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  Method and apparatus for purifying a gas containing contaminantsUSPTO Application #: 20070187226Title: Method and apparatus for purifying a gas containing contaminants Abstract: A method and an apparatus for purifying a gas containing contaminants are disclosed. The gas is irradiated with an ultraviolet ray and/or a radiation ray so as to produce microparticles of the contaminants. The resultant microparticles of the contaminants are contacted with a photocatalyst. Then, the photocatalyst is irradiated with light so as to decompose the contaminants held in contact with the photocatalyst. Organic compounds organosilicon compounds, basic gas and the like can be decomposed by the action of the photocatalyst. Even when these species are present at a low concentration, they can be concentrated locally by transforming into microparticles, and hence can be removed. (end of abstract)
Agent: Oblon, Spivak, Mcclelland, Maier & Neustadt, P.C. - Alexandria, VA, US Inventor: Toshiaki FUJII USPTO Applicaton #: 20070187226 - Class: 204157150 (USPTO) Related Patent Categories: Chemistry: Electrical And Wave Energy, Non-distilling Bottoms Treatment, Processes Of Treating Materials By Wave Energy The Patent Description & Claims data below is from USPTO Patent Application 20070187226. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] The disclosures of the Japanese Patent Applications Nos. Hei-8-235832 filed on the 20 Aug., 1996 and Hei-9-31441 filed on the 31 Jan., 1997 are incorporated herein by reference. FIELD OF THE INVENTION [0002] The present invention relates to a method and an apparatus for purifying a gas containing contaminants. More specifically, the present invention relates to a method and an apparatus for purifying a gas by producing microparticles of the contaminants present in a gas and decomposing the resultant microparticles of contaminants with a photocatalyst for facilitating the removal thereof. RELATED ART [0003] It was considered satisfactory in semiconductor industries in the past to remove only solid particles such as dust from a gas such as air in a clean room. Methods for removing solid particles can be classified broadly into 2 categories: (1) mechanical filtration methods (e.g. HEPA (High Efficiency Particulate Air) filter); and (2) methods for trapping microparticles electrostatically (e.g. MESA filter). Methods included in the category (2) comprise charging microparticles electrically with a high electrical voltage and filtering the charged microparticles with an electrically conductive filter. Gaseous contaminants, however, cannot be removed by any method of either category. [0004] Development of semiconductors of higher quality and finer precision has made it necessary to remove not only dust-like solid particles but also gaseous contaminants Gaseous contaminants include: organic compounds including phthalic esters; organosilicon compounds including siloxane; acidic gases including sulfur oxides (SOx), nitrogen oxides (NOx), hydrogen chloride (HCl) and hydrogen fluoride (HF); as well as basic gases including NH.sub.3 and amines. Amines may be included among organic compounds also. Anions such as NO.sub.3.sup.-, NO.sub.2.sup.-, SO.sub.4.sup.2-, etc. have characteristics and exert adverse effects similar to acidic gases, and therefor, are considered as a member of acidic gases out of convenience. Likewise, cations such as NH.sub.4.sup.+, etc. have characteristics and exert adverse effects similar to basic gases, and therefor, are considered as a member of basic gases for convenience. [0005] Organic compounds or organosilicon compounds, when deposited onto the surface of a wafer (substrate), may have a negative effect on the affinity (drapability) of a substrate for a resist. Decreased affinity may exert a harmful influence on both the film thickness of a resist and the adhesion of a substrate to a resist ("Air Cleaning", Vol. 33, No. 1, pp. 16-21, 1995). For example, SOx may bring about defective insulation in an oxide layer. NH.sub.3 may produce ammonium salts that are responsible for the blooming (poor resolution) of a wafer (Realize Inc., "Saishin Gijyutsu Koza, Shiryo-shu", 29 Oct. 1996, pp. 15-25, 1996). For the aforementioned reasons, such gaseous contaminants may diminish the productivity (yield) of semiconductor products. [0006] It was also considered satisfactory in the past to remove gaseous contaminants to a level of ppm. It has become required now to remove gaseous contaminants to a level of ppb. Among organic compounds, alkanes such as methane and the like are not so reactive as to exert an unfavorable influence on a semiconductor, and hence are not required to be removed to a level of ppb. [0007] Removal of contaminants including organic compounds, especially gaseous organic compounds is described below in more detail. [0008] Known methods for removing organic compounds include decomposition by combustion, catalytic decomposition, removal by adsorption, decomposition with O.sub.3 and the like. These known methods, however, are not effective in removing organic compounds present in low concentrations in air for feeding a clean room. [0009] In a clean room, contamination with organic compounds of an extremely slight concentration cannot be ignored. External organic compounds may be introduced into a clean room. For example, outdoor air is contaminated with organic compounds originating from exhaust gas of cars or those resulting from degassing of polymer products. On the other hand, internal organic compounds may be generated in a clean room. For example, polymer materials (e.g. polymeric plasticizers, releasers, antioxidants and the like) which are used for constructing a clean room are producers of organic gases ("Air Cleaning", Vol. 33, No. 1, pp. 16-21, 1995). Synthetic polymers are used in packing materials, sealants, adhesives and wall-forming materials in a clean room. In addition, plastic containers are disposed in a clean room. These synthetic polymers may evolve a trace amount of organic gases. More particularly, sealants and the production units thereof may give off gaseous siloxane, and plastic containers may give off gaseous phthalic esters. It has recently been found that gas evolves also from polymer materials employed in a production unit. A process unit is partially or entirely surrounded by plastic plates which also produce organic gas. A variety of solvents (e.g. alcohols, ketones, etc., which are necessary for operations in a clean room are also a contamination source. [0010] As stated above, a clean room is contaminated variously and heavily with not only organic compounds attributable to external air but also with organic compounds and organosilicon compounds that are generated internally. [0011] In view of energy saving considerations, recycling of air in a clean room has become more frequent recently. In consequence, organic gases are progressively concentrated in a clean room, leading to heavier contamination of the base materials of a wafer and a substrate. These organic compounds are likely to deposit onto the bodies (e.g., starting materials and semi-fabricated products of a semiconductor wafer, a glass substrate, etc.) placed in a clean room, adversely affecting them. [0012] A contact angle indicates a degree of contamination on a wafer substrate with organic compounds and organosilicon compounds. The contact angle refers to the angle formed by the water and the surface of a substrate when the surface is wet with water. The surface of a substrate, when covered with a hydrophobic (oily) substance, becomes more water-repellent and less wettable, hence the contact angle of water on the surface of a substrate becomes larger. In other words, when the contact angle is larger, the degree of contamination is higher. On the contrary, when the contact angle is smaller, the degree of contamination is lower. [0013] When a substrate is contaminated with organic compounds and organosilicon compounds, its affinity (drapability) for a resist decreases, imparting an unfavorable influence on the resist and the film thickness or on the adhesion of the substrate to the resist, that may result in lower quality and a lower yield. [0014] Techniques in the high-technology field have made remarkable progress in realizing semiconductor devices of a maximal precision and a minimal size. In consequence, it has become necessary for a clean room to be free from organic compounds normally present in the air of the level that had conventionally been able to be ignored (an extremely low concentration of the ppb level) [Preparatory Manuscripts for the 39th Meeting of the Applied Physical Society, p. 86 (1992, Spring); "Air Cleaning", Vol. 33, No. 1, pp. 16-21, (1995)], as well as gaseous contaminants including SO.sub.2, HF, NH.sub.3 ["Ultra Clean Technology", Vol. 6, pp. 29-35 (1994)]. Because, it has been revealed that the presence of these gaseous contaminants diminished remarkably the productivity (yield). The present invention is aiming to efficiently remove these gaseous contaminants. [0015] The present inventors have proposed a method for removing hydrocarbons present in a gas comprising the steps of: irradiating the gas with an ultraviolet ray and/or a radiation ray so as to produce microparticles from the hydrocarbon; and trapping the resultant hydrocarbon microparticles with a filter or charging the hydrocarbon microparticles electrically with a photoelectron and trapping the resultant charged microparticles (Laid Open Japanese Patent Application No. Hei-5-96125). A similar method can be applied also to noxious matter present in a gas (Laid Open Japanese Patent Application No. Hei-4-243517). [0016] Using the methods mentioned above, however, trapped microparticles become accumulated on the filter or in the part for trapping the charged microparticles, thus requiring frequent changing of the filter or the trapping part. Further, when the accumulated microparticles fall from the filter or from the trapping part, the fallen microparticles, even if they are in extremely small amounts, inadvertently contaminate a gas to be purified. Therefore, it is considered preferable to decompose contaminants than to remove them. [0017] A conventional removing method is now described with reference to FIGS. 16 and 17. As shown in FIG. 16, the air which is fed to a clean room 1 in a recycled manner is composed of the external air that is fed via a pipe 2 and is cleared of coarse particles through a prefilter 3 and the internal air that is drawn out of the clean room 1 through an air outlet 4. Both airs are combined in a fan 5, conditioned in temperature and moisture with an air conditioner 6 and cleared of microparticles with a HEPA filter 7. The air in the clean room is kept at a purity (class) of the order of 10,000. In this specification, the term "class" refers to the number of particles having a particle diameter of not less than 0.1 .mu.m that are present per cubic feet. [0018] A clean bench 51 is disposed in the clean room 1 to trap and remove a trace amount of hydrocarbons and microparticles (particulate matter). [0019] Organic compounds present in the clean room 1 may consist presumably of those that originating in external air introduced through the pipe 2 (those that are presumably discharged from cars and synthetic resins) and those that are produced during operations in the clean room. [0020] The clean bench 51 comprises mainly a microparticle-producing 48, a microparticle-charging section 49 and a section for trapping charged microparticles 50. A highly pure air (of class 10) that is freed of both organic compounds and coexistent microparticles is fed above a working table 53, where operations are being carried out. [0021] In other words, air having a purity (class) in the order of 10,000 and containing a trace amount of organic compounds originating in the clean room 1 is directed with a fan (not shown) toward the clean bench 51. At the clean bench 51, the microparticle-producing section 48 is provided for irradiating the air with an ultraviolet radiation of a short wavelength so as to produce microparticles of organic compounds contained in the air. Then, in the microparticle-charging section 49, the microparticles are electrically charged efficiently with photoelectrons emitted by a photoelectron-emitting material as described hereinbelow. The resultant charged microparticles are trapped and removed in the section for trapping charged microparticles 50 that follows. In this manner, air above the working table 53 can be maintained to be highly pure and free of organic compounds. Continue reading... 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