposition (ALD) were examined using Ta(NBu)(NEt) (TBTDET) and Ta(NBu)(NEt)Cp (TBDETCp) as Ta-precursors, where Bu, Et, and Cp represent tert-butyl, ethyl, and cyclopentadienyl groups, respectively, along with water vapor as oxygen source. The grown TaO films were amorphous with very smooth surface morphology for both the Ta-precursors. The saturated ALD growth rates of TaO films were 0.77 Å cycle at 250 °C and 0.67 Å cycle at 300 °C using TBTDET and TBDETCp precursors, respectively. The thermal decomposition of the amido ligand (NEt) 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 TaO film using TBDETCp, which was possibly due to readsorption of dissociated ligands as small organic molecules in the growth of TaO film by ALD. Despite the presence of high carbon concentration which might be an origin of large leakage current under electric fields, the TaO 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 TaO 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.