Atomic Layer Deposition and in-Situ Characterization of Yttrium Oxide and Yttria-Stabilized Zirconia

Y₂O₃ is an attractive rear-earth oxide due to its large permittivity (~14-18), high refractive index above 1.9, optical transparency over a wide spectral range (0.2-8µm), large bandgap of 5.8eV and excellent thermodynamic stability. Additionally, when co-deposited with ZrO₂, it leads  to yttria-stabilized zirconia (YSZ), a dense ionic conductor widely used as an SOFC electrolyte. The depositions of Y₂O₃and YSZ by thermal ALD are therefore important in the context of achieving dense pinhole-free thin films (<50nm) that can potentially be applied with excellent conformallity over high aspect ratio 3D nanostructures either for electronic, optical or energy applications.

In this paper, Y₂O₃ and YSZ thin films are thermally deposited by ALD in a commercial Ultratech Savannah reactor. Tris(ethylcyclopentadienyl)yttrium -Y(EtCp)₃  - heated at 120˚C (0.15 Torr vapor pressure) - is used as a precursor with  N₂ carrier bubbled through via  a Low Vapor Pressure Delivery system. Additionally, tetrakis(dimethylamido)zirconium TDMAZr is selected for ZrO₂ deposition while H₂O is the co-reactant. A quartz crystal microbalance has been implemented in order to rapidly characterize the ALD process space from 150 to 300˚C as a function of Y(EtCp)₃ and H₂O doses.  In-situ spectroscopic ellipsometry data are also collected in order to measure the evolution of the optical film properties with temperature. 

 In the case of Y₂O₃, a soft saturation of the growth per cycle vs. Y(EtCp)₃ bubbling time is observed with a GPC of 1.6Å/cycle at 250˚C. The non-uniformity across 150 mm wafers decreases to ~2% at an optimal Y(EtCp)₃ dose.   The GPC increases monotonically with temperature with no apparent plateau between 150 to 300˚C. The optical index remains stable at 1.92 above 200˚C. The Y₂O₃film stoichiometry is confirmed by RBS while SIMS measurements indicate a carbon content below 0.1%.

Uniform YSZ thin films were deposited and the yttria to zirconia composition is accurately controlled by adjusting the ratio of TDMAZr to Y(EtCp)₃ cycles. In-situ measurements confirm the growth of Y₂O₃ on ZrO₂, and vice-versa, without evidence of nucleation retardation when alternating the chemistries. The Y:Zr mass ratio monitored by QCM appears consistent with the Y₂O₃ to ZrO₂ ALD cycle ratio and is verified with ex-situ XPS. XRD measurements were done on Y₂O₃ and YSZ samples to characterize the film crystallinity.

Overall both Y₂O₃ and YSZ ALD processes prove robust and reproducible with negligible contamination and no apparent sign of thermal decomposition of the precursor.