CA1185056A - Method for formation of coating film of silicon oxide - Google Patents

Abstract

Abstract:
The invention provides a method for the formation of a coating film of silicon oxide on the surface of an inorganic substrate by applying a coating composition comprising a silicon compound onto the surface to form a coating composition layer, and then subjecting the surface having the coating composition layer thereon to a heat treatment to form a coating film of silicon oxide. The coating composition comprises a polar organic solvent having silicic acid dissolved therein, the molar ratio of water to silicic acid therein being not more than 5.
The coating film thus formed is uniform and does not have any pinholes or cracks.

Description

~ ~ ~5~i6 Method for formation of coating film of silicon_ox1de The presen~ invention relates to a method for the formation of a coating film of silicon oxide (SiO2) on the surface of an inorganic substrate.
Until now, silicon oxide films have usually been formed on the surface of an inorganic substrate by a sputtering method or a gas phase growth method. These methods require complex apparatus and do not usually form a coating film of uniform quality over a broad area.
In order to overcome these drawbacks, a method has been proposed wherein a coating composition comprising the reaction product of an alkoxysilane or a halogenated silane with a lower alkanoic acid and a lower alkanol is applied onto the surface of an inorganic substrate, followed by a heat treatment (cf. Japanese Patent 15 Publications (unexamined) Nos. 103533/75, lD4521/77, 34258/80 and 34276/8Q). However, the Eilm of silicon oxide formed by such method is not very satisfactory because it has pinholes, cracks, etc. Further, an extremely long time (e.g. 24 hours or more) is needed for the production of the reaction product, and therefore the productivity of the method is low.
Another method which has been proposed involves making use of a coating composition comprising the reaction product easily obtainable by hydrolyzing an alkoxysilane with water in an amount of not more than 8 mol per mol ..~

oE the alkoxysilane (cf. Japanese Patent P~lblication (unexamined) No. 93922/76); however, the coating ~ilm thereby formed is also not very satisfactory because it has pinholes, cracks, etc.
In all of the conventional methods mentioned above, the coating compositions comprise insufficiently hydro-lyzed products, and therefore cratering or unevenness is readily produced and a uniform coating film is dificult to obtain. In addition, the coating compositions tend to decompose during storage due to moisture in the air.
As a result of an extensive study, it has now been found that a coating composition comprising silicic acid dissolved in a polar organic solvent is quite suitable for the formation of a silicon oxide coating film of uniform quality.
According to one aspect of the invention there is provided a coatiny composition which comprises a polar organic solvent having silicic acid dissolved therein, the molar ratio of water to silicic acid being not more than 5.
According to another aspect of the present invention, there is provided a method for the formation of a coating film of silicon oxide on the surface of an inorganic substrate by applying a coating composition containing silicon compound onto the said surface to form a coating composition layer, and subjecting the said sur~ace having the coating composition layer thereon to a heat treatment to ~orm a silicon oxide coating film, wherein the coating composition comprises a polar organic solvent having silicic acid dissolved therein, the molar ratio of water 3~ (H2O) to silicic acid (Si(OH)4) being not more than S.
The coating composition of the invention, at least in the preferred forms, can produce a coating film having a uniform surface of high hardness having substantially no pinholes or cracksO Further t the coating composition can be prepared within a short period of time and the method can be carried out with high productivity. Moreover, the resulting coating composition is highly stable arld can be stored over a long period of time without any significant changeO
The coating composition may be prepared, for example, by acidifying a salt of silicic acid in an aqueous medium and extracting silicic acid from the resulting aqueous solution with a polar organic solvent.
The silicon compound preferably used as the starting material is a salt of silicic acid. Specific examples are synthetic silicates (eOg. silica sol, lithium silicate, potassium silicate, sodium silicate, ammonium silicate, calcium silicate, magnesium silicate, water glass, lithium water glass, potassium water glass, ~eolite X, ~eolite Y, zeolite A, zeolite L), natural silicate minerals (e~g.
chrysolite, hornblende, mical talc, wollastonite 9 analcite, serpentine, asbestos), etc.
For the preparation of the coating composition, the silicon compound is first acidified. This may be accom-plished, for example, by treatment of the silicon compound with an acid or a cationic exchange resin which can react with the base portion of the silicon compound in an aqueous medium. Examples of the acid are hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, sulfonic acid, chlorosulfonic acid, bromic acid, acetic acid, chloroacetic acid, formic acid, oxalic acid, propionic acid, acrylic acid, methacrylic acid, aceto-acetic acid, citric acid~ gluconic acid, glycollic acid, caprylic acid, caproic acid, etc. The amount of the acid depends upon the kind of the silicon compound, the acidity of the acid, the treating conditions, etc. and should be sufficient to convert the silicon compound into silicic acid, and is usually from 1 to 100 equivalents to the silicon compound. When the silicon compound is insoluble or only sparingly soluble in water, the amount of the acid may be increased to enhance the yield of silicic acid.

The acidification is normally effected at a temperature of 0 to 100C within a period o~ 10 minutes to 5 hours, and is ordinarily carried out at a pH of not more than 4, because silicic acid gel tends to be produced at a higher pH.
The resulting aqueous solution o~ silicic acid is then treated with a polar organic solvent to absorb silicic acid therein. When a precipitate of fine particles is present in the aqueous silicic acid solution, it should preferably be removed, for example by filtration prior to the extraction. Examples of the organic solvent are methanol, ethanol, propanol, butanol, tetrahydrofuran, dioxane, acetone, methylethylketone, methylisobutylketone, diethylketone, methyl acetate, ethyl acetate, etc. Of theseS methanol and tetrahydrofuran are preferred. The amount of the organic solvent should be sufficient to achieve a satisfactory transfer of the silicic acid from the water layer to the organic solvent layer, and is usually from 5 to 100 parts by weight per part by weight of silieic acid calculated as silicon oxide.
The kind and amount of the organic solvent determines whether or not the organic solvent layer can be properly separated from the water layer. When the separation is poor, an inorganic electrolyte may be incorporated therein so as to attain good or at least sufficient separation.
Thus, the presence of an inorganic electrolyte during the extraetion is effeetive in promoting easy and thorough separation between the organie solvent layer and the water layer. It also aeeelerates the transfer of silicic acid 3~ from the water layer to the organic solvent layer. Exam-ples of such inorganie electrolytes are sodium chloride, potassium chloride, ammonium chloride, calcium chloride, sodium sulfate, potassium sulfate, ammonium sulfate, magnesium sulfate, etc. The amount of the inorganic electrolyte employed should be sufficient to accomplish good separation and is normally from 1 to 20 parts by weight per part by weight of silicic acid calculated as f~ 3S6 silicon oxide.
In addition, or alternatively, the extraction may be effected in the presence of a solid dehydrating agent, or the aqueo~s silicic acid solution or the organic solvent layer containing the silicic acid may be treated with a solid dehydrating agent, so that water can be eliminated.
Examples of the solid dehydrating agent are silica gel, activated alumina, molecular sieve, dehydrated sodium sulfate, dehydrated cupric sulfate, etc.
In order to use the resultant organic solvent solu~
tion containing silicic acid as a coating composition, it should have a molar ratio of water to silicic acid of not more than 5, and preferably of not more than 3. When the molar ratio is more than 5, the formation of a coating film having no pinholes or eraeks eannot be assured. In addition, the film tends to be uneven. Therefore, when the organie solvent solution eontaining silicic acid has a molar ratio of water to silieie acid of more than 5, additional silieic acid should be added, or the solution should be subjeeted to treatment for water elimination in order to make a molar ratio 5 or less.
When desired, one or more additives may be ineor-porated into the organic solvent solution containing silieie acid. Examples of such additives are a poly-valent alcohol (e.g. ethylene glyeol, propylene glyeol,triethylene glyeol, glycerol), a chelate-forming compound (e.g. acetylacetone, ethyl acetoaeetate, diaeetone-aleohol), an oxygenated high boiling point solvent (e.g.
ethyleneglyeol monoethyl ether, diethyleneglyeol monoethyl ether, diethyleneglyeol diethyl ether)~ a high moleeular film forming agent (e.g. polyvinyl butyral, polyvinyl pyrrolidone, polyvinyl acetate), etc. Also, any modifier which is soluble in the organic solvent and can be baked to give an oxide may be ineorporated into the solution.
Examples of sueh modifiers are boron compounds (e.g. boric aeid, boron oxide, alkyl borate), phosphorus compounds (e.g. phosphoric acid, phosphorous acid, phosphorus "~:

pentoxide, pyrophosphoric acid, alkyl phosphate, alkyl-phosphin), arsenic compounds (e.g. arsenic acid, arsenous acid, alkyl arsenate, alkyl arsenic), antimony compounds (e.g. antimonic acid, antimony oxide, alkyl antimonate, alkyl antimony), etc.
The concentrations of the additives and the modifiers may be varied according to the conditions during use of the coating composition, the properties required for the coating film to be formed, etc. Usually, the concentra-tions of an oxygenated high boiling point solvent and ofa high molecular film forming agent are respectively not more than 40 % by weight and not more than 10 % by weight on the basis of the weight of the coating composition.
The modifier may be used in an amount of not more than 15 100 parts by weight, preferably not more than 50 parts by weight, calculated as its oxide to 100 parts by weight of silicic acid calculated as silicon oxide.
The thus obtained coating composition may be, as applied onto the surface of an inorganic substrate either such or after dilution with a polar organic solvent.
During application, the concentration of silicic acid in the composition is usually from 0.1 to 20 % by weight calculated as silicon oxide. The organic solvent used for the dilution may be the same as the one used for the extraction of silicic acid, or a different one.
Examples of the inorganic substrate to which the eoatlng composition is applieable are glass, ceramics, minerals, single crystals e.g. silicon and GaAs, etc.
Glass is particularly favorable.
Prior to the application of the coating composition, the surface of the inorganic substrate is preferably cleaned, for example, by washing, so that a more satis-factory coating film can be formed. The application may be accomplished by a conventional proeedure, e.g. a spraying method, dipping method, spin-on method or roll coater method. If necessary, the moisture content and the solvent vapor concentration in the atmosphere wherein the 5~;

application is effected may be appropriately controlled so as to assure uniformity of the coating film to be formed.
The inorganic substrate coated with the composition is ~hen subjected to a heat treatment (i.e. baking) at a tem-perature of not lower than 150C to produce a coating filmof silicon oxide.
A coating film of silicon oxide having a thickness of 0.02 to 5 ~m which is transparent and uniform and has substantially no pinholes or cracks, can be formed by the use of the coating composition of the invention.
The coating film of silicon oxide thus for~ed effect ively protects, both chemically and physically, the surface of glass, prevents the diffusion of impurities from semi-conductors or glass, provides electric insulation, controls electro-conductivity due to doping with impurities, assures the smoothness of the surface, etc. It is particularly useful as a film for protecting the dissolution of sodium from a glass used for liquid crystal displays, a film for the stabili~ation and separation of layers at the surface of a semi-conductor, a diffusion source film, a surface protecting film, a smoothing film for IC substrate, etc.
- Practical and presently preferred embodiments of the present invention are illustratively shown in the follow ing Examples wherein parts and percentages are by weight

2~ unless otherwise indicated.
Examples 1 to 5 and Comparative Examples 1 and 2 Sulfuric acid (0.64 mol/liter; 200 ml) was charged to a 500 ml volume flask equipped with a stirrer and water glass (JIS No. 2) (0.8 mol/liter; 100 ml~ was added dropwise thereto over 10 minutes to form an aqueous silicic acid solution having a molar ratio of water to silicic acid of 80.
Tetrahydrofuran (200 ml) and an inorganic electrolyte as shown in Table 1 were added to the aqueous silicic acid solution, and the resultant mixture was stirred ~igorously and then allowed to stand. The tetrahydrofuran layer containing silicic acid was separated and recovered. The .~, thus obtained organic silicic acid solution had a molar ratio of water to silicic acid as shown in Table 1.
The organic silicic acid solution was diluted with tetrahydrofuran to a concentration of 3 ~ of silicic acid calculated as silicon oxide. A glass plate was dipped in the diluted silicic acid solution, removed at a constant rate and then baked at 450C for 30 minutes to form a coating film of silicon oxide having a thickness of 1000 A.
The coating film was treated with an etching solution comprising 55 ~ hydrofluoric acid, 60 ~ nitric acid and water in a weight proportion of 15 : 10 : 600, and the etching rate was measured. The surface condition of the coating film was also observed by the naked eye and under an optical microscope. The results are shown in Table 1, in which cases wherein the molar ratio of water to silicic acid was more than 5 are also shown for comparison.
Table 1 _ _ _ _ Example Inorganic elec- Molar ratio Physical property trolyte of H O/Si(OH) KindAmount ¦Etching Surface (g) rate state 1 Sodium80 0.5 400Good chloride 2 Sodium40 2 450Good chloride

3 Ammonium 80 1 350 Good chloride

4 Ammonium 40 2.5 500 Good chloride Ammonium 60 4 650 Good sulfate Compara- Sodium 20 10 850Partly tive 1 chloride uneven Compara- Magnesium 20 50 1000 Partly tive 2 ~IE~ ~ , uneven, s~

Example 6 Dry silica gel (20 g) was added to the organic silicic acid solution (100 g) obtained in Comparative Example 1 and the resultant mixture was allowed to stand overnight, followed by filtration. Using the filtrate (the molar ratio of water to silicic acid being 3) as the coating composition, a coating film of silicon oxide was formed 2S in Example 1. The coating film had a good surface condition, and the etching rate was 600 A/min.
Example 7 A column having a diameter of 25 mm0 and a length of 500 mm equipped with a stop cock at the lower position was packed with ~-type cationic exchange resin "Amberlite IR-120B" (Trade Mark), and water glass (JIS No. 2) (0.8 mol/liter) was passed through the column for acidifica-tion. The aeidified solution (the molar ratio of water to silieic aeid being 24) (100 ml) was admixed with tetra-hydrofuran (200 ml) and transferred into a separation .f': ' funnel. After addition of sodium chloride (80 g)~ the Eunnel was shaken vigorously and then allowed to stand.
The tetrahydrofuran layer containing silieie aeid was then separated and recovered. The resulting organic silicic aeid solution had a molar ratio of water to silicic acid of not more than 0.5.
The organie silicic acid solution was diluted with tetrahydrofuran to a concentration of 3 % of silicic acid ealculated as silicon oxide. Using the diluted silicic acid solution as a coating composition, a coating film of silicon oxide was formed as in Example 1. The coating film had a good surface condition, and the etching rate was 450 ~/min.
Example 8 Concentrated hydrochloric acid (500 ml) and synth2tic zeolite A-4 of less than 200 mesh (20 g) were charged to a 1000 ml volume flask equipped with a stirrer, and stirring was continued at room temperature for 2 hours. The p~ was adjusted to 1 - 1.5 with aqueous ammonia. The resultant '~

- ~o mixture was filtered to eliminate precipitated materials.
The filtrate (the molar ratio of water to silicic acid being 125) was mixed with tetrahydrofuran (200 ml) and sodium chloride (120 9), stirred vigorously and allowed to stand. The tetrahydrofuran layer containing silicic acid was separated and recovered. The resulting organic silicic acid solution had a molar ratio of water to silicic acid of 2.
The organic silicic acid solution was diluted with tetrahydrofuran to a concentration of 3 % of silicic acid calculated as silicon oxide. Using the diluted silicic acid solution, a coating film of silicon oxide was formed as in Example 1. The coating film had a good surface condition and the etching rate was 600 A/min.
Examples 9 and 10 Boron oxide or phosphorus pentoxide as a modifier was added to the organic silicic acid solution obtained in Example 1 to a concentration of 0.1 ~ (i.e. 3.3 ~ to SiO2). The resulting organic silicic acid solution was diluted with tetrahydrofuran to a concentration of 3 ~
of silicic acid calculated as silicon oxide. Using the diluted solution, a coating film of silicon oxide was formed as in Example 1. The etching rate and the specific resistance of the coating film are shown in Table 2.
Comparative Example 3 Salicylic acid (10 g) was added at room temperature with stirring to a mixture of propyl orthosilicate (264 g), propionic acid (296 g) and propanol (240 g), and stirring was continued for 3 hours. After allowing the mixture to stand at room temperature for 4 days, a mix-ture of methanol and acetone (3 : 1 by weight) was added thereto to form a concentration of 5.9 % of silicic acid calculated as silicon oxide. Phosphorus pentoxide (loOl g) was dissolved in the resulting solution to form a coating composition. Using the coating composition, a coating film of silicon oxide was formed as in Example 1. The etching rate and the specific resistance of the coating ,~

5~6 ~ilm are shown in rrable 2.
Table 2 Example Modifier Film Etching Specific Surface thick- rate resistance state ness(~/min)(Q.cm) 9 B2O3 1200380 1 x 1015 Good P2o5 1300600 6 x 1014 Good 10 tive 3 P2O5 11008 2 03 x 1 Olr Goo d Comparative Example 4 A solution of water glass (JIS No. 3) (207 g) in water (93 9) was added dropwise to sulfurie aeid (3.5 mol/liter;
100 ml) at 15C. The reaction mixture was transferred to a separation funnel, tetrahydrofuran (400 ml) and sodium chloride (100 9) were added thereto, and the resultant mixture was shaken vigorously. The tetrahydrofuran layer eontaining silieie acid was separated and reeovered. The organie silieie aeid solution (37.4 g) was mixed with a solution of methyltriethoxysilane (21.4 g) in tetra-hydrofuran (44.8 g) to form a eoating eomposition.
Using the eoating eomposition, a coating film of silieon oxide was formed as in Example 1~ The eoating film had a good surfaee condition. Sinee it was not wettable to an etching solution, the etehing rate eould not be measured.

,~h=, ~ ~ i

Claims (12)

Claims:

1. A coating composition which comprises a polar organic solvent having silicic acid dissolved therein, the molar ratio of water to silicic acid being not more than 5.

2. A coating composition according to claim 1, wherein the silicic acid is prepared by acidifying a salt of silicic acid with an acid in an aqueous medium.

3. A coating composition according to claim 2, wherein the silicon compound is chosen from water glass, sodium silicate, potassium silicate and potassium water silicate.

4. A coating composition according to claim 2, wherein the acid is chosen from hydrochloric acid, sulfuric acid and nitric acid.

5. A coating composition according to claim 1, prepared by acidifying a salt of silicic acid with an acid in an aqueous medium and treating the resultant aqueous silicic acid solution with a polar organic solvent to extract the resulting silicic acid.

6. A coating composition according to claim 1, wherein the treatment of the aqueous silicic acid solution with the polar organic solvent is carried out in the presence of an inorganic electrolyte.

7. A coating composition according to claim 1, which further comprises at least one modifier chosen from boron compounds, phosphorus compounds, arsenic compounds and antimony compounds.

8. A coating composition according to claim 7, wherein the modifier is used in an amount of not more than 100 parts by weight calculated as its oxide to 100 parts by weight of silicon oxide after baking.

9. A method for the formation of a coating film of silicon oxide on the surface of an inorganic substrate which comprises applying a coating composition containing a silicon compound onto the said surface to form a coating composition layer, and subjecting the said surface having the coating composition layer thereon to a heat treatment to form a silicon oxide coating film, wherein the coating composition comprises a polar organic solvent having silicic acid dissolved therein, the molar ratio of water to silicic acid being not more than 5.

10. A method according to claim 9, wherein the inorganic substrate is a glass or silicon wafer.

11. An article comprising an inorganic substrate having a substantially pinhole- and crack-free, uniform coating film of silicon oxide formed thereon by the method according to claim 9.

12. An article according to claim 11, which is a glass or silicon wafer.