Cerium Dioxide CeO2
Cerium Dioxide is a coating material with high refractive index in the visible, and transparency to IR wavelengths >10 μm. Layers are deposited by electron-beam evaporation, preferably with IAD. Thick layers exhibit very low stress and good adhesion to glass, fused silica, silicon, and Ge substrates. However, the growth microstructure is strongly columnar, and therefore the films are soft and porous. This growth property permits the growth of structured (sculptured) layers including birefringent films and films that might be useable in photocatalytic applications.
Fully oxidized CeO2 films are absorption free over the wavelength range to ~500 nm to at least 12 μm in the IR. Adhesion is excellent to glass and to most other oxide compounds. Film layers exhibit large inhomogeneity in index: index is highest near the substrate and can decrease by >10% toward the top of the layer (wavelength dependent). Films in thick layers have rough surfaces because they grow with a coarse columnar structure. Such films are easily damaged by abrasion, for example the moderate abrasion test of the MIL-STD C-48497. The upper thickness of the film can be burnished with cloth. Durability can be increased, however, by over coating with Y2O3 or SiO2, depending on application. Roughness of thick film layers can be reduced by interposing thin layers of Y2O3, or for the case of a multi-layer structure, alternating with layers of SiO2. Cerium Dioxide layers are insoluble in boiling water and in most acids.
The columnar growth form tapers to smaller diameter with height away from the substrate, causing the refractive index to decrease with thickness as void volume increases and packing density decreases. High ion-energy (keV) at high current densities reduce this low-density effect and produce higher indices. CERAC can prepare special compositions based on admixtures of glass-forming oxides to promote increased packing density. The refractive index curve presented below was measured for a 900 nm thick film deposited using IAD (~200 eV) and 200 substrate temperature. Data from glass and Germanium substrates are combined in the curve.
The material sublimates and is best evaporated by E-beam. Evaporation is smooth and rate is stable. Beam sweep should be used, and power and rate should be limited to reduce particulate ejection. Evaporation causes only a small oxygen loss that will be restored with the addition of a partial pressure of oxygen during reactive deposition.
|Evaporation temperature||~1600° C|
|Source Container||Tantalum or graphite liner for E-beam|
|Partial pressure of oxygen||~5 x 10-5 Torr|
|Substrate temperature||200° C to 400° C.|
Physical Properties of Solid Material
|Molecullar Weight||172 g/mol|
|Melting Point||>2600° C|
Forms and Sizes Available
Materion offers materials for evaporation as well as sputtering targets.