Coating solution for use in the formation of metal oxide film

A coating solution for use in the formation of a metal oxide film is described. It is composed of (a) a .beta.-diketone, (b) at least one element or compound selected from the group consisting of elements capable of forming complexes with said .beta.-diketone, salts of the elements and hydrolysates of alkoxides of the elements, and (c) an aprotic polar solvent. As an alternative, it is composed of a metal complex of a .beta.-diketone and an aprotic polar solvent.

FIELD OF THE INVENTION 
This invention relates to a coating solution for use in the formation of a 
metal oxide film on a substrate through its coating and baking. 
The present invention is concerned with a coating solution for use in 
forming, for example, (1) a metal oxide film on a substrate such as glass 
or ceramic substrate of a liquid crystal display or the like for the 
purpose of passivation, (2) a transparent conductive film on a liquid 
crystal display, (3) an insulating film over one or more electrodes of a 
semiconductor device, or (4) a film intended to improve the 
weatherability, corrosion resistance, chemical resistance, optical 
characteristics, surface strength, etc. of a ceramic, plastic, metal, or 
the like. 
More specifically, the present invention pertains to a coating solution for 
use in the formation of a metal oxide film, which permits formation of a 
highly-uniform metal oxide film by any suitable coating method such as 
dipping, spin coating, spraying, roll coating or printing, is effective 
especially as a coating solution for roll coating, printing or the like in 
which high viscosity and slow dryability are required, and has high 
storage stability. 
BACKGROUND OF THE INVENTION 
The application field of metal oxide films has been diversified widely in 
recent years. They are being used, for example, as insulating films and 
orientation controlling films in liquid crystal displays, protective films 
for ceramics and metals, and insulating films for semiconductor devices. 
In particular, a liquid crystal display is fabricated by forming a 
patterned transparent conductive film on an insulating substrate such as 
glass substrate, forming a metal oxide film over the conductive film to 
construct an electrode substrate, arranging a pair of such electrode 
substrates, with such a metal oxide film formed thereon, side by side with 
a spacer interposed therebetween along the peripheries thereof so as to 
form a cell, and sealing a liquid crystal material within the cell. 
As characteristics required for such metal oxide films, they are supposed 
to have high adhesion to insulating substrates and transparent conductive 
films and moreover, they should themselves be free of defects such as 
pinholes and should be uniform films. Since metal oxide films are used in 
various ways in recent years, the above-mentioned characteristics of metal 
oxide films must of course satisfy their individual application purposes. 
It is also a critical requirement that they have excellent mass 
producibility upon their formation. As processes known presently for the 
formation of a metal oxide film, may be mentioned vacuum deposition, CVD 
(chemical vapor deposition), sputtering, etc. However, these processes are 
all insufficient in mass producibility and can hardly be said to be 
suitable processes. For example, vacuum deposition employs a vacuum 
apparatus. In particular, when one wants to form a metal oxide film on a 
large substrate, the film thickness varies too much to obtain a uniform 
film. In addition, a large and expensive vacuum apparatus is indispensable 
so that the production cost increases. Such large metal oxide films lack 
mass producibility. CVD requires to heat a substrate upon formation of a 
metal oxide film thereon and is difficult to form a uniform film. An 
expensive apparatus is required for the practice of CVD. CVD is 
insufficient in mass producibility. On the other hand, sputtering has 
substantially the same drawbacks as vacuum deposition. 
Coating processes have been proposed as processes which have improved the 
problems or drawbacks of the film-forming processes described above. In 
such coating processes, a coating solution for use in the formation of a 
metal oxide film is applied on a substrate and is then heated to form a 
metal oxide film. Coating processes have such advantages that a film can 
be formed easily irrespective of the size of a substrate and owing to 
simple procedures required for the formation of a film, the mass 
producibility is extremely high. 
As a conventional coating solution known to be suitable for use in the 
formation of a metal oxide film by such a coating process, there is a 
coating solution prepared by adding polyethylene glycol or nitrocellulose 
to a solution composed of a metal complex of a .beta.-diketone and as a 
solvent, a .beta.-diketone-containing organic or inorganic solvent, e.g., 
a low-viscosity alcohol such as methanol, ethanol or propanol, a ketone 
such as acetone, or an ester (Japanese Patent Laid-Open No. 149920/1980). 
This coating solution permits easy formation of films on various 
substrates. However, films formed of this coating solution have high 
porosity. Decomposition intermediates and carbonic residues of 
polyethylene glycol or nitrocellulose which is contained in the coating 
solution may thus remain in pores even after a heat treatment applied 
subsequent to the coating of the solution, thereby preventing the 
formation of a uniform metal oxide film. Furthermore, the coating solution 
has poor storage stability so that a deposit occurs in the coating 
solution when it is left over for a long period of time. 
It has heretofore been difficult to provide a coating solution which may be 
applied to any one of coating processes such as dip coating, spin coating, 
roll coating and transfer coating. A metal oxide film formed by a coating 
process which requires a coating solution of a relatively high viscosity 
is inferior in uniformity and/or poor in adherence to an insulating 
substrate or transparent conductive film. In order to obtain a thick metal 
oxide film, it is effective to apply a high-viscosity coating solution by 
a coating process such as roll coating or transfer printing. It is 
generally indispensable to incorporate a thickening agent such as organic 
polymer in order to prepare a high-viscosity coating solution. A coating 
solution with a thickening agent incorporated therein however involves 
such problems that the coating solution has poor storage stability and 
undergoes variations upon standing, thereby being gelled or developing a 
deposit and moreover, a metal oxide film to be formed will have low 
adhesion to the surface of its corresponding substrate, thereby causing 
peeling-off or clouding. 
OBJECT AND SUMMARY OF THE INVENTION 
An object of this invention is to improve the drawbacks of the conventional 
coating solutions for forming metal oxide films and hence to provide a 
coating solution for use in the formation of a metal oxide film, which has 
good uniformity, adhesion and strength and containing no pinholes, without 
being repelled by a substrate. 
Another object of this invention is to provide a coating solution for use 
in the formation of a metal oxide film, which has excellent storage 
stability. 
The present inventors have carried out an extensive investigation in order 
to attain the above objects. As a result, it has been found that (1) a 
liquid mixture composed of (a) a .beta.-diketone, (b) at least one element 
or compound selected from the group consisting of elements capable of 
forming complexes with said .beta.-diketone, salts of the elements and 
hydrolysates of alkoxides of the elements, and (c) an aprotic polar 
solvent or (2) a liquid mixture composed of a metal complex of a 
.beta.-diketone and an aprotic polar solvent can provide a metal oxide 
film having excellent film characteristics and has superb storage 
stability, leading to completion of the present invention. 
In one aspect of this invention, there is thus provided a coating solution 
for use in the formation of a metal oxide film, comprising: 
(a) a .beta.-diketone; 
(b) at least one element or compound selected from the group consisting of 
elements capable of forming complexes with said .beta.-diketone, salts of 
the elements and hydrolysates of alkoxides of the elements; and 
(c) an aprotic polar solvent. 
In another aspect of this invention, there is also provided a coating 
solution for use in the formation of a metal oxide film, comprising a 
metal complex of a .beta.-diketone and an aprotic polar solvent. 
In a further aspect of this invention, there is also provided a process for 
the formation of a metal oxide film, which comprises the following 
consecutive steps: 
(i) coating a substrate with either one of the above coating solution; and 
(ii) subjecting the thus-coated substrate to a heat treatment at a 
temperature of at least 100.degree. C.

DETAILED DESCRIPTION OF THE INVENTION 
Features of the present invention will hereinafter be described in detail. 
[Elements capable of forming a complex with a .beta.-diketone] 
As exemplary elements capable of forming a complex with a .beta.-diketone, 
may be mentioned Group Ib elements of the periodic table such as copper; 
Group IIa elements of the periodic table such as beryllium, magnesium, 
calcium, strontium and barium; Group IIb elements of the periodic table 
such as zinc and cadmium; Group IIIa elements of the periodic table such 
as lanthanum, cerium, scandium and yttrium; Group IIIb elements of the 
periodic table such as aluminum, gallium, indium and thallium; Group IVa 
elements of the periodic table such as titanium, zirconium and hafnium; 
Group IVb elements of the periodic table such as silicon, germanium, tin 
and lead; Group Va elements of the periodic table such as vanadium, 
niobium and tantalum; Group Vb elements of the periodic table such as 
antimony and bismuth; Group VIa elements of the periodic table such as 
chromium, molybdenum and tungsten; Group VIb elements of the periodic 
table such as selenium and tellurium; Group VIIa elements of the periodic 
table such as manganese and rhenium; and Group VIII elements of the 
periodic table such as iron, cobalt and nickel. These elements may be used 
either singly or in combination. In addition, salts of these elements and 
hydrolysates of alkoxides of these elements may also be used as will be 
described next. 
[Salts of elements] 
Illustrative examples of the above salt of the element, which are useful in 
the practice of this invention, may include inorganic salts such as 
hydrochlorides, nitrates and sulfates; organic salts such as acetates and 
octylates; .beta.-diketone complexes such as acetyl acetone complexes; 
biscyclopentadienyl complexes; etc. These salts may be used either singly 
or in combination. 
[Hydrolysates of alkoxides of elements] 
Illustrative examples of the above hydrolysate of the alkoxide of the 
element, which are useful in the practice of this invention, may include 
hydrolysates of alkoxides of at least one element belonging to any one of 
Groups Ib, IIa, IIb, IIIa, IIIb, IVa, IVb, Va, Vb, VIa, VIb, VIIa and VIII 
of the periodic table. Taking alkoxides of silicon by way of example, 
hydrolysates of tetraalkoxysilanes, monoalkyltrialkoxysilanes, 
monoaryltrialkoxysilanes and the like may be mentioned. As specific 
examples, may be mentioned hydrolysates of tetramethoxysilane, 
tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, 
monomethyltrimethoxysilane, monomethyltriethoxysilane, 
monomethyltripropoxysilane, monomethoxytributoxysilane, 
monoethyltrimethoxysilane, monoethyltriethoxysilane, 
monophenyltrimethoxysilane, monophenyltriethoxysilane and the like. The 
hydrolysis of these alkoxides may be effected using, as a hydrolytic 
catalyst, an inorganic acid, e.g., sulfuric acid, hydrochloric acid, 
nitric acid or phosphoric acid, or an organic acid, e.g., monochloroacetic 
acid, monofluoroacetic acid or an organic sulfonic acid. For easier 
application, it may be recommended to use, as a coating solution, a 
solution which has been obtained by adding the above-described hydrolytic 
catalyst and a small amount of water to a liquid mixture of an alkoxide of 
any one of the above-mentioned elements, a .beta.-diketone and an aprotic 
polar solvent and then hydrolyzing the alkoxide in the liquid mixture. 
These alkoxide hydrolysates may be used either singly or in combination. 
[.beta.-Diketones] 
As exemplary .beta.-diketones useful in the practice of this invention, may 
be mentioned acetyl acetone, trifluoroacetyl acetone, hexafluoroacetyl 
acetone, benzoyl acetone, benzoyl trifluoroacetone, dibenzoylmethane, 
methyl acetoacetate, ethyl acetoacetate, butyl acetoacetate and so on. 
The above-exemplified .beta.-diketones may be used either singly or in 
combination. 
[Metal complexes of .beta.-diketones] 
Illustrative examples of the metal complex of the .beta.-diketone, which 
are useful in the practice of this invention, may include metal complexes 
of the above-exemplified elements and .beta.-diketones. The metal 
complexes may each be obtained by reacting any one of the 
above-exemplified elements, its salt (other than the metal complexes of 
any one of the above-exemplified .beta.-diketones) or a hydrolysate of any 
one of the above-exemplified alkoxide with the .beta.-diketone. Such metal 
complexes of .beta.-diketones may be used either singly or in combination. 
[Aprotic polar solvents] 
As exemplary aprotic polar solvents useful in the practice of this 
invention, may be mentioned N,N-dimethylformamide, N,N-dimethylacetamide, 
acetonitrile, dimethylsulfoxide, N,N,N',N'-tetraethylsulfamide, 
hexamethylphosphoramide, N-methylmorpholine, N-methylpyrrole, 
N-ethylpyrrole, N-methyl-.DELTA..sup.3 -pyrroline, N-methylpiperidine, 
N-ethylpiperidine, N,N-dimethylpiperazine, N-methylimidazole, 
N-methyl-4-piperidone, N-methyl-2-piperidone, N-methyl-2-pyrrolidone, 
1,3-dimethyl-2-imidazolidinone, 1,3-dimethyltetrahydro-2- 
(1H)-pyrimidinone, etc. They may be used either singly or in combination. 
Among these aprotic polar solvents, N,N,N',N'-tetraethylsulfamide, 
N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone and 1,3-dimethyl 
tetrahydro-2(1H)-pyrimidinone may be used preferably for their low 
toxicity, ready availability and easy handling. 
[Preparation method of coating solutions] 
Coating solutions according to this invention may each be prepared, for 
example, by any one of the following various methods, since a 
.beta.-diketone forms a metal complex with an element, which is capable of 
forming a complex with the .beta.-diketone, or a compound of the element. 
(1) At least one element or compound (b), which is selected from the group 
consisting of elements capable of forming complexes with said 
.beta.-diketone, salts of the elements and hydrolysates of alkoxides of 
the elements, is dissolved in a liquid mixture composed of (a) a 
.beta.-diketone and (c) an aprotic polar solvent. 
(2) A metal complex of a .beta.-diketone is dissolved in a liquid mixture 
composed of the .beta.-diketone and an aprotic polar solvent. 
(3) A metal complex of a .beta.-diketone is dissolved in an aprotic polar 
solvent. 
(4) A hydrolytic catalyst and a small amount of water are added to a liquid 
mixture containing an alkoxide of any one of the above elements, a 
.beta.diketone and an aprotic polar solvent, followed by hydrolysis of the 
alkoxide in the liquid mixture to prepare a coating solution. 
[Proportions of components] 
As proportions of the above components (a), (b) and (c) in a coating 
solution according to this invention, the components (a), (b) and (c) may 
be 1 wt. %-60 wt. %, 1 wt. %-60 wt. % and 10 wt. %-80 wt. %, preferably, 1 
wt. %-50 wt. %, 1 wt. %-50 wt. % and 10 wt. %-70 wt. %, respectively, in 
the preparation method (1). 
In the preparation method (2) or (3) where the metal complex of the 
.beta.diketone is used instead of the components (a) and (b), the metal 
complex of the .beta.diketone may preferably be used in a proportion of 1 
wt.%-60 wt. while the preferably proportion of the component (c) is 40 
wt.%-99 wt.%. 
In the preparation method (4), the proportions of the alkoxide, 
.beta.diketone and aprotic polar solvent are the same as those give above 
with respect to the preparation method (1). [Organic solvents] 
An organic solvent may be added as desired to a coating solution of this 
invention in order to improve it film-forming characteristics. Organic 
solvents, which are usable in the present invention, may preferably be 
those represented by any one of the following general formulae: 
EQU HO--R--OH; 
EQU HO--R--O--R; 
EQU HO--R--O--R--OH; 
EQU HO--R--O--R--O--R; and 
EQU R--O--R--O--R--O--R 
wherein Rs mean independently an alkyl, alkylene, aryl, arylene or benzyl 
group. As specific examples, may be mentioned methyl alcohol, ethyl 
alcohol, isopropyl alcohol, n-propyl alcohol, n-butyl alcohol, ethylene 
glycol, propylene glycol, butylene glycol, hexylene glycol, octylene 
glycol, diethylene glycol, dipropylene glycol, dihexylene glycol, ethylene 
glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol 
monobutyl ether, ethylene glycol monopropyl ether, ethylene glycol 
monophenyl ether, ethylene glycol monobenzyl ether, propylene glycol 
monomethyl ether, propylene glycol monoethyl ether, propylene glycol 
monobutyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl 
ether, ethylene glycol methyl ethyl diether, ethylene glycol dibutyl 
ether, ethylene glycol dipropyl ether, ethylene glycol diphenyl ether, 
ethylene glycol dibenzyl ether, propylene glycol dimethyl ether, propylene 
glycol diethyl ether, propylene glycol dibuthyl ether, methylcarbitol, 
ethylcarbitol, butylcarbitol, phenylcarbitol, benzylcarbitol, 
dimethylcarbitol, diethylcarbitol, dibutylcarbitol, diphenylcarbitol, 
dibenzylcarbitol, methylethylcarbitol, dipropylene glycol dimethyl ether, 
dipropylene glycol diethyl ether, dipropylene glycol dibutyl ether, etc. 
These organic solvents may be used either singly or in combination. The 
organic solvent may be added preferably in an amount not greater than 80 
wt. % based on the liquid mixture of the components (a), (b) and (c) or 
the liquid mixture of the metal complex of the .beta.-diketone and the 
aprotic polar solvent, with a range of 30-70 wt. % being particularly 
preferred. An addition of the organic solvent in an unduly large amount 
will lead to a coating solution reduced in coatability and also to a 
coating film reduced in both adhesion and strength. It is therefore not 
preferable to add the organic solvent too much. 
[Coating method and method for the formation of metal oxide film] 
Coating solutions according to this invention can each form a metal oxide 
film excellent in adhesion and uniformity on a substrate, such as plastic 
substrate, glass substrate, ceramic substrate, or substrate obtained by 
sintering powder of a metal nitride, metal carbonate and/or the like, 
metal substrate or substrate for a semiconductor when it is applied on the 
substrate and thereafter heated. Any suitable coating method may be used, 
such as dip coating, spray coating, spin coating, brush coating, roll 
coating or printing. It is preferable to use a printing method in view of 
the efficiency of work especially when one wants to form a pattern with a 
metal oxide film. 
As an exemplary heating method, may be mentioned hot plate heating, hot air 
heating, burner heating, infrared ray heating, laser heating, 
high-frequency wave heating or the like. Regarding the heating 
temperature, it is preferable to conduct the heating at a temperature of 
at least 100.degree. C., preferably, 300.degree. C. or higher from the 
viewpoint of forming a chemically-stable film. It is however necessary to 
choose the heating temperature suitably within a temperature range which a 
substrate to be coated will be able to withstand, since there are some 
substrates having limited heat resistance due to their materials. 
ADVANTAGES 
A coating solution of this invention is excellent in storage stability and 
can provide a metal oxide film having excellent properties, i.e., having 
good uniformity, adhesion and strength and containing no pinholes, without 
being repelled by a substrate. The present invention can therefore provide 
film-coated substrates and the like with good mass producibility, thereby 
bringing about a significant advantage from the industrial viewpoint. 
EMBODIMENTS OF THE INVENTION 
The present invention will hereinafter be described specifically by the 
following Examples and Comparative Examples. Needless to say, the present 
invention is not necessarily limited to the following Examples only. 
Incidentally, all designations of "part" or "parts" will mean part or 
parts by weight. 
Example 1: 
After washing a soda glass substrate of 1.1 mm thick, it was dipped in a 
coating solution composed of the following components and then pulled out 
at a constant rate of 10 cm/min so as to perform its coating. 
______________________________________ 
(Composition of the coating solution) 
______________________________________ 
Ethyl acetoacetate aluminum 
15 parts 
diisopropylate 
Acetyl acetone 3 parts 
N--methyl-2-pyrrolidone 25 parts 
______________________________________ 
The thus-coated substrate was thereafter placed in a recirculating hot 
dryer, in which it was dried at 140.degree. C. for 1 minutes. It was then 
transferred into an electric furnace, where it was baked at 500.degree. C. 
for 60 minutes to form an Al.sub.2 O.sub.3 film of 80 nm thick. The 
thus-obtained film was transparent, was free of any appreciable pinholes, 
had high uniformity and was good in both adhesion and strength. In 
addition, the coating solution did not develop any change in components 
and remained uniform even after stored for 12 months. An additional 
coating test was performed in the same manner, using the coating solution 
which had been stored over 12 months. No substantial differences were 
observed in rating between a film thus formed and the film formed 12 
months ago. 
EXAMPLE 2: 
A 5-inch silicon wafer was spin-coated at 3,000 rpm with a coating solution 
which had been obtained by mixing the following components thoroughly and 
then hydrolyzing the tetraethoxysilane. 
______________________________________ 
(Composition of the coating solution) 
______________________________________ 
Tetraethoxysilane 40 parts 
Acetyl acetone 57 parts 
N--Methyl-2-pyrrolidone 50 parts 
Purified water 7 parts 
Phosphoric acid (hydrolytic catalyst) 
0.1 part 
______________________________________ 
After drying the thus-coated wafer at 200.degree. C. for 10 minutes on a 
hot plate, it was transferred into an electric furnace and then baked at 
800.degree. C. for 30 minutes, thereby to obtain a PSG (silicon oxide) 
film of 120 nm thick. The film was good in uniformity, adhesion and 
strength. Neither pinholes noticeable to the eye nor film irregularity 
caused due to repelling of the coating solution by the substrate upon 
coating were observed. Further, the coating solution did not develop any 
change in components and remained uniform even after stored at room 
temperature for 12 months. An additional coating test was performed in the 
same manner, using the coating solution which had been stored over 12 
months. No substantial differences were observed in rating between a film 
thus formed and the film formed 12 months ago. 
EXAMPLE 3: 
After washing a borosilicate glass substrate of 1.1 mm thick, it was 
subjected to transfer printing with a coating solution composed of the 
following components. 
______________________________________ 
(Composition of the coating solution) 
______________________________________ 
Serium(III) nitrate 40 parts 
Acetyl acetone 28 parts 
l,3-Dimethyl-2-imidazolidinone 
20 parts 
______________________________________ 
Drying of the thus-coated substrate was effected at 200.degree. C. for 10 
minutes in a conveyor-equipped electric furnace immediately after the 
transfer printing. The thus-dried substrate was thereafter baked at 
600.degree. C. for 60 minutes, thereby forming a Ce.sub.2 O.sub.3 film of 
65 nm thick. Neither pinholes noticeable to the eye nor film irregularity 
caused due to repelling of the coating solution by the substrate upon 
coating were observed on the film. The film was also good in both adhesion 
and strength. Further, the coating solution did not develop any change in 
components and remained uniform even after stored at room temperature for 
12 months. In addition, a similar coating test was performed using the 
coating solution which had been stored over 12 months. No substantial 
differences were observed in rating between a film thus formed and the 
film formed 12 months ago. 
EXAMPLE 4: 
After washing a metal aluminum substrate of 1.0 mm thick, it was coated 
with a coating solution composed of the following components by the 
roll-coating method. 
______________________________________ 
(Composition of the coating solution) 
______________________________________ 
Iron(III) acetyl acetonate 
25 parts 
Hexamethylphosphoramide 20 parts 
Ethyl alcohol 5 parts 
______________________________________ 
Drying of the thus-coated substrate was effected at 140.degree. C. for 15 
minutes in a recirculating hot air dryer immediately after the roll 
coating. The thus-dried substrate was thereafter transferred into an 
electric furnace, where it was baked at 300.degree. C. for 60 minutes, 
thereby forming an Fe.sub.2 O.sub.3 film of 56 nm thick. Neither pinholes 
noticeable to the eye nor film irregularity caused due to repelling of the 
coating solution by the substrate upon coating were observed on the film. 
The film was also good in both adhesion and strength. Further, the coating 
solution did not develop any change in components and remained uniform 
even after stored at room temperature for 12 months. In addition, a 
similar coating test was performed using the coating solution which had 
been stored over 12 months. No substantial differences were observed in 
rating between a film thus formed and the film formed 12 months ago. 
EXAMPLE 5: 
After washing a ceramic substrate of 1.0 mm thick, it was spin-coated at 
1,000 rpm with a coating solution composed of the following components. 
______________________________________ 
(Composition of the coating solution) 
______________________________________ 
Dibutyltin diacetate 5 parts 
Ethyl acetoacetate 1.9 parts 
Tin(II) trifluoroacetylacetonate 
12 parts 
Dimethylsulfoxide 60 parts 
Propylene glycol 40 parts 
Ethyl alcohol 20 parts 
______________________________________ 
After the spin coating, the thus-coated substrate was dried at 100.degree. 
C. for 10 minutes in a recirculating hot air dryer. The thus-dried 
substrate was thereafter transferred into an electric furnace, where it 
was baked at 700.degree. C for 60 minutes, thereby forming an SnO film of 
68 nm thick. The film has a sheet resistance of 8.3 k.OMEGA./.quadrature.. 
Neither pinholes noticeable to the eye nor film irregularity caused due to 
repelling of the coating solution by the substrate upon coating were 
observed. The film was also good in both adhesion and strength. Further, 
the coating solution did not develop any change in components and remained 
uniform even after stored at room temperature for 12 months. In addition, 
a similar coating test was performed using the coating solution which had 
been stored over 12 months. No substantial differences were observed in 
rating between a film thus formed and the film formed 12 months ago. 
Example 6: 
Following the procedure of Example 1, a soda glass substrate was coated 
with a coating solution composed of the following components, followed by 
its drying and baking. 
______________________________________ 
(Composition of the coating solution) 
______________________________________ 
Pentaethoxy tantalum 15 parts 
Nickel(II) nitrate 50 parts 
Acetyl acetone 45 parts 
l,3-Dimethyltetrahydro-2(1H)- 
30 parts 
pyrimidine 
Ethylene glycol monobutyl ether 
30 parts 
______________________________________ 
A Ta.sub.2 O.sub.5 -NiO base film having a thickness of 128 nm was formed 
on the substrate. Neither pinholes noticeable to the eye nor film 
irregularity caused due to repelling of the coating solution by the 
substrate upon coating were observed on the film. The film was also good 
in both adhesion and strength. Further, the coating solution did not 
develop any change in components and remained uniform even after stored at 
room temperature for 12 months. In addition, a similar coating test was 
performed using the coating solution which had been stored over 12 months. 
No substantial differences were observed in rating between a film thus 
formed and the film formed 12 months ago. 
EXAMPLE 7: 
After washing a soda glass substrate of 1.1 mm thick, its coating was 
conducted by dipped it in a coating solution, which had been obtained by 
mixing the following components thoroughly and then hydrolyzing the 
tetraethoxysilane, and thereafter pulling it out of the coating solution 
at a constant rate of 30 cm/min. 
______________________________________ 
(Composition of the coating solution) 
______________________________________ 
Tetraethoxysilane 30 parts 
Monomethyltriethoxysilane 
10 parts 
l,3-Dimethyl-2-imidazolidinone 
50 parts 
Acetyl acetone 54 parts 
Purified water 9.1 parts 
Hydrochloric acid 0.1 part 
(hydrolytic catalyst) 
______________________________________ 
After placing the thus-coated substrate in a recirculating hot air dryer 
and drying same at 140.degree. C. for 15 minutes, it was transferred into 
an electric furnace and then baked at 450.degree. C. for 60 minutes there, 
thereby to obtain a silica-type oxide film of 180 nm thick. 
The film was transparent. Pinholes noticeable to the eye were not observed. 
The film had high uniformity and was good in both adhesion and strength. 
The coating solution did not develop any change in components and remained 
uniform even after stored at room temperature for 12 months. 
An additional coating test was performed in the same manner, using the 
coating solution which had been stored over 12 months. No substantial 
differences were observed in rating between a film thus formed and the 
film formed 12 months ago. 
Comparative Example 1: 
A 5-inch silicon wafer was spin-coated at 3,000 rpm with a coating solution 
which had been obtained by mixing the following components thoroughly and 
then hydrolyzing the tetraethoxysilane. 
______________________________________ 
(Composition of the coating solution) 
______________________________________ 
Tetraethoxysilane 40 parts 
N--Methyl-2-pyrrolidone 30 parts 
Ethylene glycol monoethyl ether 
60 parts 
Purified water 7 parts 
Phosphoric acid (hydrolytic catalyst) 
0.1 part 
______________________________________ 
After drying the thus-coated wafer at 200.degree. C. for 10 minutes on a 
hot plate, it was transferred into an electric furnace and then baked at 
800.degree. C. for 30 minutes, thereby to obtain a PSG (silicon oxide) 
film of 135 nm thick. The film was good in uniformity, adhesion and 
strength. After the coating solution was stored at room temperature for 1 
week, it was however converted into a gel-like mixture of a milky white 
color and was no longer usable as a coating solution. 
COMATIVE EXAMPLE 2: 
After washing a soda glass substrate of 1.1 mm thick, it was dipped in a 
coating solution composed of the following components and then pulled out 
at a constant rate of 10 cm/min so as to perform its coating. 
______________________________________ 
(Composition of the coating solution) 
______________________________________ 
Triisopropoxy aluminum 10 parts 
Isopropyl alcohol 60 parts 
Dimethylformamide 20 parts 
______________________________________ 
After pulling out the soda glass substrate, it was dried at 100.degree. C. 
for 10 minutes in a recirculating hot dryer. The film became cloudy and 
fogging was observed all over the film. In addition, deposit of a white 
precipitate was observed in about 2 weeks when the coating solution was 
left over at room temperature.