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

Tuesday, October 13, 2015: 17:00
Phoenix East (Hyatt Regency)
L. Lecordier (Ultratech-Cambridge NanoTech)
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 Y2Oand 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.