Process for coating a substrate with a silica precursor

Abstract: The present invention relates to a method of forming a silica coating on a substrate. The method comprises coating a substrate with a silica precursor having a melting point between about 50 and about 450°C. The coating is heated to a temperature above its melting point in an inert environment to allow the coating to melt and flow. The melted coating is then heated in an environment which facilitates conversion to silica for a time sufficient to convert it to silica.

Description: The present invention relates to a method of forming a silica coating on a substrate. The method comprises applying a silica precursor coating to the substrate and heating it to a temperature above its melting point in an inert environment. The melted coating is then heated in an environment which facilitates conversion to silica for a time sufficient to convert it to silica. The use of ceramic coatings for their protective and dielectric effect is known in the electronics art. For instance, Haluska et al. in U.S. Patent No. 4,756,977 disclose silica coatings produced by applying a solution of hydrogen silsesquioxane resin to an electronic device and then ceramifying by heating in air at temperature of 200-1000°C. The ceramic coatings produced thereby have many desirable characteristics such as microhardness, moisture resistance, high adhesion, etc. The document EP-A-0198976 discloses a process wherein an alkyl polysilsesquioxane resin is applied on a substrate, dried and then melted and cured under nitrogen at 350°C. The present inventors have now discovered that sequential heating first in an inert environment and then in a reactive environment which facilitates conversion to silica produces coatings having improved planarity and reduced cracks in comparison to conventionally formed silica coatings. The present invention relates to a method of forming a silica layer on a substrate as defined in claim 1 comprising: (a) forming a coating on a substrate by a method comprising dissolving a silica precursor which is a hydrogen silsesquioxane resin having a melting point between 50 and 450°C in a solvent to form a solution, applying the solution to the substrate, and evaporating the solvent, (b) heating the coating to a temperature above the melting point of the silica precursor in an inert environment for a time sufficient to allow the coating to flow; and (c) after the coating has flowed, exposing the coating to a reactive environment and heating at a temperature of 50°C to 1000°C for a time sufficient to convert the coating to a silica layer. Fig 1 and 2 show a coating obtained by the present invention and by a process of the prior art. The present invention is based on the discovery that silica precursors having melting points between about 50 and about 450°C. will reproducibly melt and flow under an inert environment prior to ceramification. This discovery is used in this invention to cause the silica precursor to controllably melt and evenly flow across the surface of a substrate prior to its conversion to silica. This process is particularly advantageous over the prior art which suggest heating the coating in an oxidizing environment since such a process results in either no or very erratic flow due to the simultaneous initiation of melting along with oxidation and cross-linking in the coating. Similarly, this process is also advantageous over the prior art which completes the conversion of the precursor to silica entirely in an inert environment since high temperatures are often necessary in such a process. The sequential use of an inert environment and an oxidative environment as described herein thus allows for desirable flow characteristics and low temperature ceramification. The coatings taught herein are useful as protective and dielectric layers on a variety of substrates. For instance, the planarizing and dielectric properties of these coatings make them ideally suited for use on electronic devices as interlevel dielectrics in multilayered devices. Similarly, the high density and low defect rate of these coatings makes them useful for protecting electronic devices from environmental contaminants such as moisture and ionic impurities. Despite the specific applications set forth above, however, it is contemplated that the coatings taught herein can be used on any substrate requiring such a coating. Moreover, the choice of such substrates and devices is limited only by the need for thermal and chemical stability of the substrate at the temperature and in the environment utilized in the present invention. As used in the present invention, the expression "silica" refers to those pyrolyzed materials containing Si-O-Si bonds which may or may not be fully free of residual Si-C, Si-H, Si-OH and/or Si-OC but which are otherwise ceramic in character; the expression "planarizing coating" refers to a coating which is less irregular than the surface prior to application of the coating; and the expressions "electronic device" or "electronic circuit" include, but are not limited to silicon based devices, gallium arsenide based devices, focal plane arrays, opto-electronic devices, photovoltaic cells and optical devices. Specific examples of such devices include transistor-like devices, capacitor and capacitor-like devices, multilayer devices, 3-D devices, silicon on insulator devices, super lattice devices and the like. The silica coatings taught herein are produced by a process which comprises applying a preceramic coating comprising a hydrogen silsesquioxane resin as silica precursor having a melting point between about 50 and about 450°C. to the substrate. The preceramic coating is then heated above the melting point of the silica precursor in an inert environment for a time sufficient to allow the coating to flow but at a temperature and for a time insufficient to cause the coating to ceramify. The melted coating is then heated in an environment which facilitates conversion to silica for a time and at a temperature sufficient to convert the melted preceramic coating to silica. The silica precursor materials which are useful herein are hydrogen silsesquioxane resins which have a melting point between about 50 and about 450°C. under an inert atmosphere and which can be converted to silica at temperatures less than about 1000°C. The term hydrogen silsesquioxane resin or H-resin is used in this application to describe various hydridosilane resins of the formula HSi(OH) x (OR) y O 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. Though not represented by this structure, these resins may contain a small number of silicon atoms which have either 0 or 2 hydrogen atoms attached thereto due to various factors involved in their formation or handling. The above hydridosilane resins are generally produced by a process comprising the hydrolysis and condensation of silanes of the formula HSiX 3 , wherein X is a hydrolyzable group. These reactions may result in a fully condensed (HSiO 3/2 ) n resin or the hydrolysis and/or condensation may be interrupted at an intermediate point such that partial hydrolyzates (containing Si-OR groups) and/or partial condensates (containing SiOH groups) are formed. Such methods include that of Collins et al. in U.S. Patent No. 3,615,272. It is described therein that a nearly fully condensed H-resin (which may contain up to 100-300 ppm silanol) can be produced by hydrolyzing trichlorosilane in a benzenesulfonic acid hydrate hydrolysis medium and then washing the resultant resin with water or aqueous sulfuric acid. Bank et al. in U.S. Patent No. 5,010,159 teach an alternative method comprising hydrolyzing hydridosilanes in an arylsulfonic acid hydrate hydrolysis medium to form a resin which is then contacted with a neutralizing agent. A preferred embodiment of this latter process uses an acid to silane ratio of about 6/l. The hydrogen silsesquioxane silica precursor is first applied to the substrate to form a preceramic coating by coating the substrate with a solution comprising a solvent and the silica precursor. Removal of the solvent by evaporation yields the preceramic coating. Using the above solution method, the silica precursor is first dissolved in a solvent. Various facilitating measures such as stirring and/or heating may be used to aid in the dissolution. Solvents which may be used include any which will dissolve the silica precursor to form a solution or suspension without affecting the coating. These solvents can include, for example, alcohols such as ethyl or isopropyl, aromatic hydrocarbons such as benzene or toluene, alkanes such as n-heptane or dodecane, ketones, esters, glycol ethers or cyclic dimethylpolysiloxanes, in an amount sufficient to dissolve the above materials to low solids. Generally, enough of the above solvent is used to form a 0.1-50 weight percent solution. A platinum, rhodium or copper catalyst may also be added herein to increase the rate and extent of conversion to silica. Generally, any platinum, rhodium or copper compound or complex which can be solubilized will be functional. For instance, a composition such as platinum acetylacetonate, rhodium catalyst RhCl 3 [S(CH 2 CH 2 CH 2 CH 3 ) 2 ] 3 , obtained from Dow Corning Corporation, Midland, Michigan, U.S.A., or cupric naphthenate are all within the scope of this invention. These catalysts are generally added in an amount of between about 5 to 1000 ppm platinum, rhodium or copper based on the weight of hydridosilane. The coating solution is applied by techniques such as spin coating, dip coating, spray coating or flow coating. Evaporation of the solvent by any suitable means such as simple air drying by exposure to an ambient environment or by the application of a vacuum or mild heat yields the desired preceramic coating. The resultant preceramic coating is then heated above the melting point of the silica precursor in an inert environment for a time sufficient to allow the coating to flow but at a temperature and for a time insufficient to cause the coating to ceramify. The expression "melting point" is used herein to describe that temperature at which an amorphous solid is converted to a viscoelastic resin which will flow across the surface of the substrate. This process allows the silica precursor to evenly distribute on the surface of the substrate and planarize any uneven topography. In addition, it allows the surface of the substrate to be sealed and assists in healing and/or preventing pinholes, cracks or other surface anomalies. Generally, any inert environment which does not react with the coating can be used during this heating step. For instance, environments comprising argon, helium or nitrogen are all functional herein. Similarly, the use of vacuum will also provide the necessary environment for flow. The temperature used during this step should be one which melts the silica precursor without initiating ceramification during the required flow time. Temperatures in the range of about 50 to about 450°C. are generally sufficient, preferably 150 to 400°C. The heating time necessary during this step should be one which allows the resin to flow and yet does not initiate ceramification at the chosen temperature. Generally, this time can vary from a fraction of a second to several hours. For practical reasons, it is preferred that the time be less than about 1 hour and more preferred that the time be less than about 15 minutes. After the resin has melted and flowed, the environment is altered to facilitate conversion of the melted silica precursor to silica. Reactive environments such as those comprising air, O 2 , oxygen plasma, ammonia, amines, etc. may be used. It is especially preferred, however, to heat under a wet ammonia atmosphere to hydrolyze the Si-H bonds and then under a dry ammonia atmosphere to effect removal of any remaining Si-OH groups. The melted preceramic coating is subjected to such reactive environments at a temperature sufficient for ceramification. Generally, this temperature is about 20 to about 1000°C. depending on the reactive atmosphere. Preferred temperatures are in the range of about 50 to about 800°C. Higher temperatures usually result in quicker and more complete ceramification, but said temperatures also may have detrimental effects on various temperature sensitive substrates. For temperature sensitive substrates, temperatures in the range of about 100 to about 400°C. are often preferred. The preceramic coatings are usually subjected to these temperatures for a time sufficient to ceramify the coating, generally up to about 6 hours, with a range of up to about 2 hours being preferred. Any source of heat such as a hot plate, a convection oven, rapid thermal processing or radiant or microwave energy can be used during either pyrolysis step. By the above methods a silica planarizing coating is produced on the substrate. The coating smooths the irregular surfaces of various substrates and has excellent adhesion. This coating may also be covered by other coatings such as further SiO 2 coatings, SiO 2 /modifying ceramic oxide layers, silicon containing coatings, silicon carbon containing coatings, silicon nitrogen containing coatings, silicon nitrogen carbon containing coatings and/or diamond like carbon coatings. Such multilayer coatings are taught in U.S. Patent No. 4,973,526. Example 1 Patterned silicon wafers were coated with 6 mL of a 25 weight percent solution of hydrogen silsesquioxane resin in methyl isobutyl ketone and spun at 2000 rpm for 10 seconds in air. The coated wafers were then placed in a nitrogen environment and heated on a hot plate at 150°C. for 1 minute, 200°C. for 1 minute and 300°C. for 1 minute. The wafers with the melted coating were then heated by rapid thermal processing in oxygen at 400°C. for 10 minutes. The resultant coating was a completely planar silica coating as shown in Figure 1. By contrast, Figure 2 shows a cross-section of a commercially available device coated with Accuglass™ spin-on glass and processed according to the instructions provided in the product literature. It is evident that the process of the present invention provides a more planar surface.

Claims: A method of forming a silica layer on a substrate comprising: (a) forming a coating on a substrate by a method comprising dissolving a silica precursor in a solvent to form a solution, wherein the silica precursor is a hydrogen silsesquioxane resin of the formula HSi(OH) x (OR) y O 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 and x+y+z=3, and having a melting point between 50 and 450°C; applying the solution to the substrate, and evaporating the solvent; (b) heating the coating to a temperature above the melting point of the hydrogen silsesquioxane resin in an inert environment for a time sufficient to allow the coating to flow; and (c) after the coating has flowed, exposing the coating to a reactive environment and heating at a temperature of from 50°C to 1000°C for a time sufficient to convert the coating to a silica layer. A method according to claim 1, wherein the inert environment comprises helium, argon, or nitrogen. A method according to claim 1 or 2, wherein the reactive environment is selected from air, oxygen, oxygen plasma, ammonia and amines. A method according to claim 1, wherein the hydrogen silsesquioxane resin is heated to a temperature of 150 to 400°C. A method according to any of claims 1 to 4, wherein the silica precursor is a fully condensed (HSiO 3/2 ) n resin or a partial hydrolyzate, containing Si-OR groups where R is as defined in claim 1, and/or a partial condensate containing SiOH groups. A method according to claim 5, wherein 5 to 1000 ppm of metal of a metal catalyst, selected from platinum, rhodium, or copper catalysts, is added based on the weight of the resin. A method according to claim 6, wherein a wet ammonia atmosphere is first used to hydrolyze SiH bonds of the silica precursor, and then a dry ammonia atmosphere is used to effect removal of any remaining SiOH groups.

Inventors: BALLANCE DAVID STEPHEN, CAMILLETTI ROBERT CHARLES, GENTLE THERESA EILEEN, BALLANCE, DAVID STEPHEN, CAMILLETTI, ROBERT CHARLES, GENTLE, THERESA EILEEN
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1992
Main classification: H01L21
IPC codes: H01L21, B05D7, C04B41, C23C18, B05D3