Contents:
Film Properties
Refractive Index
Material Behavior
Evaporation Parameters
Physical Properties of Solid Material
Applications
Forms and Sizes Available

YTTRIUM OXIDE, Y₂O₃
FOR OPTICAL COATING

Introduction
Yttrium oxide, 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
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 (packing density <0.9) to exhibit index changes when vented to moist air. This manifests itself as absorption bands in the 2.9 µm (O-H stretching) and 6.9 µm (O-H-O vibration) [1]. When deposition is done by sputtering, ion assist, or at very high substrate temperatures, the absorption bands are nearly absent.

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 50° C to nearly 300° C [2]. 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.

back to top

Refractive Index
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 [3] 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.

back to top

Material Behavior
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.

back to top

Evaporation Parameters

Evaporation Temperature	~ 2400° C.
Source Container	Thick tungsten boat for resistance-heated evaporation. Tungsten will react and must be replaced after one evaporation; graphite liner for E-beam.
Rate	1-2 Å/sec.
Partial Pressure of Oxygen	5-8 x 10-5 Torr
Substrate Temperature	50° C to 300° C.

back to top

Physical Properties of Solid Material

Molecular Weight	225.81
Melting Point	2410° C
Color	White
Crystal Density	5.01 g/cc

back to top

Applications
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.

back to top

Forms and Sizes Available

Item No.	Purity	Description
Y-1043	99.9%	8-9 mm dia. x 4-5 mm thick sintered tablets
Y-1046	99.9%	3-12 mm sintered pieces

CERAC offers other particle sizes for evaporation as well as sputtering targets. To view pricing on the items listed above, please visit our on-line catalog and look-up by item number or chemical name. If you require a custom manufactured item, please contact our sales department at 414-289-9800 or ceracsales@beminc.com with your specific requirements. You can also fill out our quotation request form.

back to top

References

1. D. F. Bezuidenhout and R. Pretorius, Thin Solid Films 139, 121 (1986).
2. George Atanassov, Roland Thielsch, and Dimitar Popov, Thin Solid Films 223, 288 (1993).
3. E. Masetti, S. Scaglione, L. Caneve, and G. Emialiani, SPIE Vol. 1019 Thin Film Technol. III (1988).
   

Phone:  414-289-9800 /  FAX: 414-289-9805  /  ceracinfo@beminc.com