A coating fluid for forming an oxide coating

Abstract: A coating fluid for forming an oxide coating, having a good thermal stability and superior coating-forming properties is provided, which fluid comprises a reaction product obtained by subjecting (A) an alkoxysilane or aryloxysilane expressed by the formula RmSi(OR) 4-m wherein R is 1-4C alkyl or aryl and m is 0 to 2 and (B) a metal alkoxide or aryloxide compound expressed by the formula M(OR′) n wherein M is Mg, B, P, Zr, Y, Ti or Ba, R′ is 1-4C alkyl or aryl and n is a valence of the metal atom to hydrolysis and condensation using a catalyst in the presence of a solvent.

Description: This invention relates to a coating fluid for forming an oxide coating which is thermally stable and has good coating properties and a method for forming an oxide coating using the same. Heretofore, as a process for layer insulation of semiconductors such as IC, LSI, etc., processes for forming an oxide coating on a substrate by calcining hydrolyzed and condensed products of silanol compounds have been well known. Among these processes, a process using tetrafunctional silanes such as tetraethoxysilane (ethylsilicate), etc. has been most often studied, but according to such a process using tetrafunctional silanes only, there is a drawback that when a silica coating is formed by calcination, the resulting three-dimensional crosslinked structure is so dense and rigid that the resulting coating is thick to cause cracks. As a process for overcoming such a drawback, a process of cohydrolyzing bifunctional or trifunctional silanes together with tetrafunctional silanes is disclosed in JP-A-57191219, but according to such a process, there is also a drawback that a large quantity of carbon is contained in the resulting coating. If carbon is left in the coating after calcination, cracks are liable to occur in the coating at the step of semiconductor production. Further, in order to eliminate the carbon contained in the coating, calcination at high temperature of 500°C or higher is required and since the coating shrinks due to the elimination of carbon or the difference between the thermal expansion coefficient of the coating and that of a substrate such as silicon, aluminum, etc. after the elimination is so large, there is also a drawback that the coating is cracked. EP-A-0008215 discloses a method of preparing alkoxide coatings on substrates. A clear solution is prepared by reacting metal alkoxides with a mixture of critical amounts of water and/or acid in an alcohol diluted medium. The alkoxides can be Ti(OR)₄ or Ta(OR)₅, or Si(OR)₄ in admixture with these alkoxides. The coatings are deposited by applying the alkoxide solution to the substrate then heating the coating at over 350°C. US-A-3847,583 discloses a process for producing glassy, crystalline or glassy crystalline oxide multi-component substances, which are produced without going through a molten phase. An alkali or alkali earth metal compound and at least one other metal compound, both dissolved in an organic solvent are reacted in the solvent, the solvent is evaporated to form a precipitate, and the precipitate is heated to form the multi-component substance. These compositions are applied as coatings to substrates e.g. glasses. The present invention provides a coating fluid for forming an oxide coating on a substrate, which comprises a reaction product obtained by subjecting: (A) a silane compound expressed by the formula RmSi(OR) 4-m wherein R represents an alkyl group of 1 to 4 carbon atoms or an aryl group and m represents an integer of 0 to 2 and (B) an organic metal compound expressed by the formula M(OR') n wherein M represents a atom, R' represents an alkyl group of 1 to 4 carbon atoms or an aryl group and n represents a valence of the atom, to hydrolysis and condensation by the use of a catalyst in the presence of a solvent, characterised in that the atom is selected from magnesium, boron, phosphorous, zirconium, yttrium or barium. The present invention also provides a process for forming a coating oxide on a substrate which comprises A process for forming a coating oxide on a substrate which comprises a step of coating a coating fluid on a substrate, said coating fluid comprising a reaction product obtained by subjecting (A) a silane compound expressed by the formula RmSi(OR) 4-m wherein R represents an alkyl group of 1 to 4 carbon atoms or an aryl group and m represents an integer of 0 to 2 and (B) an organic compound expressed by the formula M(OR') n wherein M represents a atom, R' represents an alkyl group of 1 to 4 carbon atoms or an aryl group and n represents a valence of the atom, to hydrolysis and condensation by the use of a catalyst in the presence of a solvent; and a step of calcining said coated substrate at a temperature of 400-800°C: characterised in that the atom is selected from magnesium, boron, phosphorous, zirconium, yttrium or barium; and in that, before the calcining step, there is a step of drying said coated substrate at a temperature of 50-200°C. The preferred embodiments of the present invention will now be described by way of example only with reference to the numbered examples. The silane compound used in the present invention is expressed by the formula RmSi(OR) 4-m and its concrete examples are tetrafunctional silanes such as Si(OCH₃)₄, Si(OC₂H₅)₄, Si(OC₃H₇), etc., trifunctional silanes such as CH₃Si(OCH₃)₃, CH₃Si(OC₂H₅)₃, CH₃Si(OC₃H₇)₃, C₂H₅Si(OCH₃)₃, C₆H₅Si(OCH₃)₃, CH₃Si(OC₆H₅)₃, etc. and bifunctional silanes such as (CH₃)₂Si(OCH₃)₂, (CH₃)₂Si(OC₂H₅)₂, (CH₃)₂Si(OC₃H₇)₂, (C₂H₅)₂Si(OCH₃)₂, (C₆H₅)₂Si(OCH₃)₂, (CH₃)₂Si(OC₆H₅)₂, etc. These alkoxysilanes may be used singly or in admixture of two or more members thereof. The organic metal compound used in the present invention is expressed by the formula M(OR') n and its concrete examples are B(O i-C₃H₇)₃, Mg(OC₃H₇)₂, P(O i-C₃H₇)₃, etc. These metal compounds may be used singly or in admixture of two or more members thereof. Further, the above-mentioned R and R' may be the same or different. As to the proportions of the silane compound and the organic metal compound used in the present invention, it is preferred in the aspects of coating properties, carbon residue, etc. that the proportion of the silane compound be in the range of 70 to 90% by mol and that of the organic metal compound be in the range of 10 to 30% by mol. Further, the silane compound is preferred to be a tetrafunctional silane Si(OR)₄ singly or a mixture of 20 to 40% by mol of a tetrafunctional silane Si(OR)₄, 20 to 60% by mol of a trifunctional silane RSi(OR)₃ and 0 to 40% by mol of a bifunctional silane R₂Si(OR)₂. As the solvent used in the present invention, amide solvents such as N,N-dimethylformamides which do not react with alkyl groups or aryl groups,alcohol solvents which have the same carbon atoms as those of the alkyl or aryl group in the silane compound, etc. are preferably used in the aspect of coating properties. These solvents may be used in admixture. Examples of the reaction catalyst used in the present invention are inorganic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, boric acid, hydrofluoric acid, etc., oxides such as phosphorus pentoxide, boron oxide and organic acids such as oxalic acid, etc. The quantity of the catalyst added is preferably in the range of 0.1 to 5% by weight based on the total weight of the silane and the organic metal compound. The coating fluid of the present invention is obtained by subjecting the silane compound and the organic metal compound to hydrolysis and condensation by the use of a catalyst as described above in the presence of a solvent. Further, the thermal expansion coefficient of the oxide coating obtained using the resulting solution can be optinally varied by selecting the kind and quantity of the organic metal compound. In forming the oxide coating with the coating fluid of the present invention, the coating fluid is coated on the surface of a substrate such as silicon, glass, ceramics, aluminium, etc. by means of spinner, brush, spray, etc., followed by drying usually at 50 to 200°C, preferably 100 to 150°C and then calcining usually at 400 to 800°C, preferably 400 to 500°C. The oxide coating obtained using the coating fluid of the present invention is smaller in the carbon content than oxide coatings obtained using conventional silanol condensates, and an oxide of e.g. Mg, P, Zr, Y, or Ba is contained therein as a second component to form a reaction product with SiO₂, whereby the resulting coating is thermally stable and good coating properties is obtained. Example 1 Si(OCH₃)₄ (51g), CH₃Si(OCH₃)₃ (45g), (CH₃)₂Si(OCH₃)₂ (12g), B(O i-C₃H₇)₃ (31g) and Mg(OC₃H₇)₂ (10g) were dissolved in a mixed solvent of N,N-dimethylformamide (160g) and methanol (40g), followed by adding to the solution, a solution (55g) of oxalic acid (0.6g) in water and subjecting the mixture to hydrolysis and condensation to prepare a solution of the reaction product. This solution was coated on a Si wafer by means of a spinner at 3,000 rpm, followed by drying at 150°C for one hour and then calcining in an electric oven at 400°C for one hour to obtain a colorless, transparent silica coating without any crack. The coating thickness of the silica coating was measured by means of a surface roughness meter (TALYSTEP, trademark of product made by RANK TAYLOR HOBSON Co. LTD.) to give 0.7 µm. Further, when the inflared absorption spectra of the coating was measured by means of an inflared spectrophotometer, absorptions of Mg-O and B-O bonds were observed besides Si-O-Si absorption; thus it was confirmed that the coating was a complete oxide coating. Further, when the oxide coating was treated by means of a barrel type oxygen plasma ashing device (PR-501A, tradename of product made by Yamato Kagaku Co. LTD.) at 400W for 20 minutes, no crack was observed in the coating. Further, when coating of the above solution was carried out onto a Si wafer having an aluminum pattern having a thickness of 0.7 µm and a line and space width of 0.5 to 5 µm deposited thereonto under the same conditions as the above, a colorless, transparent oxide coating without any crack was obtained. Further, the above solution was dried at 150°C for 3 hours, followed by subjecting the resulting powder to compression molding into the form of pellets of 12 mm in diameter and calcining the pellets in an electric oven at 1,000°C for one hour. The thermal expansion coefficient of the resulting sample was measured by means of a thermophysical tester (TMA 8,150 type, tradename of product made by Rigaku Denki Co. LTD.) to give an average linear thermal expansion coefficient at room temperature to 450°C of 7.0 × 10⁻⁶. Example 2 Si(OC₂H₅)₄ (145g), P(OC₃H₇)₃ (41g) and Mg(OC₃H₇)₂ (14g) were dissolved in ethyl alcohol (300g), followed by adding to the solution, a solution (66g) of oxalic acid (0.8g) in water and subjecting the mixture to hydrolysis and condensation to prepare a solution of the reaction product. When the solution was coated onto a Si wafer, followed by drying and calcining under the same conditions as in Example 1 to obtain a colorless, transparent silica coating having a coating thickness of 0.5 µm and no crack. Further, when coating of the above solution was carried out on a Si wafer having an aluminum pattern deposited thereon under the same conditions as in Example 1, a colorless, transparent oxide coating without any crack was obtained. Example 3 Si(OC₂H₅)₄(69g), CH₃Si(OC₂H₅)₃ (59g), (CH₃)₂Si(OC₂H₅)₂ (31g) and B(O i-C₃H₇)₃ (31g) were dissolved in a mixed solvent of ethanol (26g) and N,N-dimethylformamide (105g), followed by adding to the solution, a solution of oxalic acid (0.6g) in water (56g) and subjecting the mixture to hydrolysis and condensation to prepare a solution of the reaction product. When coating of this solution was carried out onto a Si wafer and a Si wafer having an aluminum pattern deposited thereon under the same conditions as in Example 1, a colorless, transparent oxide coating having a coating thickness of 0.7 µm without any crack was obtained. Example 4 Si(OCH₃)₄ (51g), CH₃Si(OCH₃)₃ (30g), C₆H₅Si(OCH₃)₃ (22g), (CH₃)₂Si(OCH₃)₂ (12g), B(O i-C₃H₇)₃ (31g) and Mg(OC₃H₇)₂ (10g) were dissolved in diethylene glycol diethyl ether (200g), followed by adding to the solution, a solution (55g) of phosphoric acid (0.5g) in water and subjecting the mixture to hydrolysis and condensation to prepare a solution of the reaction product. This solution was coated on a Si wafer and a Si wafer having an aluminum pattern deposited thereon, followed by drying and calcining under the same conditions as in Example 1 to obtain a colorless, transparent oxide coating having a coating thickness of 0.8 µm without any crack. Comparative example 1 Si(OC₂H₅)₄ (35g) was dissolved in a mixed solvent of ethanol (64g) and ethyl acetate (26g), followed by adding to the solution, a solution of oxalic acid (0.5g) in water (12g), followed by subjecting the mixture to hydrolysis and condensation to prepare a silanol oligomer solution. When this solution was coated onto a Si wafer in the same manner as in Example 1, a coating of about 0.4 µm thick was obtained, but a large number of cracks were observed in the coating. Comparative example 2 Si(OCH₃)₄ (17g), CH₃Si(OCH₃)₃ (25g) and (CH₃)₂Si(OCH₃)₂ (5g) were dissolved in a mixed solvent of N,N-dimethylformamide (48g) and methanol (6g), followed by adding to the solution, a solution of phosphoric acid (0.5g) in water (20g) and subjecting the mixture to hydrolysis and condensation to prepare a silanol oligomer solution. When this solution was coated onto a Si wafer in the same manner as in Example 1, a coating of about 0.7 µm thick was obtained. When the absorption spectra of the coating were measured by means of an infrared spectrophotometer, a strong absorption of Si-CH₃ was observed besides the absorption of Si-O-Si, that is, it was confirmed that a complete SiO₂ coating was not formed. Further, when the coating was subjected to an oxygen plasma treatment at 400W for 20 minutes, cracks occurred in the coating. The coating fluid for forming an oxide coating of the present invention is thermally stable and superior in the coating properties; hence cracks do not occur in the oxide coating of even about 1.5 µm or more formed on the surface of a substrate using the coating fluid. Thus, the coating fluid for forming an oxide coating of the present invention is effective for coating electronic parts, particularly coating for step-covering on multilevel inter connection of semiconductors, planarizing the element surface of magnetic bubble domain memory, etc. The present invention aims to provide a coating fluid for forming an oxide coating having overcome the above-mentioned drawbacks of the prior art and having a good thermal stability and superior coating properties. The inventors of the present Invention have made extensive research in order to achieve the above-mentioned aims and as a result have noted that in order to form an oxide coating without any cracks on a substrate such as silicon, aluminum, etc. and further without any occurrence of cracks even at the time of the subsequent oxidizing step such as oxygen plasma treatment, it is necessary to use a coating fluid satisfying conditions of (1) reducing the strain of curing shrinkage at the time of calcination, (2) bringing the thermal expansion coefficient of the coating close to that of the substrate, and (3) making the carbon content in the coating very low or nil and such a coating fluid is obtained by subjecting a specified compound to hydrolysis and condensation by the use of a catalyst in the presence of a solvent; thus we have achieved the present invention.

Claims: A coating fluid for forming an oxide coating on a substrate, which comprises a reaction product obtained by subjecting: (A) a silane compound expressed by the formula RmSi(OR) 4-m wherein R represents an alkyl group of 1 to 4 carbon atoms or an aryl group and m represents an integer of 0 to 2 and (B) an organic compound expressed by the formula M(OR') n wherein M represents an atom, R' represents an alkyl group of 1 to 4 carbon atoms or an aryl group and n represents a valence of the atom, to hydrolysis and condensation by the use of a catalyst in the presence of a solvent, characterised in that the atom is selected from magnesium, boron, phosphorous, zirconium, yttrium or barium. A coating fluid for forming an oxide coating on a substrate according to claim 1, wherein said silane compound is selected from Si(OC₂H₅)₄, Si(OC₃H₇), CH₃Si(OCH₃)₃, CH₃Si(OC₂H₅)₃, CH₃Si(OC₃H₇)₃, C₂H₅Si(OCH₃)₃, C₆H₅Si(OCH₃)₃, CH₃Si(OC₆H₅)₃, (CH₃)₂Si(OCH₃)₂, (CH₃)₂Si(OC₂H₅)₂, (CH₃)₂Si(OC₃H₇)₂, (C₂H₅)₂Si(OCH₃)₂, (C₆H₅)₂Si(OCH₃)₂ and (CH₃)₂Si(OC₆H₅)₂. A coating fluid for forming an oxide coating on a substrate according to claim 1, wherein said organic metal compound is selected from B(O i-C₃H₇)₃, Mg(OC₃H₇)₂ and P(O i-C₃H₇)₃ A coating fluid for forming an oxide coating or, a substrate according to claim 1, wherein the proportion of said silane compound is in the range of from 70 to 90% by mol and that of said organic compound is in the range of from 10 to 30% by mol. A coating fluid for forming an oxide coating on a substrate according to claim 4, wherein said silane compound consists of a tetrafunctional silane or a mixture of from 20 to 40% by mol of a tetrafunctional silane from 20 to 60% by mol of a trifunctional silane and from 0 to 40% by mol of a bifunctional silane. A coating fluid for forming an oxide coating on a substrate according to claim 1, wherein said solvent is selected from amides and alcohols. A coating fluid for forming an oxide coating on a substrate according to claim 1, wherein said catalyst is selected from hydrochloric acid, sulfuric acid, phosphoric acid, boric acid, hydrofluoric acid, phosphorus pentoxide, boron oxide and oxalic acid. A process for forming a coating oxide on a substrate which comprises a step of coating a coating fluid on a substrate, said coating fluid comprising a reaction product obtained by subjecting (A) a silane compound expressed by the formula RmSi(OR) 4-m wherein R represents an alkyl group of 1 to 4 carbon atoms or an aryl group and m represents an integer of 0 to 2 and (B) an organic compound expressed by the formula M(OR') n wherein M represents an atom, R' represents an alkyl group of 1 to 4 carbon atoms or an aryl group and n represents a valence of the atom, to hydrolysis and condensation by the use of a catalyst in the presence of a solvent; and a step of calcining said coated substrate at a temperature of 400-800°C: characterised in that the atom is selected from magnesium, boron, phosphorous, zirconium, yttrium or barium; and in that, before the calcining step, there is a step of drying said coated substrate at a temperature of 50-200°C. A process for forming a coating oxide on a substrate according to claim 8, wherein said silane compound is selected from Si(OC₂H₅)₄, Si(OC₃H₇), CH₃Si(OCH₃)₃, CH₃Si(OC₂H₅)₃, CH₃Si(OC₃H₇)₃, C₂H₅Si(OCH₃)₃, C₆H₅Si(OCH₃)₃, CH₃Si(OC₆H₅)₃, (CH₃)₂Si(OCH₃)₂, (CH₃)₂Si(OC₂H₅)₂, (CH₃)₂Si(OC₃H₇)₂, (C₂H₅)₂Si(OCH₃)₂, (C₆H₅)₂Si(OCH₃)₂ and (CH₃)₂Si(OC₆H₅)₂. A process for forming a coating oxide on a substrate according to claim 8, wherein said calcining temperature is in the range of 400°C to 500°C.

Inventors: MORISHIMA HIROYUKI YAMAZAKI WO, UCHIMURA SHUN-ICHIRO YAMAZAKI, MORISHIMA, HIROYUKI YAMAZAKI WORKS OF, UCHIMURA, SHUN-ICHIRO YAMAZAKI WORKS OF
Applicants: HITACHI CHEMICAL CO LTD, HITACHI CHEMICAL CO., LTD.
Application year: 1989
Main classification: C23C18
IPC codes: C09D183, H01L23, H01L21, C08G77, C08G79, C09D185, C23C18