Thick opaque ceramic coatings

Thick opaque ceramic coatings are used to protect delicate microelectronic devices against excited energy sources, radiation, light, abrasion, and wet etching techniques. The thick opaque ceramic coating are prepared from a mixture containing phosphoric anhydride, i.e., phosphorous pentoxide (P.sub.2 O.sub.5), and a pre-ceramic silicon-containing material. It is preferred to also include tungsten carbide (WC) and tungsten metal (W) in the coating mixture. The coating is pyrolyzed to form a ceramic SiO.sub.2 containing coating. A second coating of plasma enhanced chemical vapor deposited (PECVD) silicon carbide (SiC), diamond, or silicon nitride (Si.sub.3 N.sub.4), can be applied over the thick opaque ceramic coating to provide hermeticity. These coatings are useful on patterned wafers, electronic devices, and electronic substrates. The thick opaque ceramic coating is unique because it is resistant to etching using wet chemicals, i.e., acids such as H.sub.3 PO.sub.4 and H.sub.2 SO.sub.4, or bases.

BACKGROUND OF THE INVENTION 
This invention is directed to a method of forming coatings using 
compositions preferably containing a methyl silsesquioxane resin, 
colloidal silica, and certain other opaque materials and/or obstructing 
agents. These protective coatings are useful on a variety of substrates, 
but especially electronic substrates. 
The use of hydrogen silsesquioxane resin (HSiO.sub.3/2).sub.n derived 
ceramic coatings on substrates such as electronic devices is known. Thus, 
U.S. Pat. No. 4,756,977 (Jul. 12, 1988), discloses a process of forming a 
silica coating on an electronic substrate. According to the process, 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.-1,000.degree. C. The '977 patent, however, does not 
teach the use of opaque materials or obstructing agents within the 
coating, nor is the (HSiO.sub.3/2).sub.n resin derived ceramic coatings on 
the substrate taught to be resistant to wet etching techniques. 
The use of silica within a protective coating is known. Thus, U.S. Pat. No. 
3,986,997 (Oct. 19, 1976), describes a coating composition containing an 
acidic dispersion of colloidal silica, and partial condensate of 
methylsilanetriol in an alcohol-water medium. The coating composition can 
be used to apply transparent abrasion resistant coatings on a variety of 
substrates. The '997 patent, however, does not describe the use of 
tungsten carbide, tungsten metal, or phosphoric anhydride, as an opaque 
material or obstructing agent; it does not describe applying the coating 
on an electronic substrate for the purpose of providing resistance to wet 
etching; nor does it describe pyrolysis of the coating at the high 
temperatures (i.e., 400.degree.to 1,000.degree. C.) as contemplated 
herein. 
Accordingly, what we have discovered is that useful coatings can be formed 
from compositions containing phosphoric anhydride, a pre-ceramic 
silicon-containing material, and certain other additional opaque materials 
or obstructing agents; and that these coatings provide protection and 
resistance against various intrusion techniques, especially wet etching. 
BRIEF SUMMARY OF THE INVENTION 
Our invention relates to a method of forming a coating on a substrate and 
to substrates coated thereby. The method comprises applying a coating 
composition containing an opaque material or obstructing agent and a 
pre-ceramic silicon-containing material on the substrate. The coated 
substrate is then heated at a temperature sufficient to convert (pyrolyze) 
the coating composition to a ceramic coating. The method is especially 
valuable for forming protective coatings on a variety of electronic 
devices. 
Our invention also relates to the coating composition containing an opaque 
material or obstructing agent and a pre-ceramic silicon-containing 
material. 
While methods and compositions described herein are preferably prepared 
using a resin derived from an aqueous alkanol dispersion of colloidal 
silica and partial condensate of methylsilanetriol, other preceramic 
silicon-containing materials can be employed, such as: 
(i) a hydrogen silsesquioxane resin, 
(ii) a hydrolyzed or partially hydrolyzed compound of the formula R.sub.n 
Si(OR).sub.4-n in which R is independently an aliphatic, alicyclic, or 
aromatic substituent of 1-20 carbon atoms, and n is 0-3, 
(iii) a hydridopolysilazane resin, 
(iv) a methyl polydisilylazane resin, or 
(v) a boron modified hydropolysilazane. 
These and other features and objects of our invention will become apparent 
from a consideration of the detailed description. 
DETAILED DESCRIPTION 
Our invention is concerned with preparation of a thick opaque protective 
ceramic coating that is effective against various forms of energy, and 
which contains a solid reactive material that is a deterrent for 
preventing examination of an integrated circuit. The coating is effective 
against ozone, UV-ozone, gaseous free radicals and ions, any vapor or 
liquid borne reactive species, or plasmas. In addition, the coating is 
resistant to abrasion and is opaque to radiation and light. 
The coating preferably contains a combination of colloidal silica and 
partial condensate of methylsilanetriol, which is more resistant to being 
wet chemical etched with either an acid or a base, because it presents a 
surface that is more hydrophobic than a hydrogen silsesquioxane resin. 
This enhances the tamper-proof qualities of the coating. The coating also 
contains solid P.sub.2 O.sub.5 which obstructs and acts as a deterrent to 
any attack by wet etching techniques. It functions in this capacity by 
generating phosphoric acid that would readily destroy the integrated 
circuit and its memory. 
In the most preferred embodiment, the coating is prepared from a 
combination of colloidal silica and partial condensate of 
methylsilanetriol, tungsten carbide (WC), tungsten metal (W) and solid 
P.sub.2 O.sub.5. These components of the coating composition provide 
protection for any microelectronic device, and are useful for application 
on gallium arsenide (GaAs), silicon, metallic, or other electronic 
substrates, devices, or patterned wafers. If desired, a second hermetic 
coating, such as PECVD silicon carbide, diamond, or silicon nitride, can 
be applied to provide hermeticity. 
As used herein, the expression "ceramic coating" is intended to describe a 
hard coating obtained after heating or pyrolyzing the coating composition. 
The ceramic coating may contain both amorphous silica (SiO.sub.2) 
materials, as well as amorphous silica-like materials that are not fully 
free of residual carbon and silanol (.tbd.SiOH), which are obtained upon 
heating the coating composition. 
The expression "electronic device" is meant to include electronic 
substrates or electronic circuits such as silicon-based devices, gallium 
arsenide based devices, focal plane arrays, opto-electronic devices, 
photovoltaic cells, and optical devices. 
In our process, a ceramic coating is formed on a substrate by applying a 
coating composition base containing a pre-ceramic silicon-containing 
material onto the substrate, and then heating and pyrolyzing the coated 
substrate at a temperature sufficient to convert the pre-ceramic 
silicon-containing material to a ceramic SiO.sub.2 containing coating. 
One suitable coating composition base and method for its preparation is 
described in U.S. Pat. No. 3,986,997 (Oct. 19, 1976), incorporated herein 
by reference. This coating composition is a dispersion containing 
colloidal silica and a partial condensate of methylsilanetriol. 
The silica component of the coating composition is present as an aqueous 
colloidal silica dispersion having a particle diameter of 5-150 
millimicrons (nanometers). The partial condensate component of the coating 
composition is predominately derived from methylsilanetriol, but may 
contain other silanols of the formula RSi(OH).sub.3 in which R is an alkyl 
radical of 2 or 3 carbon atoms, the vinyl radical CH.sub.2 .dbd.CH--, the 
3,3, 3-trifluoropropyl radical CF.sub.3 CH.sub.2 CH.sub.2 --, the 
3-glycidoxypropyl radical, or the 3-methacryloxypropyl radical CH.sub.2 
.dbd.C(CH.sub.3)COOCH.sub.2 CH.sub.2 CH.sub.2 --. At least 70 weight 
percent of the silanol is CH.sub.3 Si(OH).sub.3. 
Preferably, the coating composition contains 10-50 weight percent solids, 
with the distribution being 10-70 weight percent colloidal silica and 
30-90 weight percent of the partial condensate of the silanol. The partial 
condensate of the silanol is carried in a lower aliphatic alcohol-water 
solvent, i.e., methanol, ethanol, isopropanol, n-butyl alcohol, or 
mixtures thereof. 
The coating composition is prepared by adding a trialkoxysilane 
RSi(OCH.sub.3).sub.3 to a colloidal silica hydrosol, and adjusting the pH 
to 3-6 with an organic acid such as acetic acid. Trisilanols are generated 
in situ by adding the trialkoxysilane to the acidic aqueous dispersion of 
colloidal silica. Upon generation of the silanol in the acidic aqueous 
medium, there occurs a partial condensation of hydroxyl substituents to 
form .tbd.Si--O--Si.tbd. bonding. Alcohol is also generated by hydrolysis 
of the alkoxy substituents of the silane. Depending upon the percent 
solids desired, additional alcohol, water, or water-miscible solvent can 
be added, to adjust the level of solids in the coating composition. This 
coating composition is a clear or slightly hazy low viscosity fluid which 
is stable for several days, but condensation of .tbd.SiOH will continue at 
a very slow rate, and the coating composition eventually forms a gel 
structure. 
If desired, the coating composition according to our invention may contain 
other ceramic oxide precursors, examples of which are compounds of various 
metals such as aluminum, titanium, zirconium, tantalum, niobium, or 
vanadium; and non-metallic compounds such as boron or phosphorous; any of 
which may be dissolved in solution, hydrolyzed, and subsequently pyrolyzed 
to form ceramic oxide coatings. 
These ceramic oxide precursor compounds generally have one or more 
hydrolyzable groups bonded to the metal or non-metal depending on the 
valence of the metal. The number of hydrolyzable groups included in such 
compounds is not critical provided the compound is soluble in the solvent. 
Likewise, selection of an exact hydrolyzable substituent is not critical 
since it will be either hydrolyzed or pyrolyzed out of the system. Typical 
hydrolyzable substituents include alkoxy groups such as methoxy, propoxy, 
butoxy and hexoxy; acyloxy groups such as acetoxy; and other organic 
groups bonded to the metal or non-metal through an oxygen atom such as 
acetylacetonate. Some specific compounds that can be used are zirconium 
tetracetylacetonate, titanium dibutoxy diacetylacetonate, aluminum 
triacetylacetonate, and tetraisobutoxy titanium. 
When the coating composition is combined with one of these ceramic oxide 
precursors, generally it is used in an amount such that the final ceramic 
coating contains 0.1-30 percent by weight of the modifying ceramic oxide 
precursor. 
Our coating composition may contain a platinum, rhodium, or copper 
catalyst, to increase the rate and extent of conversion of the precursors 
to silica. Generally, any platinum, rhodium, or copper compound or complex 
which can be solubilized is appropriate, 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 a product of Dow Corning Corporation, Midland, Mich. or cupric 
naphthenate. These catalysts can be added in amounts of 5 to 1,000 parts 
per million of platinum, rhodium, or copper, based on the weight of the 
methyl silsesquioxane resin. 
Our coating composition may optionally contain other types of opaque 
materials or obstructing agents, in addition to P.sub.2 O.sub.5, if 
desired. For purposes herein, the terms opaque material or obstructing 
agent are used synonymously to describe a finely divided solid phase, 
which is distributed within the resin and the final ceramic coating, and 
which tends to (i) obstruct inspection, (ii) hinder inspection, or (iii) 
prevent reverse engineering of the device. 
Specific optional materials useful in the instant invention include, but 
are not limited to, optically opaque materials, radiopaque materials, 
luminescent materials, oxidizing materials, abrasion resistant materials, 
magnetic materials, and conductive materials. Various inorganic and 
organic types of materials can be used in a variety of morphologies, 
including but not limited to, powders, particles, flakes, and 
microballoons. 
Optically opaque materials are agents that when mixed with the preceramic 
silicon-containing material, render the resulting coating impenetrable to 
visual light. Optically opaque materials include but are not limited to, 
oxides, nitrides and carbides of silicon, alumina, metals, and inorganic 
pigments. Some preferred optional optically opaque materials are plasma 
alumina microballoons having an average particle size of about 6 microns, 
silica microballoons having an average particle size of about 5-40 
microns, silicon nitride (Si.sub.3 N.sub.4) powder or whiskers, tungsten 
metal (W), silicon carbide (SiC) powder or whiskers, aluminum nitride 
(AlN) powder, and black inorganic pigments such as black Ferro.RTM. F6331 
having an average particle size of about 0.4 microns. 
Radiopaque materials are agents that when mixed with the preceramic 
silicon-containing material, render the resulting coating impenetrable to 
radiation, such as x-rays, UV, IR, and visible light, as well as sound 
waves. Radiopaque materials include but are not limited to heavy metals 
such as lead, and insoluble salts of heavy metals such as barium, lead, 
silver, gold, cadmium, antimony, tin, palladium, strontium, and bismuth. 
The salts can include for example, carbonates, sulfates, and oxides. As 
noted previously, tungsten metal is the most preferred radiopaque 
material. 
Luminescent materials are agents that when mixed with the preceramic 
silicon-containing material, result in a coating that will absorb energy 
and emit electromagnetic radiation in excess of thermal radiation. These 
materials are typically phosphors such as sulfides (i.e., zinc sulfide and 
cadmium sulfide); selenides; sulfoselenides; oxysulfides; oxygen dominate 
phosphors such as borates, aluminates, gallates, and silicates; and halide 
phosphors such as alkali metal halides, alkaline earth halides, and 
oxyhalides. Preferred are sulfides, and most preferred is zinc sulfide. 
The phosphor compound may be doped with an activator. Activators include, 
but are not limited to manganese, silver, and copper; and rare earth ions 
in the form of halides. The activator is generally present in amounts of 
about 0.1-10 mol percent based on the weight of the phosphor. 
Abrasion resistant materials are agents that when mixed with the preceramic 
silicon-containing material, render the resulting coating impenetrable to 
removal by frictional means such as scraping. Abrasion resistant materials 
are exemplified by, but not limited to diamond, diamond dust, titanium 
nitride (TiN), tantalum carbide (TaC), tungsten carbide (WC), and fibers 
of graphite, KEVLAR.RTM. (a DuPont trademark for aromatic polyamide 
fibers), NEXTEL (a tradename of 3M Company, St. Paul, Minn. for 
boria-modified alumina-silica fibers), and aluminum oxide (Al.sub.2 
O.sub.3). As noted previously, tungsten carbide is the most preferred 
abrasion resistant material. 
Energy resistant materials are agents that when mixed with the preceramic 
silicon-containing material, react with energy sources such as ozone, 
UV-ozone, gaseous free radicals and ions, any vapor or liquid borne 
reactive species, and plasmas, to effectively cause degradation of the 
circuit and the memory. Energy resistant materials are exemplified by, but 
not limited to heavy metals such as lead, tungsten, tantalum, and 
antimony. 
Magnetic materials are agents that when mixed with the perceramic 
silicon-containing material, render the resulting coating magnetic (i.e., 
magnetized by a magnetic field having a net magnetic moment). Magnetic 
materials are exemplified by carbon alloy ferrites, iron carbonyl, and 
alloys of metals such as iron, manganese, cobalt, nickel, copper, 
titanium, vanadium, molybdenum, magnesium, aluminum, chromium, zirconium, 
lead, and zinc. Some specific agents are Fe.sub.2 O.sub.3, MnZn, NiZn, and 
CuZn. 
Conductive materials are agents that when mixed with the perceramic 
silicon-containing material, render the resulting coating either 
electrically or thermally conductive. Conductive materials are exemplified 
by gold, silver, copper, aluminum, nickel, zinc, chromium, cobalt, and 
diamond. 
Other optional opaque materials or obstructing agents useful herein include 
synthetic and natural materials such as oxides, nitrides, borides, and 
carbides of various metals and non-metals such as glass, phosphorous 
oxynitride (PON), alumina, titanium dioxide, zinc oxide, tungsten oxide, 
zirconium oxide (ZrO.sub.2), and ruthenium oxide (RuO.sub.2); titanates 
such as potassium titanate and barium titanate; niobates such as lithium 
niobate (LiNbO.sub.3) and lead niobate Pb(NbO.sub.3).sub.2 ; barium 
sulfate; calcium carbonate; precipitated diatomite; aluminum silicate or 
other silicates; pigments and dyes such as crystal violet (C.sub.25 
H.sub.30 N.sub.3 Cl) and the cyanines; phosphors; metals such as silver, 
aluminum, or copper; wollastonite (CaSiO.sub.3); mica; kaolin; clay; talc; 
organic materials such as cellulose, polyimides, phenol resins, epoxies, 
polybenzocyclobutanes; fluorocarbon polymers such as 
polytetrafluoroethylene (C.sub.2 F.sub.4).sub.n, vinylidene fluoride 
H.sub.2 C.dbd.CF.sub.2 and hexafluoropropylene CF.sub.3 CF:CF.sub.2 ; high 
dielectric constant materials such as titanate, niobate, or tungstate 
salts of metals such as strontium, zirconium, barium, lead, lanthanium, 
iron, zinc, and magnesium, i.e., barium titanate (BaTiO.sub.3), potassium 
titanate (K.sub.2 TiO.sub.3), lead niobate, lithium titanate, strontium 
titanate, barium strontium titanate, lead lanthanium zirconium titanate, 
lead zirconium titanate, and lead tungstate. 
The type of optional opaque material or obstructing agent employed will 
depend on the intended use of the coating. Thus, if the coating is to be 
used as an interlayer dielectric, a material such as alumina is desirable, 
so that the coating will have a low dielectric constant (DK). If a coating 
having a high DK is desired, a material such as barium titanate or lead 
niobate should be employed. If a conductive coating is desired, other 
types of metals can be added. 
The particle size and shape of the opaque material or obstructing agent can 
vary over a wide range, depending on factors such as type of material and 
desired thickness of the coating. Our coatings are preferably less than 
about 500 microns in thickness, so a particle size of about 100 to less 
than about 500 microns is generally sufficient. A smaller particle size 
can be used, such as 50-100 microns, or even particle size from submicron 
(i.e., 5-150 millimicrons) to 10 microns. 
The amount of opaque material or obstructing agent can be varied, depending 
on quality and electrical characteristics desired in the final coating. 
Generally, the opaque material or obstructing agent is used in amounts of 
from one weight percent to less than about 94 weight percent, based on the 
weight of pre-ceramic silicon-containing material. Sufficient resin must 
be employed to insure that enough resin is present to bind the opaque 
material or obstructing agent. Lesser amounts of opaque material or 
obstructing agent can also be employed, i.e., 1-5 weight percent. 
If desired, the coating composition can contain a silane coupling agent to 
modify the surface of the opaque material or obstructing agent for better 
adhesion. Generally, silane coupling agents conform to the formula 
A.sub.(4-n) SiY.sub.n where A is a monovalent organic radical such as an 
alkyl group, an aryl group, or a functional group such as methacryl, 
methacryloxy, epoxy, vinyl, or allyl; Y is a hydrolyzable radical such as 
an alkoxy group with 1-6 carbon atoms, an alkoxyalkoxy group with 2-8 
carbon atoms, or an acetoxy group; and n is 1, 2, or 3, preferably 3. 
Some examples of suitable silane coupling agents are 
3-methacryloxypropyltrimethoxysilane of the formula CH.sub.2 
.dbd.C(CH.sub.3)COO(CH.sub.2).sub.3 Si (OCH.sub.3).sub.3 ; 
3-glycidoxypropyltrimethoxysilane; 3-mercaptopropyltrimethoxysilane of the 
formula HSCH.sub.2 CH.sub.2 CH.sub.2 Si(OCH.sub.3).sub.3 ; 
vinyltriacetoxysilane of the formula CH.sub.2 .dbd.CHSi(OOCCH.sub.3).sub.3 
; vinyltriethoxysilane of the formula CH.sub.2 .dbd.CHSi(OC.sub.2 
H.sub.5).sub.3 ; vinyl-tris(2-methoxyethoxy)silane of the formula CH.sub.2 
.dbd.CHSi(OCH.sub.2 CH.sub.2 OCH.sub.3).sub.3 ; and 
2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane. Reference may be had to U.S. 
Pat. No. 4,689,085 (Aug. 25, 1987) for these and other suitable silane 
coupling agents that are appropriate. 
According to our process, the coating composition is applied to the surface 
of a substrate such as an electronic device. Various facilitating measures 
such as stirring and heating may be used to dissolve or disperse the 
opaque material or obstructing agent in the base coating composition, and 
create a more uniform coating solution. For example, the coating solution 
can be prepared by mixing the resin, opaque material or obstructing agent, 
solvent, and silane coupling agent, with a homogenizer, sonic probe, or 
colloid mill, to obtain a coating solution suitable for application to the 
surface of an electronic device. 
Solvents which may be used include any agent or mixture of agents which 
will dissolve or disperse the resin and opaque material, to form a 
homogenous liquid mixture without affecting the resultant coating. These 
solvents can include, for example, alcohols such as ethyl alcohol, 
isopropyl alcohol, and n-butanol; ketones; esters; and glycol ethers. The 
solvent should be present in an amount sufficient to dissolve or disperse 
the materials to the concentration desired for the application. Generally, 
enough solvent is used to form a 0.1-80 weight percent mixture, but 
preferably a 1-50 weight percent mixture. 
The coating mixture is then coated onto the substrate. The coating 
composition can be applied by spin coating, dip coating, spray coating, or 
flow coating. Any other equivalent coating method can also be employed 
such as by applying the coating composition on a substrate or device by 
silk screening, screen printing, meniscus coating, or wave solder coating. 
The solvent is allowed to evaporate from the coated substrate resulting in 
deposition of the resin and opaque material containing coating 
composition. Any means of evaporation may be used such as air drying by 
exposure to an ambient environment, or by application of vacuum or mild 
heat (i.e., less than 50.degree. C.) during early stages of the heat 
treatment. When spin coating is used, any additional drying period is 
minimized as spinning drives off the solvent to some extent. 
Following removal of solvent, the resin and opaque material containing 
coating composition is converted to a ceramic coating by pyrolysis, i.e., 
heating it to a sufficient temperature to ceramify. Generally, this 
temperature is in the range of about 400.degree. C. to about 1,000.degree. 
C. depending on the pyrolysis atmosphere. Preferred temperatures are in 
the range of 400.degree. C. to 800.degree. C. Heating is conducted for a 
time sufficient to ceramify the coating composition. Generally, heating 
will require one to 6 hours, but typically less than about 3 hours will be 
adequate. 
Heating to ceramify the coating composition may be conducted at atmospheric 
pressures varying from vacuum to superatmospheric, and under an oxidizing 
or non-oxidizing gaseous environment such as air, oxygen, an inert gas 
such as nitrogen, ammonia, an amine, moisture, or nitrous oxide. Heating 
can be carried out using a convection oven, a rapid thermal process, a hot 
plate, radiant energy, or microwave energy. The rate of heating is not 
critical although it is practical to heat the coating composition as 
rapidly as possible. 
Pyrolysis results in removal of organic substituents and their replacement 
by oxygen. By this method, a ceramic coating is produced on the substrate, 
generally according to the scenario: 
##STR1## 
The thickness of the ceramic coating can vary over a wide range, but is 
typically from 1-500 microns. These ceramic coatings are able to smooth 
irregular surfaces of various substrates. The coatings are (i) relatively 
defect free, (ii) have excellent adhesive properties, and (iii) have a 
variety of electrical properties, i.e., DK's less than four and up to 
conductive coatings. As such, such coatings are particularly useful for 
electronic applications such as dielectric layers, protective layers, and 
conductive layers. 
If desired, additional coatings may be applied over the ceramic coating. 
These additional coatings include SiO.sub.2 coatings, SiO.sub.2 /ceramic 
oxide layers, silicon-containing coatings, silicon carbon containing 
coatings, silicon nitrogen containing coatings, silicon oxygen nitrogen 
containing coatings, silicon nitrogen carbon containing coatings, and 
diamond-like carbon coatings. Methods for applying such coatings are 
described in U.S. Pat. No. 4,756,977, referred to previously. An 
especially preferred additional coating is silicon carbide. 
The method of applying an additional coating such as silicon carbide is not 
critical, and such coatings can be applied by any chemical vapor 
deposition technique such as thermal chemical vapor deposition (TCVD), 
photochemical vapor deposition, plasma enhanced chemical vapor deposition 
(PECVD), electron cyclotron resonance (ECR), and jet vapor deposition. It 
could also be applied by physical vapor deposition techniques such as 
sputtering or electron beam evaporation. These processes involve either 
the addition of energy in the form of heat or plasma to a vaporized 
species to cause the desired reaction, or they focus energy on a solid 
sample of the material to cause its deposition. 
For example, in thermal chemical vapor deposition, the coating is deposited 
by passing a stream of the desired precursor gas over a heated substrate. 
When the precursor gas contacts the hot surface, it reacts and deposits 
the coating. Substrate temperatures in the range of 25.degree. C. to 
1,000.degree. C. are sufficient to form these coatings in several minutes 
to several hours, depending on the precursor gas and the thickness of the 
coating. Reactive metals can be used in such a process to facilitate 
deposition. 
In PECVD techniques, a precursor gas is reacted by passing it through a 
plasma field. Reactive species are formed and focused at the substrate 
where they readily adhere. The advantage of a PECVD process over a thermal 
CVD process is that in the former, lower substrate and processing 
temperatures can be used, i.e., 25.degree.-600.degree. C. 
Plasma used in a PECVD process can be energy derived from electric 
discharges, electromagnetic fields in the radio-frequency or microwave 
range, lasers, or particle beams. In most plasma deposition processes, it 
is preferred to use radio frequency (i.e., 10 kHz to 10.sup.2 MHz) or 
microwave energy (i.e., 0.1-10 GHz or 10.sup.9 hertz) at moderate power 
densities (i.e., 0.1-5 watts/cm.sup.2). The frequency, power, and pressure 
are tailored to the precursor gas and equipment being used. 
Some precursor gases that can be used include (1) mixtures of silanes or 
halosilanes such as trichlorosilane (HSiCl.sub.3) in the presence of an 
alkane of 1-6 carbon atoms such as methane, ethane, and propane; (2) an 
alkylsilane such as methylsilane (CH.sub.3 SiH.sub.3), dimethylsilane 
(CH.sub.3).sub.2 SiH.sub.2, trimethylsilane (CH.sub.3).sub.3 SiH, and 
hexamethyldisilane (CH.sub.3).sub.3 Si--Si(CH.sub.3).sub.3 ; or (3) a 
silacyclobutane or a disilacyclobutane of the type described in U.S. Pat. 
No. 5,011,706 (Apr. 30, 1991) incorporated by reference. 
Examples of such silacyclobutanes (1) and disilacyclobutanes (2) are shown 
below. R1 is hydrogen, fluorine, or a hydrocarbon radical having 1-4 
carbon atoms. R2 is hydrogen or a hydrocarbon radical having 1-4 carbon 
atoms. A preferred disilacyclobutane is 1,3-dimethyl-1,3-disilacyclobutane 
shown in formula (3). 
##STR2## 
When silicon carbide is used as a coating layer, it is capable of forming a 
hermetic and electrical barrier over the surface of the silica-containing 
ceramic layer on the electronic device, and inhibits damage from chemical 
and mechanical means. 
In a preferred coating composition according to our invention, the matrix 
resin is colloidal silica and partial condensate of methylsilanetriol. 
Opaque material or obstructing agents for the coating composition are 
tungsten carbide, tungsten, and P.sub.2 O.sub.5. Tungsten metal is the 
material that functions as an energy barrier. Tungsten metal and tungsten 
carbide are each effective against radiation and light. Tungsten carbide 
has a MoHs' hardness rating greater than 9.5, and tungsten metal has a 
MoHs' hardness of about 6.5-7.5. Tungsten carbide is therefore the more 
effective against abrasion, although each are included in the coating 
composition and function to provide abrasion resistance. 
MoHs' hardness, it is noted, is a comparison test involving the assignment 
of a relative number to known materials based on their ability to scratch 
one another. Some examples of the MoHs' hardness scale which ranks 
materials from 1-10, are graphite with a MoHs' hardness of 1, manganese 
with a MoHs' hardness of 5, quartz with a MoHs' hardness of 7, and diamond 
with a MoHs' hardness of 10. 
As noted previously, P.sub.2 O.sub.5 protects the integrated circuit and 
its memory from invasion by wet etching techniques, and the silane 
coupling agent can be included in the coating composition to improve 
adhesion of the coating to the electronic device.

Following is an example illustrating our invention in terms of the method 
for the preparation of a coating composition containing (WC), (W), and 
(P.sub.2 O.sub.5). 
EXAMPLE I 
The materials listed below were mixed in a container for four periods of 
twenty seconds with a sonic probe to prepare a solution useful as a 
coating composition. 
______________________________________ 
Amount Component 
______________________________________ 
1.0 g Aqueous alkanol dispersion of colloidal silica and 
partial condensate of methylsilanetriol 
15.0 g Tungsten Carbide (WC) 
0.83 um average particle diameter 
3.0 g Tungsten Metal (W) 
0.6-0.9 um average particle diameter 
0.08 g P.sub.2 O.sub.5 powder 
0.4 g 3-glycidoxypropyltrimethoxysilane 
1.0 g Isopropyl alcohol 
20.48 g Total 
______________________________________ 
The resin component of this coating composition was a solution prepared 
generally according to the procedure set forth in Example 1 of U.S. Pat. 
No. 3,986,997. This component contained 31 percent solids, 40 percent of 
which was SiO.sub.2 and 60 percent partial condensate of CH.sub.3 
Si(OH).sub.3. The remaining 69 percent of the resin component was a 
mixture containing water, acetic acid, isopropanol, and n-butanol. 
A 4.5 inch square alumina panel with a thickness of 40 mils was coated with 
the solution using a 2 mil drawdown bar. The coated alumina panel was 
allowed to air dry for 50 minutes. The coating was then pyrolyzed at 
400.degree. C. for one hour in air. The coating was examined with a 
microscope under 1000.times. magnification and no cracks were observable. 
The thickness of the coating was 7.2 um. The coating had a pencil hardness 
of 6H, a Vickers hardness of 633 N/mm.sup.2, and a modulus of 23.6 GPa 
(i.e., giga or 10.sup.9 Pascals). 
The pencil test used to measure the hardness of the coating is a standard 
qualitative method of determining scratch resistance of coatings. 
According to the test procedure, a coated panel is placed on a firm 
horizontal surface. A pencil is held firmly against the coated film at a 
45.degree. angle pointing away from the technician conducting the test. It 
is pushed away by the technician in one-quarter inch (6.5 mm) strokes. The 
technician starts the process with the hardest lead pencil, i.e., 9H, and 
continues down the scale of pencil hardness, i.e., 6B, to the pencil that 
will not cut into or gouge the film. The hardest pencil that will not cut 
through the film to the substrate for a distance of at least one-eighth of 
an inch (3 mm) is recorded, using the Berol scale, i.e., Berol 
Corporation, Brentwood, Tenn. 
##STR3## 
The Vickers test used in our evaluation of the coating is a measure of the 
resistance of a material to deformation. The test procedure is described 
in the American Society for Testing and Materials, Philadelphia, Pa., 
(ASTM) Standard Number E92. According to the test procedure, a pyramidal 
diamond is used as an indenter. The test is conducted using a flat 
specimen on which the indenter is hydraulically loaded. When the 
prescribed number of indentations have been made, the specimen is removed, 
the diagonals of the indentations are measured using a calibrated 
microscope, and then the values are averaged. A Vickers hardness number 
(VHN) can be calculated, or the VHN can be taken from a precalculated 
table of indentation size versus Vickers Hardness Numbers. The Vickers 
scale for VHN ranges from a low value of 100 which is a measure of the 
softest, to a high value of 900 which is a measure of the hardest. 
Coating compositions most suitable for use according to our invention 
should contain 0.05-20 percent by weight of the combination of colloidal 
silica and partial condensate of methylsilanetriol, 5-30 percent by weight 
of tungsten metal, 30-80 percent by weight of tungsten carbide, 0.1-5 
percent by weight of phosphoric anhydride, and 1-5 percent by weight of a 
silane coupling agent; the remainder of the coating composition being the 
solvent(s). 
The amount of tungsten carbide can be varied from 1-91 volume percent. The 
amount of tungsten metal can also be varied from 1-91 volume percent minus 
the volume percent of tungsten carbide. In addition, the amount of 
phosphoric anhydride can be varied from 1-30 volume percent, minus the 
combined volume percent of tungsten carbide and tungsten metal. 
Our invention is also applicable to the formation of coatings using 
phosphoric anhydride in combination with coating compositions other than 
those containing colloidal silica and partial condensate of 
methylsilanetriol. 
Thus, other useful preceramic silicon-containing materials include 
precursors to silicon oxides, especially silica. The silicon oxide 
precursors which may be used include, but are not limited to, hydrogen 
silsesquioxane resin, hydrolyzed or partially hydrolyzed R.sub.n 
Si(OR).sub.4-n, or combinations of the above, in which each R is 
independently an aliphatic, alicyclic or aromatic substituent of 1-20 
carbon atoms, preferably 1-4, such as an alkyl (i.e., methyl, ethyl, 
propyl), alkenyl (i.e., vinyl or allyl), alkynyl (i.e., ethynyl), 
cyclopentyl, cyclohexyl, or phenyl, and n is 0-3, preferably 0 or 1. 
The hydrogen silsesquioxane resin which may be used includes 
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 the R 
group include alkyls such as methyl, ethyl, propyl, and butyl, aryls such 
as phenyl, and alkenyls such as allyl or vinyl. These resins may be fully 
condensed (HSiO.sub.3/2).sub.n or they may be only partially hydrolyzed 
(i.e., containing some .tbd.Si--OR) and/or partially condensed (i.e., 
containing some .tbd.Si--OH). Although not represented by this structure, 
these resins may also contain a small number (i.e., less than about 10%) 
of silicon atoms which have either 0 or 2 hydrogen atoms attached thereto 
or a small number of .tbd.SiC bonds due to various factors involved in 
their formation or handling. 
Hydrogen silsesquioxane resins are ladder or cage polymers which can be 
shown generally by the formula: 
##STR4## 
Typically, n has a value of four or more. By way of illustration, when n is 
four, a bond arrangement for a silsesquioxane cubical octamer is shown 
below: 
##STR5## 
As this series is extended, i.e., n being ten or more, double-stranded 
polysiloxanes of indefinitely higher molecular weight are formed, 
containing regular and repeated cross-ties in their extended structure. 
Hydrogen silsesquioxane resin and methods for their production are known in 
the art. For example, U.S. Pat. No. 3,615,272, which is incorporated 
herein by reference, teaches the production of a nearly fully condensed 
hydrogen silsesquioxane 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, U.S. Pat. 
No. 5,010,159, which is hereby incorporated by reference, teaches 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 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 specific molecular weight fractions of the above 
hydrogen silsesquioxane resin may also be used in this process. Such 
fractions and methods for their preparation are taught by U.S. Pat. Nos. 
5,063,267 and 5,416,190, which are hereby incorporated by reference. A 
preferred fraction comprises material wherein at least 75% of the 
polymeric species have a molecular weight above about 1,200, and a more 
preferred fraction comprises material wherein at least 75% of the 
polymeric species have a number average molecular weight between about 
1,200 and about 100,000. 
Another type of silica precursor material useful herein includes hydrolyzed 
or partially hydrolyzed compounds of the formula R.sub.n Si(OR).sub.4-n in 
which R and n are as defined above. Some of these materials are 
commercially available, for example, under the tradename ACCUGLASS from 
Allied Signal Inc., Morristown, N.J. Specific compounds of this type 
include methyltriethoxysilane, phenyltriethoxysilane, 
diethyldiethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, 
phenyltrimethoxysilane, vinyltrimethoxysilane, tetramethoxysilane, 
tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane After 
hydrolysis or partial hydrolysis of these compounds, the silicon atoms 
therein may be bonded to C, OH or OR groups, but a substantial portion of 
the material is believed to be condensed in the form of soluble 
.tbd.Si--O--Si.tbd. resins. Compounds in which x=2 or 3 are generally not 
used alone, as volatile cyclic structures are generated during pyrolysis, 
but small amounts of said compounds may be cohydrolyzed with other silanes 
to prepare useful preceramic materials. 
Additional examples of other silicon-containing preceramic materials 
include silicon carbonitride precursors such as hydridopolysilazane resin 
and methylpolydisilylazane resin. Processes for the production of these 
materials are described in U.S. Pat. No. 4,540,803, and 4,340,619, 
respectively, both of which are incorporated herein by reference. Examples 
of silicon carbide precursors include polycarbosilanes, and examples of 
silicon nitride precursors include polysilazanes. Oxygen can be 
incorporated into the ceramics resulting from the above precursors, or the 
precursors can be converted to silica by pyrolyzing them in an 
oxygen-containing environment. Mixtures of silicon-containing preceramic 
materials may also be used. 
Yet other silicon-containing preceramic materials include polyborosilazanes 
(or borosilazanes). Processes for the production of these materials are 
described in U.S. Pat. No. 5,169,908 (Dec. 8, 1992), and U.S. Pat. No. 
5,436,084 (Jul. 25, 1995), incorporated herein by reference. An example of 
one particularly preferred polymer is a boron modified hydropolysilazane 
produced by reacting a hydrosilazane polymer with a borane complex or 
diborane, as explained in the '084 patent. 
Following are some additional examples illustrating our invention in terms 
of the method for the preparation of other coating compositions containing 
(WC), (W), and (P.sub.2 O.sub.5). 
EXAMPLE II 
The materials listed below, except for P.sub.2 O.sub.5, were mixed in a 
container for two periods of twenty seconds. P.sub.2 O.sub.5 was added, 
and the resulting mixture was mixed for an additional period of twenty 
seconds with a sonic probe to prepare a solution useful as a coating 
composition. 
______________________________________ 
Amount Component 
______________________________________ 
1.0 g Hydrogen silsesquioxane resin made by the method of 
US Pat. No. 3,615,272 
15.0 g Tungsten Carbide (WC) 
0.83 um average particle diameter 
3.0 g Tungsten Metal (W) 
0.6-0.9 um average particle diameter 
0.08 g P.sub.2 O.sub.5 powder 
0.4 g 3-glycidoxypropyltrimethoxysilane 
3.0 g Xylene 
22.48 
g Total 
______________________________________ 
A 4.5 inch square alumina panel with a thickness of 40 mils was coated with 
the solution using a 2 mil drawdown bar. The coated alumina panel was 
allowed to air dry for 3 hours. The coating was then pyrolyzed at 
400.degree. C. for one hour in air. The coating was examined with a 
microscope under 1000.times. magnification and no cracks were observable. 
The thickness of the coating was 18.7 um. 
EXAMPLE III 
The materials listed below were mixed in a container for four periods of 
twenty seconds with a sonic probe to prepare a solution useful as a 
coating composition. 
______________________________________ 
Amount Component 
______________________________________ 
1.104 
g Hydridopolysilazane (55.6 weight percent solids in 
xylene) made by the method of US Pat. No. 4,540,803 
10.5 g Tungsten Carbide (WC) 
0.83 um average particle diameter 
3.5 g Tungsten Metal (W) 
0.6-0.9 um average particle diameter 
0.5 g P.sub.2 O.sub.5 powder 
0.3 g 3 glycidoxypropyltrimethoxysilane 
2.5 g Mixture of 25 percent by weight 
octamethylcyclotetrasiloxane and 75 percent by weight 
decamethylcyclopentasiloxane 
18.4 g Total 
______________________________________ 
A 4.5 inch square alumina panel with a thickness of 40 mils was coated with 
the solution using a 2 mil drawdown bar. The coated alumina panel was 
allowed to air dry for 2 hours. The coating was then pyrolyzed at 
400.degree. C. for one hour in air. The coating was examined with a 
microscope under 1000.times. magnification and no cracks were observable. 
The thickness of the coating was 12.8 um. 
EXAMPLE IV 
The materials listed below were mixed in a container for four periods of 
twenty seconds with a sonic probe to prepare a solution useful as a 
coating composition. 
______________________________________ 
Amount Component 
______________________________________ 
1.05 g Borohydridopolysilazane (57 weight percent solids in 
xylene containing 1.5 weight percent B) made by the 
method of US Pat. No. 5,169,908 
10.5 g Tungsten Carbide (WC) 
0.83 um average particle diameter 
3.6 g Tungsten Metal (W) 
0.6-0.9 um average particle diameter 
0.5 g P.sub.2 O.sub.5 powder 
0.3 g 3-glycidoxypropyltrimethoxysilane 
2.5 g Mixture of 25 percent by weight 
octamethylcyclotetrasiloxane and 75 percent by weight 
decamethylcyclopentasiloxane 
18.45 
g Total 
______________________________________ 
A 4.5 inch square alumina panel with a thickness of 40 mils was coated with 
the solution using a 2 mil drawdown bar. The coated alumina panel was 
allowed to air dry for 1 hour and 50 minutes. The coating was then 
pyrolyzed at 400.degree. C. for one hour in air. The coating was examined 
with a microscope under 1000.times. magnification and no cracks were 
observable. The thickness of the coating was 31.7 um. 
Other variations may be made in the compounds, compositions, and methods 
described herein without departing from the essential features of our 
invention. The forms of our invention are exemplary only and not intended 
as limitations on its scope which is defined in the claims.