Method of forming a composite, article and composition

The present invention relates to a method of forming composite coatings on electronic substrates and the substrates coated thereby. The method comprises applying a coating comprising hydrogen silsesquioxane resin and a refractory fiber on an electronic substrate and heating the coated substrate at a temperature sufficient to convert the hydrogen silsesquioxane resin into a ceramic.

BACKGROUND OF THE INVENTION 
The present invention relates to composite coatings formed from hydrogen 
silsesquioxane resin and refractory fibers. These coatings are useful for 
forming opaque dielectric layers on electronic substrates. 
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 fibers within the coating. 
Similarly, ceramic composite coatings comprising ceramic fibers within 
ceramic matrices are also known in the art. The art, however, does not 
describe the use of hydrogen silsesquioxane as the matrix nor the 
application of such coatings on electronic substrates. 
The present inventors have now discovered that ceramic composite coatings 
containing refractory fibers can be formed on electronic devices for 
protection. 
SUMMARY OF THE INVENTION 
The present invention relates to a method of forming a coating on an 
electronic device and the device coated thereby. The method comprises 
first applying a composition comprising hydrogen silsesquioxane resin and 
refractory fibers onto the substrate. The coated substrate is then heated 
at a temperature sufficient to convert the hydrogen silsesquioxane into a 
ceramic. 
The present invention also relates to a coating composition comprising 
hydrogen silsesquioxane resin and refractory fibers diluted in a solvent. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention is based on the discovery that ceramic composite 
coatings can be formed from compositions comprising hydrogen 
silsesquioxane resin and refractory fibers. These coatings are useful on 
electronic devices because they provide excellent mechanical protection 
and dielectric protection. Additionally, the coatings are opaque such that 
visualization of the underlying device is impaired. The use of hydrogen 
silsesquioxane resin in this process is advantageous in that the coatings 
can be formed with low conversion temperatures and the coatings do not 
crack under stress since the hydrogen silsesquioxane resin does not 
undergo significant shrinkage. 
As used in the present invention, the expressions "silica containing 
matrix" or "ceramic matrix" are used to describe the hard coating obtained 
after heating the hydrogen silsesquioxane resin. This coating contains 
both amorphous silica (SiO.sub.2) materials as well as amorphous 
silica-like materials that may be not fully free of residual carbon (eg., 
Si--C or Si--OC), silanol (Si--OH) and/or hydrogen (which are obtained 
upon heating the silica precursor resin) and the refractory fibers. 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. 
In the process of the present invention a ceramic coating is formed on a 
substrate by a process which comprises applying a coating composition 
comprising hydrogen silsesquioxane resin and refractory fibers onto the 
substrate and then heating the coated substrate at a temperature 
sufficient to convert the composition to a 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 
(eg., 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. 
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, any 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 refractory fibers used herein are known in the art and can comprise any 
fibers which are compatible with the hydrogen silsesquioxane and which can 
withstand the heating. Many of these fibers are commercially available. 
Examples of suitable fibers include those of silicon carbide, silicon 
nitride, silicon carbide deposited on a carbon core, aluminum borate, 
aluminum oxide, silicon oxide, silicon carbide containing titanium, 
silicon oxycarbides, silicon oxycarbonitrides, carbon, graphite, aramide, 
organics and the like. These fibers may contain any desirable number of 
filaments per tow and have a size in the range of less than about 1 
micrometers (eg., 0.1 micrometers) to about 500 micrometers. 
Examples of specific fibers include silicon carbide fibers with a diameter 
in the range of 10-20 micrometers manufactured by Nippon Carbon and sold 
under the trade name "Nicalon"; fibers comprising silicon carbide 
deposited on a carbon core with a diameter of about 143 micrometers 
manufactured by Avco and designated "SCS-6"; alumina-boria-silica fibers 
with a diameter of about 10-12 micrometers manufactured by 3M and sold 
under the tradenames "Nextel 312", "Nextel 440" and "Nextel 480"; Al.sub.2 
O.sub.3 fibers with a diameter of about 20 micrometers manufactured by Du 
Pont under the designation "FP"; SiO.sub.2 fibers with a diameter of about 
8-10 micrometers manufactured by J. P. Stevens; Al.sub.2 O.sub.3 
--SiO.sub.2 fibers with a diameter in the range of about 9-17 micrometers 
manufactured by Sumitomo; silicon carbide fibers containing titanium with 
a diameter in the range of 8-10 micrometers manufactured by Ube and sold 
under the tradename "Tyranno"; silicon carbide fiber with a diameter in 
the range of about 6-10 micrometers manufactured by Avco; silicon 
oxycarbonitride fibers with a diameter in the range of about 10-15 
micrometers manufactured by Dow Corning designated "MPDZ" and "HPZ"; 
silicon carbide fibers with a diameter in the range of about 10-15 
micrometers manufactured by Dow Corning designated "MPS"; silicon nitride 
fibers such as those produced by Tonen or Rhone Poulanc, Al.sub.2 O.sub.3 
--ZrO.sub.2 fibers with a diameter of about 20 micrometers manufactured by 
Du Pont and Designated " PRD-166", carbon fibers such as those sold by 
Hitco and aramide fibers sold under the tradename "KEVLAR" by DuPont. 
The refractory fibers used herein are chopped into short lengths for ease 
in coating. Any fiber length which can be manipulated into the desired 
coating can be used herein. Generally, the lengths are less than 1 
centimeter with lengths in the range of between about 10 micrometers to 10 
millimeters being preferred. 
The amount of refractory fibers used in the present invention can also be 
varied over a wide range depending, for example, on the characteristics 
desired in the final coating. Generally, however, the refractory fibers 
are used in an amount less than about 90 volume percent to insure that 
enough resin is present to bind the refractory fibers. Obviously, smaller 
amounts of fibers (eg., 1-5 vol. %) can also be used. Preferred are fiber 
volumes in the range of between about 25 and 80%. 
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 fiber for better adhesion or 
for better release. 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 H-resin, refractory fibers 
and any optional components are applied to the surface of the electronic 
substrate. The surface of the electronic substrate can be bare (i.e., no 
passivation) or the circuit can have a passivation. Such passivation can 
be, for example, ceramic coatings such as silica, silicon nitride, silicon 
carbide, silicon oxynitride, silicon oxycarbide, etc. deposited by, for 
example, CVD, PVD, or sol-gel approaches. Such passivation is known to 
those skilled in the art. Likewise, the circuit can be pre or post 
interconnection. 
The coating composition can be accomplished in any manner, but a preferred 
method involves dissolving the H-resin in a solvent and dispersing the 
fiber and any optional components therein. This dispersion is then applied 
to the surface of the electronic substrate. Various facilitating measures 
such as stirring and/or heating may be used to dissolve or disperse the 
H-resin and fiber and create a more uniform application material. Solvents 
which may be used include any agent or mixture of agents which will 
dissolve the hydrogen silsesquioxane resin and disperse the fiber to form 
a uniform liquid mixture without affecting the resultant coating. These 
solvents can include, for example, aromatic hydrocarbons such as benzene 
or toluene, alkanes such as n-heptane or dodecane, ketones, esters, 
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 H-resin, 
refractory fiber, 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 
refractory fiber 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 (eg., 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 such as rapid thermal processing would also function herein and are 
contemplated to be within the scope of this invention. 
The hydrogen silsesquioxane resin and refractory fiber coating is then 
typically converted to the ceramic 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.-500.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, Ar, etc.), ammonia, amines, moisture, N.sub.2 O, 
hydrogen, 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 (eg., up to 500 
microns) as described above. These coatings smooth the irregular surfaces 
of various substrates (i.e. planarizing), they are relatively defect free, 
they have excellent adhesive properties, they provide mechanical and 
electrical protection and they are opaque. Moreover, the fibers provide 
added strength and toughness to the coating. 
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, organic coatings, silicone 
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 silacyclobutane. This process is described in U.S. Pat. No. 
5,011,706 which is incorporated herein by reference. 
The following non-limiting example is included so that one skilled in the 
art may more readily understand the invention.

EXAMPLE 
Hydrogen silsesquioxane resin, 1 g, produced by the method of Collins et 
al. in U.S. Pat. No. 3,615,272, 1 g silicon carbide whiskers from Tateho 
Chemical Industries Co. Ltd. (0.5.times.15.2 micrometers), 0.4 g 
glycidoxypropyltrimethoxysilane and 3.0 g cyclic polydimethylsiloxanes 
were mixed with a sonic probe to form a coating solution. An 11.25 sq. cm 
alumina panel was coated with the solution by using a 75 micrometer 
drawdown bar. The coated panel was air dried for 3 hours and pyrolyzed for 
2 hours at 400.degree. C. in air. The pyrolyzed coating was examined with 
a microscope and found to have no cracks at 1000.times. magnification. The 
coating thickness was 26 micrometers.