Film Thickness Nonuniformity and the Crucial Role of Water Dosage for Atomic Layer Deposition of Yttrium Oxide

This work uses in situ ellipsometry to explore the kinetics of atomic layer deposition (ALD) of thin yttrium oxide (Y ₂ O ₃ ) films from 200 to 350 °C to find the process window using tris(methylcyclopentadienyl)yttrium ((MeCp) ₃ Y) and water (H ₂ O) precursors. The results show that an accurate control of H ₂ O dosage with sufficient H ₂ O purge is essential. To verify the occurrence of ALD, particular attention is paid to the film thickness (non‐)uniformity corresponding to saturation conditions of the growth‐per‐cycle (GPC) curves. A decrease in thickness nonuniformity from 63% (350 °C) down to <5% (200 °C) is observed, emphasizing the need to carefully select the process temperature ( T ) of Y ₂ O ₃ within the published ALD temperature window (200–400 °C). This work demonstrates that purely relying on the GPC saturation curves does not necessarily ensure the occurrence of ALD and highlights the importance of combining the results of both growth kinetics and film thickness uniformity studies. For films deposited at T  ≤ 250 °C, X‐ray photoelectron spectroscopy results show (i) no carbon impurity, (ii) limited hydroxylation, and (iii) an oxygen‐to‐yttrium ratio of ~1.14 after applying argon pre‐sputtering (with excess removal of oxygen during the sputtering). The ~10 nm Y ₂ O _3‐x films exhibit a polycrystalline structure as confirmed by X‐ray diffractometry.