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Dow Corning litigates HSQ patents| thinfilmmfg.com
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Dow Corning litigates HSQ patents |
As we reported last week, Dow Corning has sued Tokyo Ohka Kogyo Co. Ltd. (TOK), alleging infringement of several patents related to Dow Corning's hydrogen silsesquioxane (HSQ) materials. HSQ, long used for passivation layers, is also the base material for Dow Corning's XLK low dielectric constant (k) materials. The patents at issue in the suit cover several aspects of HSQ deposition, including synthesis of the precursor compound, deposition and firing to achieve a hard coating, and purification and fractionation to control the properties of the material.
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According to its abstract, the first of the four relevant patents, US Patent #5,045,592, "provides a relatively simple synthesis procedure for the formation of silane hydrolyzate compositions of the formula [Equation 1], where R is hydrogen or a methyl group, n is an integer greater than about 8, and x is a number between 0 and 2." |
Equation 1: | |
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As the '592 patent explains, several methods can be used to synthesize HSQ resin. However, many of those methods rely on corrosive acids and aromatic hydrocarbons. Many of the resulting resins remain soluble after coating on a substrate. It's difficult to construct thick layers, such as planarization coatings, from soluble materials. Unless a catalyst or crosslinking agent is added, each coating application will re-dissolve the previous layers .
The synthesis approach described in the '592 patent starts with a reactive mixture composed of a polar organic solvent containing water or hydrogen, combined with a chlorosilane with the formula RSiCl3. The solvent must be able to form hydrogen bonds with residual silanol groups. According to the patent, solvents such as ethyl acetate and methyl isobutyl ketone offer appropriate properties. A metal oxide added to the mixture scavenges hydrogen chlorides to form an insoluble metal chloride precipitate. Appropriate oxides include CuO, ZnO, and MgO.
Adding water or hydrochloric acid to the mixture initiates hydrolysis. Metal chlorides and water precipitate out of the solution, leaving behind a metastable silane hydrolyzate.
A closely related patent, US Patent #5,085,893, describes a method for using the hydrolyzate solution to form a protective coating of silsesquioxane resin. The '893 patent's method involves coating a substrate with solution, then evaporating the solvent to form an insoluble silsesquioxane coating. An oxidizing furnace converts the coating to an SiO2-based ceramic.
The '592 patent claims the silane hydrolyzate solution itself. The '893 patent claims a process for coating a substrate with this solution. The third patent at issue, US Patent #5,370,903, describes the method for converting the HSQ film to an SiO2-based ceramic. The '903 patent's method heats an HSQ coating in an atmosphere containing inert gases with up to 20% oxygen. Oxidation continues, according to the abstract, until "the content of silicon-bonded hydrogen in the silicon oxide product has reached <= 80% of the content of silicon-bonded hydrogen in the HSQ resin."
The '903 patent also contemplates planarization of semiconductor devices with HSQ film. The SiO2 film resulting from this method is claimed to be free from cracks and pinholes, even at thicknesses in excess of 1.0 micron.
As a group, then, these three patents claim to describe a material and method for using HSQ resins to create a planarizing oxide on semiconductor devices. The fourth patent mentioned in Dow Corning's suit, US Patent #5,486,564, makes the link to low-k dielectric materials. As the '564 patent explains, the Si-bonded hydrogen present in polyhydrogen silsesquioxane (pHSQ; (HSiO3/2)n) can bond to organic resins and can allow introduction of organic functional groups. These additions can be used to tune the heat resistance, dielectric constant, and other properties of the film.
Unfortunately, the hydrolysis and condensation reactions used to synthesize pHSQ typically introduce ionic impurities from the reactor vessel and leave behind residual catalysts. The reaction products have a wide range of molecular weights, ranging from several hundred to several hundred thousand. These materials do not meet the purity or consistency requirments of microelectronic applications.
The '564 patent describes a method for fractionation and purification of pHSQ materials. First, an active-hydrogen-free nonpolar solvent dissolves the pHSQ. Appropriate solvents include toluene, hexane, and diphenyl ether. Next, an active-hydrogen-free polar solvent, such as acetonitrile or nitrobenzene, is added to the solution. While the nonpolar solvent will dissolve almost all of the pHSQ, the polar solvent will dissolve only pHSQ with a molecular weight of several thousand. The mixing ratio of the two solvents determines the molecular weight solubility limit of the mixture. PHSQ with a molecular weight greater than the solubility limit will precipitate out. Repeated applications of this procedure will produce as narrow a molecular weight distribution as desired.
The method can be implemented by removing either the high-molecular-weight or the low-molecular-weight pHSQ first. According to the '564 patent, removing the high-molecular-weight components first tends to deplete impurities from the material, while removing low-molecular-weight components first tends to concentrate impurities in the solvent. The patent's authors claim to have achieved pHSQ resins with sodium impurities of 9 ppb or less, sulfur content of 14 ppm or less, and chlorine content less than 1 ppm.
To read the full text of these patents, visit Delphion's patent database.
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