Deposition of magnesium fluoride films

A reactive precursor, for depositing a magnesium fluoride film on glass by a spray pyrolysis or chemical vapor deposition process, comprises magnesium acetylacetonate, trifluoroacetic acid, and optionally a solvent.

FIELD OF THE INVENTION 
This invention relates generally to the deposition of magnesium fluoride 
films onto glass. More particularly, the invention is directed to a 
process for depositing, by spray pyrolysis or chemical vapor deposition, 
magnesium fluoride films onto the surface of a hot glass substrate, such 
as is produced by the float-glass process. 
BACKGROUND OF THE INVENTION 
Magnesium fluoride is well-known as a coating material which imparts 
anti-reflectivity to optical articles and vision glazings, to increase the 
visible light transmissivity of same. Magnesium fluoride films may 
conveniently be deposited onto transparent substrates, either singularly 
or in combination with other metal and dielectric layers,.by conventional 
methods such as, for example, spray pyrolysis, chemical vapor deposition, 
vacuum evaporation, sputtering, and the like. 
The deposition of magnesium fluoride by spray pyrolysis or chemical vapor 
deposition requires the preparation of a proper reactive precursor which 
is then directed onto the heated surface of the substrate to be coated. 
Spray pyrolysis utilizes a liquid reactive precursor which is sprayed onto 
the surface of the hot substrate whereat the precursor thermally 
decomposes and reacts to form a film. Chemical vapor deposition utilizes a 
gaseous reactive precursor which is delivered to the surface of the hot 
substrate whereat the precursor reacts to form the film. Conveniently, a 
continuous glass ribbon being produced by the float glass process possess 
sufficient heat to activate and react both spray pyrolysis and chemical 
vapor deposition precursors to form such films thereon. 
U.S. Pat. No. 3,475,192 to Langley discloses a liquid reactive precursor 
which is coated onto a glass surface then fired to form a magnesium 
fluoride film. The precursor comprises magnesium fluoride dissolved in a 
mixture of an organic solvent and a film forming material, e.g., an oil. 
U.S. Pat. No. 4,492,721 to Joosten et al. discloses a method of providing 
magnesium fluoride layers on substrates by the disproportionation of 
fluorine-containing organic magnesium compounds containing at least six 
fluorine atoms per magnesium atom. A reactive precursor, comprising a 
fluorine-containing compound, e.g., magnesium trifluoroacetate, magnesium 
trifluoroacetylacetonate, magnesium hexafluoroacetylacetonate, etc., and 
an organic solvent is contacted with a hot glass substrate utilizing 
either spray pyrolysis or chemical vapor deposition, depending upon the 
vaporization temperature of the fluorine-containing compound used. Such 
precursors, however, are generally difficult to work with due to their 
limited solubility in various solvents, and additionally produce coatings 
on glass having relatively limited anti-reflection properties. 
It would be desirable to devise a process for depositing highly 
anti-reflective magnesium fluoride coatings on glass, utilizing reactive 
precursors which are easy to work with due to their good solubility 
properties. 
SUMMARY OF THE INVENTION 
Accordant with the present invention, a reactive precursor for preparing a 
magnesium fluoride film on glass by a spray pyrolysis or chemical vapor 
deposition process has surprisingly been discovered. The precursor 
comprises: 
A) magnesium acetylacetonate; 
B) trifluoroacetic acid; and 
C) optionally, a solvent. 
The inventive reactive precursor may be delivered to the surface of a 
heated glass substrate as either a liquid (in the spray pyrolysis process) 
or as a vapor (in the chemical vapor deposition process). 
The reactive precursors of the present invention are particularly well 
suited for placing magnesium fluoride anti-reflective films onto 
automotive and architectural glazings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A magnesium fluoride film is deposited onto a hot glass substrate by either 
of the well-known pyrolysis or chemical vapor deposition processes, 
utilizing a reactive precursor comprising a mixture of magnesium 
acetylacetonate and trifluoroacetic acid. Conveniently, the reactive 
precursor may also include a solvent. The magnesium fluoride film 
deposited by the process of the present invention additionally contains a 
minor amount of magnesium oxide which decreases as the reaction 
temperature increases. 
The glass upon which the magnesium fluoride film is deposited may be in the 
form of individual glass sheets, or in the form of a continuous glass 
ribbon produced by the well-known float glass process. Suitable glass 
substrates for use according to the present invention may include any of 
the conventional glass compositions known in the art as useful for 
preparing automotive or architectural glazings. The various chemical 
compositions which produce different glass substrates, e.g., borosilicate 
glass or soda-lime-silica glass, generally do not structurally nor 
chemically affect the deposited layer of magnesium fluoride. A preferred 
glass is commonly known in the art as soda-lime-silica glass, and may be 
of any thickness generally known as useful for providing support for a 
coating such as magnesium fluoride. 
Where the glass to be coated with magnesium fluoride is being produced by 
the float glass process, the temperature of the glass is already 
sufficiently high to either pyrolyze the liquid spray or react together 
the vaporized components of the chemical vapor deposition reactive 
precursors of the present invention. Pyrolysis or chemical vapor 
deposition utilizing the inventive reactive precursors generally is 
carried out at a temperature from about 900.degree. F. to about 
1,200.degree. F. Individual glass sheets which are to receive a coating of 
magnesium fluoride must be heated to approximately this specified 
temperature range to effect the pyrolysis or chemical vapor deposition 
reaction. 
Magnesium acetylacetonate is a well-known, commercially available chemical 
reagent which may be obtained from Amspec, Gloucester City, N.J. 
Trifluoroacetic acid likewise is a well-known commercially available 
chemical reagent, generally available in concentrations from about 97% to 
about 98.5%, from suppliers such as Fisher Scientific, Pittsburgh, Pa. 
Solvents which may optionally be included in the reactive precursors of the 
present invention include a wide variety of organic materials such as, for 
example, methanol, dimethylformamide, benzene, toluene, xylene, hexane, 
heptane, methanol, ethanol, methylene chloride, perchloroethylene, 
chloroform, carbon tetrachloride, dimethylacetamide, acetonitrile, 
nitrobenzene, acetic acid, ethylene diamine, propanol, butanol, etc., and 
water, as well as mixtures thereof. A preferred solvent is a mixture of 
methanol and water. 
The ingredients may be admixed in any conventional mixing device, and in 
any order. Generally, the solvent, if one is used, is mixed with the 
trifluoroacetic acid, and the magnesium acetylacetonate is thereafter 
added to the mixture. The magnesium acetylacetonate and trifluoroacetic 
acid are present in amounts sufficient to produce a magnesium fluoride 
film on the surface of a hot glass substrate, utilizing either a spray 
pyrolysis or chemical vapor deposition process. Generally, the magnesium 
acetylacetonate is present in a concentration from about 4% to about 20% 
by weight of the reactive precursor. Preferably, the concentration of 
magnesium acetylacetonate is from about 10% to about 18% by weight. The 
trifluoroacetic acid is generally present in a concentration from about 5% 
to about 95% by weight of the reactive precursor. Preferably, the 
concentration of trifluoroacetic acid is from about 8% to about 14% by 
weight, and is used in conjunction with a solvent. The solvent, when 
optionally present, is generally included at a concentration from about 
60% to about 80% by weight of the reactive precursor. A preferred reactive 
precursor includes a solvent at a concentration from about 70% to about 
80% by weight. The reactive precursor is delivered to the surface of the 
heated glass substrate as a liquid in a spray pyrolysis process. However, 
the reactive precursor is vaporized by conventional means to form a vapor 
which is then delivered to the heated surface of the glass substrate in a 
chemical vapor deposition process. 
The resultant magnesium fluoride film may be deposited so as to form a 
layer having virtually any thickness, depending upon the reaction time, 
reaction conditions, glass temperature, etc. Generally, film thicknesses 
from about 50 Angstroms to about 5,000 Angstroms have been found to be 
useful for various automotive and architectural glazing applications, 
depending on the intended purpose for the magnesium fluoride film. 
The inventive process is not only useful for forming a magnesium fluoride 
film directly on a glass surface, but also for forming a magnesium 
fluoride film on a previously deposited coating which is adhered to the 
glass surface. Thus, the present invention may be used to apply a 
magnesium fluoride film over a previously applied dielectric or metal 
layer. Accordingly, the term "glass substrate" as used herein is intended 
to include glass having one or more of the aforementioned dielectric or 
metal layers adhered thereto. Examples of dielectric layers upon which a 
magnesium fluoride film may be deposited according to the present 
invention include, but are not necessarily limited to, ZnO, SnO.sub.2, 
MgF.sub.2, Al.sub.2 O.sub.3, TiN, SiO.sub.2, MgO, TiO.sub.2, and the like, 
as well as mixtures thereof. Contemplated metal layers having the same 
operability and utility upon which a magnesium fluoride film may be 
deposited include, but are not necessarily limited to, Ag, Au, Cu, Ti, Al, 
Sn, etc., as well as alloys thereof. It is well known, for example, to 
manufacture a high performance automotive or architectural glazing having 
anti-reflection, infrared rejection, and/or electrical resistance 
heatability properties, comprising a glass substrate having multiple 
coatings of silver and zinc oxide thereon, including a final 
anti-reflection coating of magnesium fluoride. 
EXAMPLE 
A reactive precursor is prepared by admixing 15.6 weight percent magnesium 
acetylacetonate, 8.9 weight percent trifluoroacetic acid, 30.3 weight 
percent water, and 45.1 weight percent methanol. The resulting reactive 
precursor solution is sprayed onto a soda-lime-silica glass sheet having a 
previously applied layer of fluorine-doped tin oxide, at a temperature of 
about 1,000.degree. F., to produce a magnesium fluoride film, containing a 
minor amount of magnesium oxide, about 1,400 Angstroms thick. 
The magnesium fluoride coated substrate has a film-side reflectance of 
5.6%, compared to the film-side reflectance of the fluorine-doped tin 
oxide coated glass substrate equal to 9.4%. 
Other examples may be prepared with similar success by substituting the 
generally or specifically described reactants and/or reaction conditions 
recited herein for those actually used in the preceeding example. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention, and without departing 
from its spirit and scope, can make various changes and modifications to 
adapt the invention to various usages and conditions.