Contents:
Manufacturing Process
Analysis
Evaporation Parameters
Properties of MgF₂ Films
Forms and Sizes Available

MAGNESIUM FLUORIDE, MgF₂
FOR OPTICAL COATING

Introduction
CERAC has developed a manufacturing process for magnesium fluoride which produces a highly pure, dense material form that is particularly well suited for optical coating. MgF₂, a low index coating material, has been used for many years in anti-reflection and multilayer coatings. It is insoluble and hard if deposited on hot substrates.

CERAC's MgF₂ evaporates cleanly and completely, without outgassing or spitting, and leaves a hard, uniform coating on the substrate.

Manufacturing Process
Magnesium fluoride vacuum evaporation material is typically prepared from magnesium metal or other costly starting materials. CERAC has developed a process whereby inexpensive, readily available raw materials are converted into absolutely dry, white MgF₂ powder. The powder is then vacuum melted into clear-to-transparent ingots which are free of trapped water and other volatiles. The ingots are cleanly crushed to various particle sizes for vacuum coating.

back to top

Analysis
CERAC vacuum melted MgF₂ typically shows these elements, each less than 100 ppm: Al, Cr, Cu, Fe, Mn, Sr and Ti. Please contact us if information on any impurity is critical to your coating application.

back to top

Evaporation Parameters

Container	Graphite, Ta, Mo or Al2O3
Evaporation temperature	~950°C
Substrate temperature	150-250°C
Evaporation pressure	10-5 to 10-6 Torr

CERAC vacuum melted MgF₂ granules can be evaporated by electron beam or resistance heating. Graphite or tantalum containers are suggested. They are loaded to the top with material. For thermal evaporation, a large heater shaped like an open boat is used. The power should be increased slowly, and in the case of E-beam sources, the beam should be swept for even heating. Preconditioning for granules of MgF₂ consists of fully melting the charge. The shutter should remain closed until the melt is uniformly red hot. No outgassing is experienced during the preconditioning cycle or during evaporation with CERAC MgF₂ coating material.

The pressure should be in the low 10^-6 Torr range. When completely melted, the material can be evaporated at a smooth rate of 20Å/sec. without spitting. The temperature of the substrate should be at least 250°C for high film hardness and density on glass or zinc sulfide substrates. In difficult cases, a thin, initial bonding layer of yttrium oxide, Y₂O₃, can promote nucleation and adhesion. Films will grow and adhere to glass substrates heated to as low as 150°C. Glow discharge cleaning is recommended for low temperature depositions. The melt can be reused until reaction with the holder begins.

back to top

Properties of MgF₂ Films

Transparent region (for layers <1 µm thick)	1200Å (UV) to ~8 µm (IR)
Absorption	Begins slowly >8 µm and exceeds 2% near 10 µm for 700 nm thick layers
Adhesion to substrates	Good to glass, sapphire, zinc sulfide
Refractive index	1.390 at 300 nm 1.380 at 400 nm 1.375 at 450 nm 1.370 at 500 nm 1.365 at 600 nm 1.360 at 700 nm 1.350 at 1000 nm 1.340 at 1500 nm

The factor that limits MgF₂ layer thickness to ~1 µm is internal stress. MgF₂ can be doped with BaF₂ to reduce this stress and permit thicker layers to be grown. Please contact the CERAC sales department if this is of interest.

Films of MgF₂ that have been deposited on high temperature substrates (250-300°C) using E-beam deposition are mechanically hard but grow with densities only ~80% of bulk density. Voids in the microstructure admit water upon exposure to air, increasing the mean refractive index of the film. Absorption bands are seen in the UV spectra at 3 and 6 µm. Multilayers that include MgF₂ will shift in wavelength upon venting the deposition system to room air. Films that are bombarded by high energy ions during growth achieve higher densities, since an amorphous structure is created. This produces an environmentally stable film with a higher refractive index. Excessively high ion energies can, however, deplete the fluorine content and produce absorption in the UV.

back to top

Forms and Sizes Available

Item No.	Purity	Description
M-2010	99.999% (Ca<10 ppm)	1-4 mm pieces, vacuum melted
M-1113	99.9% (Ca<1%)	3-6 mm pieces, vacuum melted
M-1128	99.9% (Ca<1%)	0.8-3 mm pieces, vacuum melted

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.

NOTE: Facts pertaining to properties and processing parameters of MgF₂ were derived from published literature sources. Although this information is believed to be correct, CERAC does not guarantee its accuracy.

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