High dielectric constant coatings

Disclosed is a method of forming high dielectric constant coatings on substrates. The method comprises applying a coating of hydrogen silsesquioxane resin and a high dielectric constant filler onto a substrate and heating the coated substrate at a temperature sufficient to convert the hydrogen silsesquioxane resin to a silica containing ceramic matrix containing the filler.

BACKGROUND OF THE INVENTION 
The present invention relates to high dielectric constant coatings formed 
from hydrogen silsesquioxane resin and fillers. These coatings are useful, 
for example, in the manufacture of capacitors. 
The use of hydrogen silsesquioxane derived ceramic coatings on substrates 
such as electronic devices is known in the art. For instance, Haluska et 
al. in U.S. Pat. No. 4,756,977 disclose a process for forming a silica 
coating on an electronic substrate wherein a solution of hydrogen 
silsesquioxane resin is applied to a substrate followed by heating the 
coated substrate in air at a temperature in the range of 
200.degree.-1000.degree. C. This reference, however, does not describe the 
use of high dielectric constant fillers within the coating. 
Similarly, the use of fillers within ceramic coatings is also known in the 
art. For instance, U.S. Pat. No. 3,986,997 describes a composition 
comprising an acidic dispersion of colloidal silica and hydroxylated 
silsesquioxane in an alcohol-water medium which can be used to apply 
transparent abrasion resistant coatings on a variety of substrates. The 
reference, however, does not describe the use of hydrogen silsesquioxane 
nor the use of high dielectric constant fillers on electronic substrates. 
The present inventors have now discovered that high dielectric constant 
coatings can be formed from compositions comprising hydrogen 
silsesquioxane resin and fillers. 
SUMMARY OF THE INVENTION 
The present invention relates to a method of forming a high dielectric 
constant coating on a substrate and the substrate coated thereby. The 
method comprises first applying a composition comprising hydrogen 
silsesquioxane resin and a high dielectric constant filler onto the 
substrate. The coated substrate is then heated at a temperature sufficient 
to convert the composition to a ceramic coating having a high dielectric 
constant. 
The present invention also relates to a coating composition comprising 
hydrogen silsesquioxane resin and a high dielectric constant filler 
diluted in a solvent. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention is based on the discovery that coatings having a high 
dielectric constant can be formed from compositions comprising hydrogen 
silsesquioxane resin and fillers. These coatings have particular utility 
in the manufacture of electronic devices such as capacitors, but the 
choice of substrates is limited only by the need for thermal and chemical 
stability of the substrate under the conditions used. 
As used in the present invention, the expression "ceramic coating" is used 
to describe the hard coating obtained after heating the hydrogen 
silsesquioxane-filler composition. This coating contains both amorphous 
silica (SiO.sub.2) materials as well as amorphous silica-like materials 
that are not fully free of residual carbon, silanol (Si--OH) and/or 
hydrogen (which are obtained upon heating the hydrogen silsesquioxane) and 
the filler materials. The expression "filler" is used to describe a finely 
divided solid phase which is distributed within the resin and the final 
ceramic coating. The expression "electronic substrate" is meant to 
include, but is not limited to, electronic devices or electronic circuits 
such as silicon based devices, gallium arsenide based devices, focal plane 
arrays, opto-electronic devices, photovoltaic cells and optical devices. 
The expression "high dielectric constant coating" is used to describe 
coatings having a DK at least about 8. 
In the process of the present invention a high dielectric constant ceramic 
coating is formed on a substrate by a process which comprises applying a 
coating composition comprising hydrogen silsesquioxane resin and a filler 
onto the substrate and then heating the coated substrate at a temperature 
sufficient to convert the composition to a silica-containing ceramic. 
The hydrogen silsesquioxane resins (H-resin) which may be used in this 
invention include hydridosiloxane resins of the formula HSi(OH).sub.x 
(OR).sub.y O.sub.z/2, in which each R is independently an organic group or 
a substituted organic group which, when bonded to silicon through the 
oxygen atom, forms a hydrolyzable substituent, x=0-2, y=0-2, z=1-3, 
x+y+z=3. Examples of R include alkyls such as methyl, ethyl, propyl, 
butyl, etc. , aryls such as phenyl, and alkenyls such as allyl or vinyl. 
As such, these resins may be fully condensed (HSiO.sub.3/2).sub.n or they 
may be only partially hydrolyzed (i.e., containing some Si--OR) and/or 
partially condensed (i.e., containing some Si--OH). Although not 
represented by this structure, these resins may contain a small number 
(e.g., less than about 10%) of silicon atoms which have either 0 or 2 
hydrogen atoms attached thereto due to various factors involved in their 
formation or handling. 
The above H-resins and methods for their production are known in the art. 
For example, Collins et al. in U.S. Pat. No. 3,615,272, which is 
incorporated herein by reference, teach the production of a nearly fully 
condensed H-resin (which may contain up to 100-300 ppm silanol) by a 
process comprising hydrolyzing trichlorosilane in a benzenesulfonic acid 
hydrate hydrolysis medium and then washing the resultant resin with water 
or aqueous sulfuric acid. Similarly, Bank et al. in U.S. Pat. No. 
5,010,159, which is hereby incorporated by reference, teach an alternative 
method comprising hydrolyzing hydridosilanes in an arylsulfonic acid 
hydrate hydrolysis medium to form a resin which is then contacted with a 
neutralizing agent. 
Other hydridosiloxane resins, such as those described by Frye et al. in 
U.S. Pat. No. 4,999,397, hereby incorporated by reference, those produced 
by hydrolyzing an alkoxy or acyloxy silane in an acidic, alcoholic 
hydrolysis medium, those described in Kokai Patent Nos. 59-178749, 
60-86017 and 63-107122, or any other equivalent hydridosiloxane, will also 
function herein. 
It is to be noted that in a preferred embodiment of the invention, specific 
molecular weight fractions of the above H-resins may also be used in this 
process. Such fraction and methods for their preparation are taught by 
Hanneman et al. in U.S. Pat. No. 5,063,267 which is hereby incorporated by 
reference. A preferred fraction comprises material wherein at least 75% of 
the polymeric species have a molecular weight above about 1200 and a more 
preferred fraction comprises material wherein at least 75% of the 
polymeric species have a molecular weight between about 1200 and about 
100,000. 
The coating composition may also contain other ceramic oxide precursors. 
Examples of such ceramic oxide precursors include compounds of various 
metals such as aluminum, titanium, zirconium, tantalum, niobium and/or 
vanadium as well as various non-metallic compounds such as those of boron 
or phosphorous which may be dissolved in solution, hydrolyzed, and 
subsequently pyrolyzed, at relatively low temperatures and relatively 
rapid reaction rates to form ceramic oxide coatings. 
The above ceramic oxide precursor compounds generally have one or more 
hydrolyzable groups bonded to the above metal or non-metal, depending on 
the valence of the metal. The number of hydrolyzable groups to be included 
in these compounds is not critical as long as the compound is soluble in 
the solvent. Likewise, selection of the exact hydrolyzable substituent is 
not critical since the substituents are either hydrolyzed or pyrolyzed out 
of the system. Typical hydrolyzable groups include, but are not limited 
to, alkoxy, such as methoxy, propoxy, butoxy and hexoxy, acyloxy, such as 
acetoxy, or other organic groups bonded to said metal or non-metal through 
an oxygen such as acetylacetonate. Specific compounds, therefore, include 
zirconium tetracetylacetonate, titanium dibutoxy diacetylacetonate, 
aluminum triacetylacetonate and tetraisobutoxy titanium. 
When hydrogen silsesquioxane resin is to be combined with one of the above 
ceramic oxide precursors, generally it is used in an amount such that the 
final ceramic coating contains 0.1 to about 30 percent by weight modifying 
ceramic oxide. 
The coating composition may also contain a platinum, rhodium or copper 
catalyst to increase the rate and extent of conversion to silica. 
Generally, ally platinum, rhodium or copper compound or complex which can 
be solubilized will be functional. For instance, a composition such as 
platinum acetylacetonate, rhodium catalyst RhCl.sub.3 [S(CH.sub.2 CH.sub.2 
CH.sub.2 CH.sub.3 ).sub.2 ].sub.3, obtained from Dow Corning Corporation, 
Midland, Mich., or cupric naphthenate are all within the scope of this 
invention. These catalysts are generally added in an amount of between 
about 5 to 1000 ppm platinum, rhodium or copper based on the weight of 
hydrogen silsesquioxane resin. 
The fillers used herein are known ill the art for use in coatings with 
other polymers. These included various inorganic and organic fillers, 
especially inorganic fillers, in a variety of morphologies including, but 
not limited to powders, particles, flakes, microballoons and the like. 
Examples of inorganic fillers include synthetic and natural materials such 
as the oxides, nitrides, borides and carbides of various metals and 
non-metals such as glass, alumina, silica, titanium dioxide, zinc oxide, 
tungsten oxide, and ruthenium oxide, titanates such as potassium titanate 
and barium titanate, niobates such as lithium niobate and lead niobate, 
barium sulfate, calcium carbonate, precipitated diatomite, aluminum 
silicate or other silicates, pigments, phosphors, metals such as silver, 
aluminum and copper, wollostonite, mica, kaolin, clay, talc and the like. 
Also, some organic materials such as cellulose, polyamides, phenol resins 
and the like may be used. 
The preferred fillers to be used herein are those with a high dielectric 
constant (e.g., at least about 8) and, thus, produce a high dielectric 
constant coating. These include titanate, niobate or tungstate salts of 
metals such as strontium, zirconium, barium, lead, lanthanium, iron, zinc, 
and magnesium. Specific preferred compounds include barium titanate, lead 
niobate, lead titanate, strontium titanate, barium strontium titanate, 
lead lanthanium zirconium titanate, lead zirconium titanate and lead 
tungstate. In addition, the high dielectric constant fillers can include 
oxides of various metals such as tantalum oxide, titanium oxide and 
niobium oxide. 
The particle size and shape of the above fillers can vary over a wide range 
depending on factors such as the type of filler, the desired coating 
thickness, etc. 
The amount of filler used in the present invention can also be varied over 
a wide range depending, for example, on the quality and electrical 
characteristics desired in the final coating. Generally, however, the 
fillers are used in an amount less than about 90 weight percent of the 
coating to insure that enough resin is present to bind the filler. 
Obviously, smaller amounts of fillers (e.g., 1-5 wt %) can also be used. A 
preferred amount of filler is in the range of about 5 to about 80 wt. 
percent of the coating. 
If desired, other materials may also be present in the coating composition. 
For instance, it is within the scope of the present invention to use a 
material which modifies the surface of the filler for better adhesion. 
Such materials can include, for example, silanes such as 
glycidoxypropyltrimethoxysilane, mercaptopropyltrimethoxysilane, and 
vinyltriacetoxysilane. Similarly, it is within the scope of the invention 
to include suspending agents in the coating composition. These and other 
optional components are known to those skilled in the art. 
According to the process of the invention, the hydrogen silsesquioxane 
resin, filler and any optional components are applied to the surface of a 
substrate. Such substrates are well known to those skilled in the art 
(e.g., capacitor art). 
The coating according to the present invention can be applied in any 
manner, but a preferred method involves dissolving the hydrogen 
silsesquioxane resin in a solvent and dispersing the filler and any 
optional components therein. This dispersion is then applied to the 
surface of the substrate. Various facilitating measures such as stirring 
and/or heating may be used to dissolve or disperse the hydrogen 
silsesquioxane resin and filler and create a more uniform application 
material. Solvents which may be used include any agent or mixture of 
agents which will dissolve or disperse the hydrogen silsesquioxane resin 
and filler to form a homogenous liquid mixture without affecting the 
resultant coating. These solvents can include, for example, alcohols such 
as ethyl or isopropyl, aromatic hydrocarbons such as benzene or toluene, 
alkanes such as n-heptane or dodecane, ketones, esters, glycol ethers, or 
cyclic dimethylpolysiloxanes, in an amount sufficient to dissolve/disperse 
the above materials to the concentration desired for application. 
Generally, enough of the above solvent is used to form a 0. 1-80 weight 
percent mixture, preferably 1-50 wt. percent. 
If a liquid method is used, the liquid mixture comprising the hydrogen 
silsesquioxane resin, filler, solvent, and, any optional components is 
then coated onto the substrate. The method of coating can be, but is not 
limited to, spin coating, dip coating, spray coating or flow coating. 
Other equivalent means, however, are also deemed to be within the scope of 
this invention. 
The solvent is then allowed to evaporate from the coated substrate 
resulting in the deposition of the hydrogen silsesquioxane resin and 
filler coating. Any suitable means of evaporation may be used such as 
simple air drying by exposure to an ambient environment, by the 
application of a vacuum or mild heat (e.g., less than 50.degree. C.) or 
during the early stages of the heat treatment. It is to be noted that when 
spin coating is used, the additional drying period is minimized as the 
spinning drives off the solvent. 
Although the above described methods primarily focus on using a liquid 
approach, one skilled in the art would recognize that other equivalent 
means (e.g., melt coating) would also function herein and are contemplated 
to be within the scope of this invention. 
The hydrogen silsesquioxane resin and filler coating is then typically 
converted to a silica-containing ceramic matrix having the filler 
disbursed therein by heating it to a sufficient temperature. Generally, 
the temperature is in the range of about 50.degree. to about 1000.degree. 
C. depending on the pyrolysis atmosphere. Preferred temperatures are in 
the range of about 50.degree. to about 800.degree. C. and more preferably 
50.degree.-425.degree. C. Heating is generally conducted for a time 
sufficient to ceramify, generally up to about 6 hours, with less than 
about 3 hours being preferred. 
The above heating may be conducted at any effective atmospheric pressure 
from vacuum to superatmospheric and under any effective oxidizing or 
non-oxidizing gaseous environment such as those comprising air, O.sub.2, 
an inert gas (N.sub.2, etc.), ammonia, amines, moisture, N.sub.2 O etc. 
Any method of heating such as the use of a convection oven, rapid thermal 
processing, hot plate, or radiant or microwave energy is generally 
functional herein. The rate of heating, moreover, is also not critical, 
but it is most practical and preferred to heat as rapidly as possible. 
By the above methods a ceramic coating is produced on the substrate. The 
thickness of the coating can vary over a wide range (e.g., up to 500 
microns). These coatings smooth the irregular surfaces of various 
substrates, they are relatively defect free, and they have excellent 
adhesive properties. In addition the coatings have high DKs which are 
often above 10 and frequently above 15. As such, they are particularly 
useful for a variety of electronic applications such as in the manufacture 
of capacitors. 
Additional coatings may be applied over these coatings if desired. These 
can include, for example, SiO.sub.2 coatings, SiO.sub.2 /ceramic oxide 
layers, silicon containing coatings, silicon carbon containing coatings, 
silicon nitrogen containing coatings, silicon oxygen nitrogen coatings, 
silicon nitrogen carbon containing coatings and/or diamond like carbon 
coatings. Methods for the application of such coatings are known in the 
art and many are described in U.S. Pat. No.4,756,977, which is 
incorporated herein by reference. An especially preferred coating is 
silicon carbide applied by the chemical vapor deposition of an 
organosilicon precursor. One example of such a process is described in 
U.S. Pat. No. 5,011,706 which is incorporated herein by reference. A 
second example involves the chemical vapor deposition utilizing 
trimethylsilane as the source gas. The most preferred coating comprises 
silicon carbide deposited in a non-uniform thickness such that uniform 
etching is difficult.

The following non-limiting examples are included so that one skilled in the 
art may more readily understand the invention. 
EXAMPLE 1 
Eccospheres.TM. DCT-28-27 (silica glass microballoons with a particle size 
range of 10-40 microns), 0.2 g, were ground in a mortar and pestle for 
about 20 minutes to decrease the particle size. A coating composition was 
then formed by mixing the ground glass, 0.7 g of Hydrogen silsesquioxane 
resin made by the method of Collins et al. in U.S. Pat. No. 3,615,273, and 
0.012 g glycidoxypropyltrimethoxysilane and 0.36 g dodecane. The coating 
composition was applied to the surface of 2-3".times.5" aluminum panels; 
one using a 3 mil applicator to apply a 2 mil coating and the second using 
a 4 mil applicator to apply a 3 mil coating. The coatings were allowed to 
dry for 1 hour at 64.degree. C. The coated panels were then heated at 
185.degree. C. for 1 hour and at 410.degree. C. for 45 minutes. The 
dielectric constant and dissipation factor for the 3 mil coating are as 
follows: 
______________________________________ 
Frequency Dielectric Constant 
Dissipation Factor 
______________________________________ 
100 HZ 2.43 0.106 
1 KHZ 2.21 0.097 
10 KHZ 2.13 0.019 
100 KHZ 2.08 0.009 
______________________________________ 
The spheres had a dielectric constant =1.17 (1-8.6 GHZ) and dissipation 
factor =0.001 (1-8.6 GHZ). 
EXAMPLE 2 
A coating composition was formed by mixing 0.12 g Eccospheres SDT-28-27 
(not crushed), 0.12 g 5 micron Minusil (silica), 0.7 g of Hydrogen 
silsesquioxane resin made by tile method of Collins et al. in U.S. Pat. 
No. 3,615,273, 0.012 g glycidoxypropyltrimethoxysilane and 0.456 g 
dodecane. The coating composition was applied to the surface of various 
substrates and processed as set out in the following table: 
______________________________________ 
Film Ap- Air Thickness 
Substrate plicator Dry 185.degree. C. 
410.degree. C. 
(micron) 
______________________________________ 
3" .times. 6" Al Panel 
3 mil 3 hr 1.75 hr 
1.25 hr 
20 
3" .times. 6" Al Panel 
4 mil 3 hr 1.75 hr 
1.25 hr 
41 
3" .times. 6" Al Panel 
5 mil 3 hr 1.75 hr 
1.25 hr 
48 
3" Si Wafer 
3 mil 3 hr 0.75 hr 
1.00 hr 
24 
2" sq 10,000 A 
3 mil 3 hr 0.75 hr 
1.00 hr 
22 
gold on nichrome 
2" sq wafer - 
6 mil 3 hr 0.75 hr 
1.00 hr 
42 
Al coated 
Al interdigitated 
6 mil 3 hr 0.75 hr 
1.00 hr 
22 
pattern on 2" Si 
wafer 
______________________________________ 
The aluminum interdigitated pattern on the silicon wafer coated above was 
tested for porosity, pinhole density, and barrier layer ability by MIL STD 
883C, Method 2021-3, Procedure B, for determining glassivation layer 
integrity with a phosphoric acid etch solution. The Etch solution consists 
of 16 parts conc. phosphoric acid, 2 parts deionized water, 1 part 
concentrated nitric acid and 1 part glacial acetic acid. The sample was 
tested by applying 1 drop of the etch solution on the surface of the 
coating for a period of 50 minutes (30 minutes=normal exposure time). The 
sample was next rinsed with distilled water and allowed to dry. 
Examination of the test area on the coating showed the film was intact and 
there was no evidence of any corrosion. The dielectric constant and 
dissipation factor for the 3".times.6" aluminum panel coated above with 
the 41 micron thick coating are as follows: 
______________________________________ 
Frequency Dielectric Constant 
Dissipation Factor 
______________________________________ 
100 HZ 2.42 0.032 
1 KHZ 2.35 0.014 
10 KHZ 2.32 0.006 
100 KHZ 2.31 0.004 
______________________________________ 
EXAMPLE 3 
A coating composition was formed by mixing 0. 12 g Eccospheres SDT-28-27 
(not crushed) , 0. 12 g plasma alumina, average particle size =6 microns 
(Product No. 13,699, ZYP Coatings, Inc. ), 0.7 g of Hydrogen 
silsesquioxane resin made by the method of Collins et al. in U.S. Pat. No. 
3,615,273, 0. 012 g glycidoxypropyltrimethoxysilane and 0.456 g dodecane. 
The coating composition was applied to the surface of a 3".times.5" 
aluminum panel using a 3 mil applicator. The coatings were allowed to dry 
for 3 hours at 64.degree. C. The coated panels were then heated at 
185.degree. C. for 1 hour and at 400.degree. C. for 1 hour. A 31 micron 
thick coating was obtained. The dielectric constant and dissipation factor 
for the coating are as follows: 
______________________________________ 
Frequency Dielectric Constant 
Dissipation Factor 
______________________________________ 
100 HZ 2.04 0.058 
1 KHZ 1.94 0.030 
10 KHZ 1.87 0.020 
100 KHZ 1.82 0.015 
______________________________________ 
EXAMPLE 4 
A coating composition was formed by mixing 0.15 g plasma alumina, average 
particle size=6 microns (Product No. 13,699, ZYP Coatings, Inc.), 0.7 g of 
Hydrogen silsesquioxane resin made by the method of Collins et al. in U.S. 
Pat. No. 3,615,273, 0.024 g glycidoxypropyltrimethoxysilane and 0.271 g 
dodecane. The coating composition was applied to the surface of a 
3".times.5" aluminum panel using a 2 mil applicator. The coatings were 
allowed to dry for 4 hours at 64.degree. C. The coated panels were then 
heated at 185.degree. C. for 1 hour and at 400.degree. C. for 1 hour. A 36 
micron thick coating was obtained. The dielectric constant and dissipation 
factor for the coating are as follows: 
______________________________________ 
Frequency Dielectric Constant 
Dissipation Factor 
______________________________________ 
100 HZ 2.23 0.038 
1 KHZ 2.15 0.017 
10 KHZ 2.12 0.008 
100 KHZ 2.10 0.005 
______________________________________ 
EXAMPLE 5 
Nalco 84SS-258 (30 % colloidal silica with a particle size of 20 nanometer 
diluted in a glycol propyl ether), 1.08 g, 0.6 g of Hydrogen 
silsesquioxane resin made by the method of Collins et al. in U.S. Pat. No. 
3,615,273, and 1.08 g 2-(2-butoxyethoxy)ethyl acetate were mixed in 1.11 g 
methyl ethyl ketone. The coating composition was applied to the surface of 
10 Motorola 14011B CMOS devices and spun at 3000 RPM for 10 seconds. A 1 
inch square silicon wafer was also coated in the same manner. The coated 
materials were then heated at 400.degree. C. for 2.5 hours in air. The 
CMOS devices were all functional after pyrolysis. Salt atmosphere tests 
per MIL-STD-883C, method 1009 showed that 7 out of 10 passed 2 hours and 3 
of 7 passed 4 hours in the test. A similar coating of silica derived from 
H-resin failed after 10 minutes in the test. 
EXAMPLE 6 
Nalco 84SS-258 (30 % colloidal silica with a particle size of 20 nanometer 
diluted in a glycol propyl ether), 2.163 g, 2.0 g of triethoxysilane, 
0.164 g water, 9.49 g isopropyl alcohol and 3.0 g n-butanol were mixed and 
heated at 60.degree.-75.degree. C. for 30 minutes. The coating composition 
was applied to the surface of 1.times.3 inch aluminum panel. The coated 
panel was air dried for 10 minutes and pyrolyzed at 400.degree. C. for 1 
hour in air. A Motorola 14011B CMOS device and a 1 inch square silicon 
wafer were also spin coated with the above liquid mixture at 3000 RPM for 
15 seconds. The coated parts were then heated at 400.degree. C. for 4 
hours in air. FTIR showed essentially complete conversion to silica. The 
CMOS device was functional after pyrolysis and survived 4 hrs of the salt 
atmosphere tests of Example 5 (failed at 24 hrs). 
EXAMPLE 7 
Barium titanate, 3.0 g, 1.0 g of Hydrogen silsesquioxane resin made by the 
method of Collins et al. in U.S. Pat. No. 3,615,273, and 0.64 g 
glycidoxypropyltrimethoxysilane were mixed in 0.7 g xylene with a 
biohomogenizer for 4 minutes. The coating composition was applied to the 
surface of a 3".times.6" aluminum panel with a 3 mil drawdown bar. The 
coated panel was then air dried for 2 hours and heated at 400.degree. C. 
for 1 hours in air in a 12 inch Lindberg Box Furnace. The coating was 
37.+-.2 microns thick. Electrical data on the coating was as follows: 
______________________________________ 
Frequency D.K. D.F. 
______________________________________ 
10.sup.2 18.5 0.033 
10.sup.3 18.1 0.011 
10.sup.4 17.9 0.008 
10.sup.5 17.7 0.007 
10.sup.6 17.5 0.007 
______________________________________ 
EXAMPLE 8 
Tantalum oxide (Ta.sub.2 O.sub.5), 4.0 g, 1.0 g of Hydrogen silsesquioxane 
resin made by the method of Collins et al. in U.S. Pat. No. 3,615,273, and 
0.3 g glycidoxypropyltrimethoxysilane were mixed in 1 g cyclic 
polydimethylsiloxanes with a biohomogenizer for 2.5 minutes. The coating 
composition was applied to the surface of a 3".times.6" aluminum panel 
with a 3 mil drawdown bar. The coated panel was then air dried for 4 hours 
and heated at 500.degree. C. for 1 hour in air in a 12 inch Lindberg Box 
Furnace. The coating was 32 microns thick. Upon examination at 1000X, the 
coating was found to have no cracks.