ALUMINUM OXIDE, Al2O3
FOR OPTICAL COATING
Introduction
Aluminum oxide, Al2O3, is a medium
index, low absorption material usable for coatings in the near-UV
(<300 nm) to IR (~5 µm) regions. Typical applications
include near-UV laser AR and dielectric mirror designs. Alumina
can be used in combination with silicon dioxide layers to form
multilayer structures with high damage thresholds for UV laser
applications.
Film
Properties
Completely oxidized alumina films are absorption-free over the
range below 300 nm to at least 5 µm. Slight dissociation
and oxygen loss occurs during evaporation. Adhesion is good to
glass, most other oxides, some polymers, and to metals such as
aluminum and silver. The films generally grow with a crystalline
microstructure and low packing density and exhibit index changes
when vented to moist air. Adsorption of water is often evident
as rings of discoloration which reach a uniform appearance after
moisture penetration is complete. This instability effect is
exaggerated under low energy evaporation, low substrate temperature,
or excessive background pressure evaporation conditions.
The refractive index responds to high energy deposition techniques
and to high substrate temperature because both parameters decrease
the void volume by increasing the packing density of the microstructure.
Post-deposition baking in air can also raise the refractive index.
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Refractive
Index
The refractive indices are dependent on the degree of oxidation,
the substrate temperature and the film density achieved. For
example, on a substrate at temperature of 300° C, n=1.63
for 550 nm. Approximate values are plotted below [1]. Since less
dissociation occurs for Al2O3 than with
SiO2 evaporation, less background oxygen is required
to obtain low absorption film layers of alumina.
REFRACTIVE INDEX FOR
ALUMINUM OXIDE
(note scale discontinuity)
Wavelength (nm)
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Material
Behavior
Recommended preconditioning consists of slow sweeping
of the electron beam to heat the top surfaces to white hot and
fuse them. Avoid hole drilling. Replace the charge when it becomes
dark colored.
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Evaporation
Parameters
|
Evaporation Temperature |
~2100°
C |
|
Source Container |
No liner for
E-beam. Electron beam should be swept. |
|
Rate |
2-5 Å/sec. |
|
Partial Pressure of Oxygen |
1 x 10-5
Torr. |
|
Substrate Temperature |
200° C
to 300° C |
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Physical
Properties of Solid Material
|
Molecular Weight |
101.96 |
|
Melting Point |
2020°
C |
|
Color |
Clear to white
(see item description) |
|
Crystal Density |
3.97 g/cc |
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Some
Forms and Sizes Available from CERAC
|
Item No. |
Purity |
Description |
|
A-1230 |
99.99% |
2-12 mm pieces
(clear, single-crystal sapphire) |
|
A-1220 |
99.99% |
1.5-3 mm pieces,
clear, single-crystal sapphire |
|
A-1121 |
99.8% |
3-12 mm pieces
(highly sintered, opaque) |
|
A-1217 |
99.8% |
1-3 mm pieces
(highly sintered, opaque) |
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.
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Applications
Because of its low absorption below wavelengths of 300 nm,
alumina films can be combined in multilayers with silicon dioxide
(n = 1.48) for UV laser applications. Adherent coatings can be
deposited on low temperature substrates. Thin alumina films sometimes
are useful in promoting adherence between two dissimilar materials.
In contrast to the parent crystalline sapphire form, alumina
films never obtain equivalent density, hardness or water impermeability.
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Reference
T. S. Eriksson, A. Hjortsberg, G. A. Niklasson, and C. G.
Granqvist, Applied Optics 20Ý(5), 2742 (1981).
|
