Electronic coating composition

Abstract: Disclosed are coating compositions which include a preceramic silicon-containing material dissolved in short chain linear siloxanes. These compositions surprisingly have a long shelf life and are useful for forming coatings which are thick and of good quality.

Description: The present invention relates to an electronic coating composition containing a preceramic silicon-containing material and a linear siloxane fluid. Preceramic silicon-containing materials and their use in forming coatings on electronic devices is known in the art. For instance, US Patents 4,756,917 and 4,749,631 teach processes of forming coatings on electronic devices comprising diluting preceramic silicon-containing materials in solvents, applying the solutions to electronic devices and heating the coated devices to convert the preceramic materials to ceramics. The references, however, do not use the linear siloxane carrier fluids claimed herein. Similarly, patents such as US Patent 5,380,553 disclose a variety of carrier fluids which are used for preceramic silicon-containing materials. These include alcohols such as ethyl or isopropyl, aromatic hydrocarbons such as benzene or toluene, alkanes such as n-heptane or dodecane, ketones, cyclic dimethylpolysiloxanes, esters or glycol ethers. The linear siloxanes specifically claimed are not mentioned therein. We have now found that specific linear siloxanes and blends thereof will form stable solutions of preceramic silicon-containing material which can be subsequently used to deposit coatings on electronic devices. Thereafter, the resultant coatings can be ceramified to ceramic coatings on electronic devices. The present invention provides a coating composition comprising between 1 and 50 weight percent of preceramic silicon-containing material (other than a linear siloxane) which is a precursor to silicon oxides, silicon nitride, silicon oxynitride, silicon oxycarbide, silicon carbonitride, silicon oxycarbonitride or silicon carbide and 50 to 99 weight percent of linear siloxane, wherein the linear siloxane is selected from octamethyltrisiloxane, decamethyltetrasiloxane and a mixture of hexamethyldisiloxane with octamethyltrisiloxane in a ratio of 10:90 to 40:60. The present invention is based on our finding that the carrier fluid used to deposit a coating of a preceramic silicon-containing material does have a dramatic effect on the resultant solution and coating. For instance, the carrier fluid affects the stability of the solution, the quality of the resultant coating (e.g., purity, dielectric constant, etc.) and the thickness of the resultant coating. Additionally, certain carrier fluids have adverse effects on the environment because they contain volatile organic compounds (VOCs). Accordingly, we undertook an extensive review of carrier fluids and unexpectedly found that linear siloxane fluids, and blends thereof, can be used to form solutions of preceramic silicon-containing materials that have a long shelf life, even at room temperature, low VOC content and good compatibility with containers storing these materials. These stable solutions, in turn, are used to deposit coatings which have excellent quality and unexpected thickness. Because of these advantages, the coating compositions of the present invention are particularly valuable on electronic substrates. Such coatings serve, for example, as protective coatings, interlevel dielectric layers, doped dielectric layers to produce transistor like devices, pigment loaded binder systems containing silicon to produce capacitor and capacitor like devices, multilayer devices, 3-D devices, silicon on insulator devices, coatings for superconductors, super lattice devices and the like. As used herein, the expression "ceramic" describes the hard, Si containing coatings obtained after heating our preceramic silicon-containing material. These coatings may contain silicon, carbon, nitrogen, oxygen or hydrogen such as silica (SiO 2 ), silica-like materials (e.g., HSiO 3/2 , SiO, Si 2 O 3 , etc.), silicon carbide and silicon nitride which may not be fully free of residual carbon, silanol (Si-OH) and/or hydrogen. The expression "electronic substrate" includes electronic devices or electronic circuits such as silicon based devices, gallium arsenide based devices, focal plane arrays, opto-electronic devices, photovoltaic cells and optical devices. The compositions of this invention contain between 1 and 50 weight percent of a preceramic silicon-containing material and preferably between 5 and 30 weight percent of the same in a carrier fluid. The term "preceramic silicon-containing material" describes material which can be diluted in a carrier fluid, coated on the surface of an electronic device and converted to a solid layer exhibiting properties generally recognized by those skilled in the art as characteristic of a ceramic. These materials are precursors to silicon oxides, silicon nitride, silicon oxynitride, silicon oxycarbide, silicon carbonitride, silicon oxycarbonitride and silicon carbide. The preferred preceramic silicon-containing materials used in our process are precursors to silicon oxides, especially silica. These precursors include hydrogen silsesquioxane resin (H-resin), hydrolyzed or partially hydrolyzed R n Si(OR) 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 carbons, such as an alkyl (e.g. methyl, ethyl, propyl), alkenyl (e.g. vinyl or allyl), alkynyl (e.g. ethynyl), cyclopentyl, cyclohexyl or phenyl and n is 0-3, preferably 0 or 1. The H-resin used in this invention includes hydridosiloxane 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. Examples of R 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 3/2 ) n or they may be only partially hydrolyzed (i.e., containing some Si-OR) and/or partially condensed (i.e., containing some Si-OH). Although not represented by this structure, these resins may also contain a small number (e.g., less than 10%) of silicon atoms which have either 0 or 2 hydrogen atoms attached thereto or a small number of SiC bonds due to various factors involved in their formation or handling. H-resins and methods for their production are known in the art. For example, US Patent 3,615,272 claims the production of a nearly fully condensed H-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, US Patent 5,010,159 provides 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 by US Patent 4,999,397; those produced by hydrolyzing an alkoxy or acyloxy silane in an acidic, alcoholic hydrolysis medium; those described in JP-As 59-178749, 60-86017 and 63-107122; or any other equivalent hydridosiloxane, will also function herein. In a preferred embodiment of the invention, specific molecular weight fractions of the above H-resins may also be used in this process. Such fractions and methods for their preparation are revealed in US Patents 5,063,267 and 5,416,190. A preferred fraction comprises material wherein at least 75% of the polymeric species have a number average molecular weight above 1200 and a more preferred fraction comprises material wherein at least 75% of the polymeric species have a number average molecular weight between 1200 and 100,000. The H-resin coating material may also contain a platinum, rhodium or copper catalyst 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, Mich., or cupric naphthenate are all feasible. These catalysts are generally added in an amount of between 5 to 1000 ppm platinum, rhodium or copper metals, based on the weight of H-resin. A second type of silica precursor material useful herein includes hydrolyzed or partially hydrolyzed compounds of the formula R n Si(OR) 4-n in which R and n are as defined above. Some of these materials are commercially available, for example, under the name ACCUGLASS™ (Allied Signal Company). Specific compounds 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 Si-O-Si 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. In addition to the above silicon-oxide precursors, other ceramic oxide precursors may also be advantageously used herein in combination with the above silicon oxide precursors. The ceramic oxide precursors specifically contemplated herein include compounds of various metals such as aluminum, titanium, zirconium, tantalum, niobium and/or vanadium as well as various non-metallic compounds such as those of boron or phosphorous which may be dissolved in solution, hydrolyzed and subsequently pyrolyzed at relatively low temperatures to form ceramic oxides. The above ceramic oxide precursor compounds generally have one or more hydrolyzable groups bonded to the above metal or non-metal, depending on the valence of the metal. The number of hydrolyzable groups included in these compounds is not critical as long as the compound is soluble in the solvent. Likewise, selection of the exact hydrolyzable substituent is not critical since the substituents are either hydrolyzed or pyrolyzed out of the system. Typical hydrolyzable groups are alkoxy such as methoxy, propoxy, butoxy and hexoxy; acyloxy such as acetoxy; other organic groups bonded to said metal or non-metal through an oxygen such as acetylacetone or amino groups. Specific compounds, therefore, include zirconium tetracetylacetonate, titanium dibutoxy diacetylacetonate, aluminum triacetylacetonate, tetraisobutoxy titanium and Ti (N(CH 3 ) 2 ) 4 . When a silicon oxide such as SiO 2 is combined with one of the above ceramic oxide precursors, generally it is used in an amount such that the final ceramic contains 70 to 99.9 percent by weight silicon oxide. Examples of other silicon-containing preceramic materials include silicon carbonitride precursors such as hydridopolysilazane (HPZ) resin and methylpolydisilylazane (MPDZ) resin. Processes for the production of these materials are further described in US Patents 4,540,803 and 4,340,619 and are not repeated herein. 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. The carrier fluids used in the present invention are linear siloxanes selected from octamethyltrisiloxane, decamethyltetrasiloxane and a mixture of hexamethyldisiloxane with octamethyltrisiloxane. Hexamethyldisiloxane has a viscosity of 0.65 mm 2 /second at 25°C. Octamethyltrisiloxane has a viscosity of 1 mm 2 /second at 25°C. When a mixture of hexamethyldisiloxane with octamethyltrisiloxane is employed the mixing ratio is in the range of 10:90 to 40:60. These linear siloxanes and methods for their manufacture are known in the art. For instance, Noll in "Chemistry and Technology of Silicones" (Academic Press-1960) teaches these specific materials and hydrolytic and non-hydrolytic processes for obtaining them from a variety of silanes. Generally, these linear siloxanes are purified to avoid contamination of the coating solution which may result in instability. Similarly, impurities may result in detrimental properties in the resultant coating. Purification techniques are known in the art and are exemplified by filtration, distillation and a host of other separation techniques. The compositions of the present invention generally contain minor amounts (e.g., less than 5 wt%) of other carrier fluids or additives such as cyclic siloxanes, without affecting the invention. The above formulations consistently provide solutions with long shelf-life. In fact, these solutions even have reasonable shelf lives at room temperature. By contrast, other solutions of preceramic silicon-containing materials such as H-resin and similar silica precursors require refrigeration for reasonable shelf lifes and even then the shelf life is often only days as compared to the months achieved herein. Additionally, these formulations are compatible with a variety of containers used to hold such materials. By contrast, other carrier fluids require the use of specific containers which will not react with their formulations. Similarly, the solutions of this invention have low levels of VOCs. By contrast, other carrier fluids proposed for preceramic silicon-containing materials are often organic and have detrimental effects on the environment. Finally, solutions of preceramic silicon-containing materials, such as H-resin, when dissolved in the linear siloxanes of our claimed invention have the advantage of delivering coatings with thicknesses comparable to higher concentrations of preceramic material in other carrier fluids. As such, these linear siloxane solutions have a distinct economic advantage. The compositions of our invention are used to coat electronic substrates with ceramic coatings. Generally, the process of forming such coatings comprises first applying the coating composition onto an electronic substrate. If desired, the coating is then "flowed" by heating under an inert atmosphere, thus causing the coating to be leveled. The coated electronic substrate is then heated to a temperature in the range of 50 to 800°C. in an appropriate environment (e.g., oxidizing (air, ozone, ammonia, etc) or inert (e.g., nitrogen)) to convert the coating composition into a ceramic coating. The coatings which result from our claimed compositions are of high quality (e.g., purity, dielectric constant, etc.) and have desirable thicknesses. It should be recognized, however, that thickness variations in the coatings are also affected by factors such as spin speed and preceramic material concentration. These factors must be taken into account when producing coatings with these compositions. The following non-limiting examples are included so that one skilled in the art may more readily understand the invention. Example 1 - Stability Testing The following test was performed to compare the stability of hydrogen silsesquioxane resin (H-resin) in the linear siloxanes of the present invention with other carrier fluids for this material that are reported in the literature. H-resin (made by the method of US Patent 3,615,272) was diluted to 50 wt% in the carrier fluids listed in Table 1 and stored at room temperature in 29.6 ml (one ounce) glass vials. The solutions were then visually observed until they gelled. The results are presented in Table 1. Carrier Fluid Time to Gel Methyl Isobutylketone 6-117 days n-Butyl Acetate 8-43 days Octane > 4 months Octamethylcyclotetrasiloxane 98 + days Hexamethyldisiloxane > 3 months Octamethyltrisiloxane > 5 months Decamethyltetrasiloxane* 132 + days Silicone Fluid 10 mm 2 /sec viscosity 0 As is apparent, the linear siloxanes claimed herein have stabilities at least equivalent to those of the other carrier fluids. Example 2 The H-resin of Example 1 was diluted to 18 wt% solids in the carrier fluids listed in Table 2. These compositions were applied to the surface of silicon wafers and spun at 3000 RPM on a Semix™ Spin Coater for 10 seconds. The coated wafer was then heated to allow the resin to melt and flow. This heating was conducted at 150°C. for 1 minute, 200°C. for 1 minute and 350°C. for 1 minute under a nitrogen atmosphere. The coating was then converted to a ceramic by ramping the temperature up between 40 and 400°C. over 1 hour, holding the temperature at 400°C. for 1 hour and ramping the temperature down from 400 to 40°C. over 1 hour under a nitrogen atmosphere. A Nanometrics™ Nanospec AFT System was used to determine the thickness of the coatings as spun, after flowing and after conversion. The visual quality was reduced to a numerical value between 1 (good) and 5 (poor) and recorded as a response. The results of this experiment are provided in Table 2.

Claims: A coating composition comprising between 1 and 50 weight percent of preceramic silicon-containing material (other than a linear siloxane) which is a precursor to silicon oxides, silicon nitride, silicon oxynitride, silicon oxycarbide, silicon carbonitride, silicon oxycarbonitride or silicon carbide and 50 to 99 weight percent of linear siloxane, wherein the linear siloxane is selected from octamethyltrisiloxane, decamethyltetrasiloxane and a mixture of hexamethyldisiloxane with octamethyltrisiloxane in a ratio of 10:90 to 40:60. A coating composition according to claim 1, wherein the preceramic silicon-containing material is selected from hydrogen silsesquioxane resin, hydrolyzed or partially hydrolyzed R n Si(OR) 4-n , or combinations thereof, wherein each R is independently an aliphatic, alicyclic or aromatic substituent of 1-20 carbon atoms and n is 0-3. A coating composition according to claim 1 or claim 2, also containing modifying ceramic oxide precursors comprising a compound containing an element selected from titanium, zirconium, aluminum, tantalum, vanadium, niobium, boron and phosphorus wherein the compound contains at least one hydrolyzable substituent selected from alkoxy or acyloxy and the compound is present in an amount such that a silica coating formed from the coating composition contains 0.1 to 30 percent by weight modifying ceramic oxide. A coating composition according to any of the preceding claims, also containing a platinum, rhodium or copper catalyst in an amount of between 5 and 1000 ppm platinum, rhodium or copper based on the weight of preceramic silicon-containing material. A method of coating an electronic substrate with a ceramic coating comprising applying the coating composition according to any of claims I to 4 onto an electronic substrate and heating the coated electronic substrate to a temperature in the range of 50 to 800°C. to convert the coating composition into a ceramic coating. A method according to claim 5, wherein the coated substrate is heated in an inert atmosphere to a temperature above the melting point of the preceramic silicon-containing material to cause the coating to flow, before conversion into a ceramic coating.

Inventors: CURRIE ALEXANDER BRIAN, HAINES CATHRYN MARIE, LITTLE MICHAEL DEAN, MICHAEL KEITH WINTON, SCHEIBERT KRISTEN ANDREA, CURRIE, ALEXANDER BRIAN, HAINES, CATHRYN MARIE, LITTLE, MICHAEL DEAN, MICHAEL, KEITH WINTON, SCHEIBERT, KRISTEN ANDREA
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1997
Main classification: C04B41
IPC codes: C04B41, C04B35, H01L21, C09D183, H01L23, C07F7