Contents:
Applications
Physical Properties of Solid Material
Evaporation Parameters
Film Properties
Forms and Sizes Available from CERAC

ALUMINUM FLUORIDE, Alf₃ FOR
OPTICAL COATING

Introduction
Fluoride compounds are used in coatings for the UV to middle-IR regions. They provide the lowest refractive indices and therefore are essential components of anti-reflective coatings. Fluoride films display absorption bands in the IR regions 2.8-3.2 µm and 6.0-7.4 µm to varying depths, depending on the compound and the deposition method and parameters. High substrate temperatures, E-beam and IAD are methods that reduce the depths of the absorptions.

Aluminum fluoride, AlF₃, is a well-known material which has previously been used as a coating material where a low index was required.* It is not affected by water and has good mechanical strength.

Applications
AlF₃ is used as a coating material for mirrors in excimer lasers. Excimer laser technology has applications in angioplasty in medicine and lithography in semiconductor processing.

Physical Properties of Solid Material

Color	White
Melting point	1260°C sub
Crystal density	2.9 g/cc
Approx. Evaporation Temperature	1100°C sub.

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Evaporation Parameters
AlF₃ was test evaporated with an electron beam from graphite crucibles. The material was preconditioned for 5-15 minutes to dehydrate the particle surfaces and to create a melted, dense surface. AlF₃ evaporated smoothly at 10^-6 Torr at low E-beam power. No spitting occurred, and the vacuum pressure continued to decrease, indicating that no volatiles or water vapor were being emitted. A deposition rate of 10-20 Å/sec. and a substrate temperature of equal or greater than 250°C are suggested for highest density, water resistance and hardness.

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Film Properties

Film Thickness Deposited	1480 nm
Clarity IR, vis	Very Clear
Water Absorption	None
Moisture Resistance	Excellent, not hygroscopic
Eraser Test	Ge and ZnS - both excellent
Adhesion Test	Ge, ZnS, glass - all excellent
Adsorption	0 at 0.25-1 µm <1% at 8 µm 2% at 9 µm 4% at 10 µm 8% at 11 µm
Refractive index	1.36 at 600 nm 1.34 at 800 nm 1.34 at 1000 nm 1.32 at 4000 nm 1.31 at 8000 nm 1.30 at 10,000 nm 1.30 at 12,000 nm

AlF₃ coatings have a very low index compared with other fluorides. A loss of ~2% in transmission at wavelengths near 400 nm is attributed to a partial deficiency of fluorine content caused by decomposition in the E-beam evaporation. This depletion is aggravated by high energy processes, such as ion assisted deposition (IAD). The effect is minimal with resistance heated evaporation. Since the test evaporated film layer was ~1500 nm thick, this deficiency would be undetectable in the UV, where applications require layers one-tenth this thickness. AlF₃ has promise for IR use although its absorption in the 10 µm region might be a little higher than desirable for high energy applications. The durability and adhesion of AlF₃ films proved to be excellent. In the UV, AlF₃ is finding applications in high energy laser coatings at wavelengths below 250 nm.

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Forms and Sizes Available from CERAC

Item No.	Purity	Description
A-1218	99.5%	3-6 mm pieces (highly dense, pressure sintered)

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.

* "Vacuum Evaporated Films of Aluminum Fluoride," W. Heitmann, Thin Solid Films, 5 (1970) 61-67.

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