Yttrium Oxide, Y2O3 for Optical Coating References
D. F. Bezuidenhout and R. Pretorius, Thin Solid Films 139, 121 (1986).
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Yttrium oxide,Yttria Y2O3, is a medium-index, low-absorption material usable for coatings in the near-UV (300 nm) to IR (12 µm) regions. Hard, dense layers are deposited by electron-beam evaporation or sputtering. Typical applications are protection of aluminum and silver mirror coatings, intermediate layer in wide band visible AR coatings and for XeCl (308 nm) laser AR and dielectric mirror designs. Yttria can be used in combination with silicon dioxide layers to form a high index-contrast structure and also with higher index materials such as titania (TiO2) and tantala (Ta2O5).
Film Properties of Yttrium Oxide Y2O3
Yttria films are absorption-free over the range 300 nm to at least 11 µm. Under some evaporation conditions, such as low energy resistance-heated evaporation or excessive background pressure, the films grow with sufficient void volume.
The refractive index also responds to high energy deposition techniques but responds less than other oxides to substrate temperature. The index remains constant to within 1% over substrate temperature range 50c to nearly 300c. Post-deposition baking in air can raise the refractive index of electron-beam and resistance-heated depositions.
Adhesion is excellent to glass, germanium, silicon, zinc sulfide and zinc selenide, as well as to metals such as aluminum and silver. In some cases, a very thin layer of yttria can serve as an adherence promoter for multilayer coatings on non-oxide substrates.
Some amount of index inhomogeneity can appear with increasing layer thickness. The effect can be reduced by providing sufficient oxygen backfill during evaporation. The films generally grow with an amorphous microstructure, but as mentioned above, there can be void volume which will absorb water vapor upon venting the vacuum chamber.
Refractive Index of Yttrium Oxide Y2O3
The refractive indices are process dependent. Reactive thermal and e-beam depositions produce values 1.80 to 1.88 at 500 nm wavelength, while IAD and magnetron sputtering produce values >1.90. Above 2 µm wavelength, magnetron values are close to bulk crystal index values: 1.87 @ 2 µm and 1.72 @ 9 µm, while thermally evaporated values are 1.7 and 1.42 respectively. Approximate values are plotted below.
REFRACTIVE INDEX FOR YTTRIUM OXIDE
(note scale discontinuity)
Material Behavior of Yttrium Oxide Y2O3
The starting material form is either tablets or sintered pieces. Gentle preconditioning at reduced power for several minutes is recommended. The electron-beam should be swept over the surface. The material sublimes.
Evaporation Parameters of Yttrium Oxide Y2O3
Physical Properties of Solid Material Yttrium Oxide Y2O3
Applications of Yttrium Oxide Y2O3
Because the index below wavelengths 300 nm is near 1.9, yttria can be used in multilayers with silicon dioxide (n = 1.48) for UV laser applications. In the visible region, it satisfies the intermediate index value required for three-layer wide band AR coatings on glass. Environmental protection of silver and aluminum mirrors has been demonstrated from the near-UV through the far-IR. Yttria exhibits no reststrahlen bands below 18 µm, and therefore does not produce reflection loss at large incidence angles seen with silicon oxides and alumina.
Forms and Sizes Available for Yttrium Oxide Y2O3
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