| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 19.09.2000 |
| PATENT TITLE |
Air cleaning apparatus, air filter and method for manufacturing the same |
| PATENT ABSTRACT | An air cleaning apparatus such as a clean room or the like having an air circulation mechanism which circulates the humidity controlled air through a predetermined air circulation path, wherein the air circulation path is provided with a filter which has a hydrophobic zeolite layer formed on its base filter media surface and another filter which is disposed downstream from the filter having a hydrophobic zeolite layer and is used for removing particulate impurities contained in the circulating air. According to this air cleaning apparatus, gaseous organic impurities and particulate impurities contained in the air circulating in the air cleaning apparatus can be removed without lowering the humidity of the circulating air. Furthermore, since this air cleaning apparatus includes no activated charcoal which is inflammable, the apparatus is excellent from the standpoint of disaster prevention. Accordingly, the filters according to the invention and particle filters can be installed with safety in the ceiling portion of the air cleaning apparatus |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | September 28, 1998 |
| PATENT CT FILE DATE | January 30, 1998 |
| PATENT CT NUMBER | This data is not available for free |
| PATENT CT PUB NUMBER | This data is not available for free |
| PATENT CT PUB DATE | August 6, 1998 |
| PATENT FOREIGN APPLICATION PRIORITY DATA | This data is not available for free |
| PATENT REFERENCES CITED | Sematech, Forecast of Airborne Molecular Contamination Limits for the 0.25 Micron High Performance Logic Process, Technology Transfer #95052812A-TR, May 31, 1995. |
| PATENT CLAIMS |
What is claimed is: 1. A filter having a base filter media as a supporter for supporting an adsorbent, wherein synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms as the adsorbent for adsorbing gaseous organic impurities contained in an object ambience, is firmly fixed to the surface of said supporter by using an inorganic substance as binder, said inorganic substance having ability to adsorb gaseous organic impurities and having an effective pore diameter larger than that of zeolite. 2. A filter having a base filter media as a supporter for supporting an adsorbent, wherein a first adsorption layer is formed by firmly fixing synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms as the adsorbent for adsorbing gaseous organic impurities contained in an object ambience to the surface of said supporter with an inorganic substance as a binder, and a second adsorption layer is formed by firmly fixing another inorganic substance to the surface of said first adsorption layer, said another inorganic substance having ability to adsorb gaseous organic impurities and having an effective pore diameter larger than that of zeolite. 3. A filter having a base filter media as a supporter for supporting an adsorbent, wherein synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms as an adsorbent for adsorbing gaseous organic impurities contained in an object ambience, is pelletized by using an inorganic substance as a binder, and the resultant zeolite pellets are firmly fixed to the surface of said supporter, said inorganic substance having ability to adsorb gaseous organic impurities and an effective pore diameter larger than that of zeolite. 4. A filter having a base filter media as a supporter for supporting an adsorbent, wherein synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms as the adsorbent for adsorbing gaseous organic impurities contained in an object ambience, is pelletized by using an inorganic substance as a binder, and the resultant zeolite pellets are coated with another inorganic substance having ability to adsorb gaseous organic impurities and an effective pore diameter larger than that of zeolite. 5. A filter as claimed in claims 1, 2, 3, or 4, wherein said supporter is formed into a honeycomb structure. 6. A filter provided with a casing which is charged with an adsorbent, wherein said adsorbent has a shape of a pellet which is formed by pelletizing a mixture of synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms for adsorbing gaseous organic impurities contained in an object ambience and an inorganic substance as a binder, said inorganic substance having ability to adsorb gaseous organic impurities and an effective pore diameter larger than that of zeolite. 7. A filter provided with a casing which is charged with an adsorbent, wherein said adsorbent has a shape of a pellet which is formed by pelletizing a mixture of synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms for adsorbing gaseous organic impurities contained in an object ambience and an inorganic substance as a binder, and said resultant zeolite pellet is further coated with another inorganic substance which has ability to adsorb gaseous organic impurities and an effective pore diameter larger than that of zeolite. 8. A filter as claimed in claims 1, 2, 3, 4, 6 or 7, wherein said inorganic substance adsorbing gaseous organic impurities has an effective pore diameter larger than that of zeolite, and a total volume per unit weight of the pore of said inorganic substance is more than 0.2 cc/g, or a specific surface of the pore of said inorganic substance is more than 100 m.sup.2 /g, the size of said pore of said inorganic substance distributing in the range of 15 to 300 angstroms. 9. A filter as claimed in claims 1, 2, 3, 4, 6 or 7, wherein a main component of said inorganic substance adsorbing gaseous organic impurities and having an effective pore diameter larger than that of zeolite, is selected from the group consisting of porous viscous clay mineral, diatom earth, silica, alumina, mixture of silica and alumina, aluminum silicate, activated alumina or porous glass, and combinations thereof. 10. A filter as claimed in claim 9, wherein said porous viscous clay mineral is hydrated magnesium silicate clay mineral, activated clay, acid clay, activated bentonite, or a complex of micro-crystal of aluminosilicate and microscopic particles of silica, or a combination thereof. 11. A filter as claimed in claims 1, 2, 3, 4, 6 or 7, wherein said inorganic substances includes an inorganic adhesion assisting agent. 12. A filter as claimed in claim 11, wherein said adhesion assisting inorganic agent includes at least sodium silicate, silica, or alumina. 13. An air cleaning apparatus having an air circulation mechanism which circulates humidity controlled air through a predetermined air circulation path, wherein said air circulation path is provided with such a first filter as recited in claims 1, 2, 3, 4, 6 or 7, and a second filter which is disposed downstream from said first filter, said second filter capable of removing particulate impurities contained in the circulating air. 14. A filter comprising a first filter portion including synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms as an adsorbent, and a second filter portion which is disposed upstream or downstream from said first filter portion so as to be adjacent thereto, said second filter portion including an inorganic adsorbent which adsorbs gaseous organic impurities and has an effective pore diameter larger than synthetic zeolite. 15. A filter as claimed in claim 14, wherein said first filter portion includes a base filter media as an adsorbent supporter to which synthetic zeolite is firmly fixed, and said second filter portion includes a base filter media as an adsorbent supporter to which an inorganic adsorbent is firmly fixed. 16. A filter as claimed in claim 15, wherein the supporter of said first filter portion is immersed in a suspension in which synthetic zeolite is dispersed, and then dried, thereby synthetic zeolite being firmly fixed to the surface of said supporter, and the supporter of said second filter portion is immersed in a suspension in which an inorganic adsorbent is dispersed, and then dried, thereby said inorganic adsorbent being firmly fixed to the surface of said supporter. 17. A filter as claimed in claim 15, wherein pellets formed by pelletizing synthetic zeolite powder are firmly fixed to the supporter surface of the first filter portion, and pellets formed by pelletizing said inorganic adsorbent powder are firmly fixed to the supporter surface of the second filter portion. 18. A filter as claimed in claim 16 or 17, wherein when firmly fixing pellets formed by pelletizing synthetic zeolite or powder thereof to the surface of said supporter and/or when firmly fixing pellets formed by pelletizing said inorganic adsorbent or powder thereof to the surface of said supporter, there is used an inorganic adhesion assisting agent which is selected from the group consisting of talc, kaolin mineral, bentonite, sodium silicate, silica, and alumina and combinations thereof. 19. A filter as claimed in claim 15, wherein said supporter is divided into two regions by an imaginary boundary plane intersecting the air flow passing through the supporter at right angles, and pellets formed by pelletizing synthetic zeolite or powder, thereof are firmly fixed to the surface of one of said two regions, thereby forming the first filter portion, while pellets formed by pelletizing said inorganic adsorbent or powder thereof are firmly fixed to the surface of the other of said two regions, thereby forming the second filter portion. 20. A filter as claimed in claims 14, 15, 16, 17 or 19, wherein said inorganic adsorbent used for said second filter portion is selected from the group consisting of diatom earth, silica, alumina, mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, hydarated magnesium silicate clay mineral, activated clay, and activated bentonite, and combinations thereof. 21. A filter as claimed in claims 14, 15, 16, 17 or 19, wherein a total volume per unit weight of the pore of said inorganic substance is more than 0.2 cc/g or a specific surface of the pore of said inorganic substance is more than 100 m2/g, the size of said pore of said inorganic substance distributing in the range of 15 to 300 angstroms. 22. A filter as claimed in claims 14, 15, 16, 17 or 19, wherein said supporter is formed into the honeycomb structure. 23. A filter as claimed in claim 14, wherein said first filter portion is constructed in the form of a cylindrical casing which is filled with said synthetic hydrophobic zeolite pellets while said second filter portion is constructed in the form of a separate cylindrical casing which is filled with pellets which are pelletized from said inorganic adsorbent. 24. A method of manufacturing a filter having a base filter media as a supporter for supporting an adsorbent, comprising the steps of: preparing a suspension in which there are dispersed powder of synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms and powder of an inorganic substance which absorbs gaseous impurities and has an effective pore diameter larger than that of zeolite and serves as a binder; and immersing said supporter in said suspension and then subjecting said supporter to drying, thereby forming an adsorption layer on the surface of said supporter. 25. A method of manufacturing a filter having a base filter media as a supporter for supporting an adsorbent, comprising the steps of: immersing said supporter in a suspension in which there are dispersed powder of synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms and powder of an inorganic substance as a binder and then subjecting said supporter to drying, thereby forming a first adsorption layer on the surface of said supporter; and immersing said supporter already having said first adsorption layer in a suspension in which there is dispersed an inorganic substance which has ability to adsorb gaseous organic impurities and an effective pore diameter larger than that of zeolite and then subjecting said supporter to drying, thereby forming a second adsorption layer on the surface of said first adsorption layer. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
TECHNICAL FIELD This invention generally relates to an air cleaning apparatus such as a clean room, a clean bench and so forth, which is used in the manufacture of precision products such as semiconductor elements (e.g. LSI), liquid crystal displays (LCD) and so forth, and has the function of controlling the relative humidity of the air in the working space and also removing gaseous organic impurities and fine particulate impurities contained in air of the working space. More particularly, the invention relates to an air filter which can be installed in such apparatus for removing gaseous organic impurities contained in the air of the working space, and further relates to a method for manufacturing the same. In the following description, the expression `air cleaning apparatus` seems to be too general, so that it will be expressed as just `clean room` or `clean room or the like` for more practical and concrete understanding of the invention. BACKGROUND ART It is well known that a clean room, a clean bench and so on are widely made use of in the process of manufacturing LSI's and LCD's. For instance, in case of manufacturing a DRAM of 1-megabit from a bare wafer (silicon wafer), there is required a semiconductor manufacturing line consisting of the manufacturing steps of about 200. Also, in case of manufacturing a TFT of the 9.4 type from a bare glass plate (LCD substrate), there is required a LCD panel manufacturing line consisting of the manufacturing steps of about 80. In such a manufacturing line, it is not possible in practice to continuously convey the object good e.g. silicon wafer or LCD substrate from process to process up to the goal without leaving the object good in any standby position. Consequently, it may happen that semifinished products are held in a certain place or area for hours being exposed to the air of the working space. For instance, in the manufacturing line of the TFT-LCD, the half-finished LCD substrate which has been finished up to the formation of an electric circuit through the prior steps, is left in a standby position in a proper carrier or a storage compartment or room for several to several tens hours being exposed to the ordinary clean room air. As such, when the silicon wafer or the LCD substrate are left in the ordinary clean room air for a long time, it often happens that some organic substances are deposited on the surface of the wafer or the LCD substrate. If such deposition of organic substances occurs, it causes the following an inconvenient state. That is, if an insulating oxide film (SiO.sub.2) is formed on the silicon wafer surface on which organic substances are deposited, the carbon component of the organic substance is captured within the silicon dioxide film, which reduces the dielectric strength of the silicon dioxide film, thus a leakage current being increased to a great extent. Furthermore, adsorption of the organic substances disturbs the close contact between the silicon wafer surface and a photoresist film applied thereto, thus, causing failure in light exposure and etching in the photolithographic process, and resulting in incorrect formation of a circuit pattern. In addition, adsorption of the organic substances results in increasing the surface resistivity of the silicon dioxide film formed on the silicon wafer, which causes the silicon wafer to be easily charged, thereby electrostatically adsorbing the fine airborne particles thereon, and readily causing the dielectric breakdown thereof. Still further, if organic impurities contained in the clean room air are irradiated by the ultra-violet rays emitted from the optical instruments, an optical CVD reaction might take place in the clean room air, so that products generated through such reaction might stick to and cloud up the surfaces of the lens and the mirror constituting an optical instrument such as a light projector, thereby reducing the optical efficiency. Organic substances contained in the air of the working space can give a similar ill influence to the glass substrate as the LCD substrate. In case of forming an amorphous silicon film (a-Si) for use in a thin film transistor (TFT) on the LCD substrate, if organic substances contained in the air of the working space is deposited on the LCD substrate surface, it again disturbs the close contact between the a-Si film and the LCD substrate surface. As described in the above, organic substance flying about in the air of the working space is really an ill factor to be removed as complete as possible. On one hand, it is possible to remove the organic substance deposited on the substrate surface by some cleaning methods, for instance by means of ultraviolet/ozone treatment. According to this cleaning method, however, at least several minutes have to be spent for completely cleaning a single substrate. Consequently, enormous time has to be shared for cleaning a large number of substrates, thus naturally reducing productivity. Recently, therefore, in addition to problem with regard to the Si and LCD substrate surface contamination caused by metallic impurities and fine airborne particles contained in the air of the working space, the deep concern is focused on organic impurities which still exist even in the clean air of the working space and give a heavy influence over the production yield of semiconductor devices and others. For instance, in the article entitled `Forecast Of Airborne Molecular Contamination Limits For The 0.25 Micron High Performance Logic Process` of Technology Transfer #95052812A- TR published by SEMATECH (U.S.A.) on May 31, 1995, the organic contaminant control level (allowable surface contamination value) is discussed and made into the following Table 1. In the discussion, the article writer(s) forecasts that in 1998, the allowable number of carbon atoms on the semiconductor substrate surface should be limited up to 5.times.10.sup.13 carbon atoms/cm.sup.2 before processing and 1.times.10.sup.15 carbon atoms/cm.sup.2 after processing. TABLE 1 ______________________________________ Requirements (Year) 1995 1998 2001 2004 2007 2010 ______________________________________ Feature Size 0.35 0.25 0.18 0.13 0.1 0.07 (micrometer) Front End Processes Organics 1E14 5E13 3E13 1E13 5E12 3E12 (atoms of C/cm.sup.2) Back End Process Organics 1E15 1E15 1E15 <1E15 <1E15 <1E15 (atoms of C/cm.sup.2) ______________________________________ Heretofore, a chemical filter such as an activated charcoal filter has been used as means for adsorbing and removing gaseous organic impurities which are still contained in the cleaned air. A predetermined container in the form of a tower which is filled with activated charcoal is well known as the most popular and simple chemical filter. It is also well known that a chemical filter is formed in various types. For instance, some chemical filters are made in the form of a felt by interweaving fibrous activated charcoal with a binder of polyester having a low melting point or nonwoven polyester fabric, and the others are formed by adhering granular activated carbon on an urethane foam or nonwoven fabric with a proper adhesive and then fabricating it in the shape of a block or cutting it in the shape of a sheet. In case of a clean room of which the ceiling is used as a clean air outlet, the most effective filter arrangement for removing gaseous organic impurities in the air may be achieved by arranging the chemical filters on the upstream side of the particle filters installed in the ceiling portion so as to entirely cover it. However, according to the Fire Laws of Japan, activated charcoal is classified into an inflammable material group, and it strictly requires careful attention to fire. Accordingly, in view of the disaster prevention, it should be as avoided as possible to arrange the chemical filter using activated charcoal on the backside of the ceiling of the clean room. The clean room for use in the manufacture of LSI's and LCD's supplies the clean air to the working space, the clean air being typically controlled to have a temperature of 23 to 25 .degree.C. and a relative humidity of 45.+-.10%, respectively. In general, however, activated charcoal has a very weak hydrophobic nature, so that it adsorbs not only gaseous organic impurities but also a considerable amount of moisture even if it is placed in the air conditioned circumstances. The adsorption quantity of moisture in the air by activated charcoal is rapidly increased with a small increase of the relative humidity. For instance, after holding fresh activated charcoal adsorbing none of moisture in a container filled with a dry air, if this activated charcoal is abruptly transferred to the clean room and exposed to the clean room air of which the temperature is 23 to 25.degree. C. and the relative humidity is 45.+-.10%, activated charcoal would adsorb a large quantity of moisture before reaching its saturated state. Accordingly, if the chemical filter using fresh activated charcoal is installed on the air supply side of the clean room, the humidity level of the supplied air is lowered due to the large amount of moisture adsorption by activated charcoal, even though the humidity is controlled at a preset level on the upstream side of the filter. If the humidity is lowered exceeding a preset level, it would facilitate generation of static electricity, which often interferes with the manufacture of LSI's and LCD's. In order to solve this problem, the manufacturer of chemical filters using activated charcoal is requested to carry out a very complex work prior to their shipping the filter. That is, they first survey the temperature and humidity of their client's clean room air before their shipping the filter, and then they intentionally humidify the filters so as to meet the air temperature and humidity level in the client's clean room, and further air-tightly pack the humidified filter for shipping thereof. A chemical filter of the tower type generally has a high adsorption efficiency, but it has a high pressure loss (vent resistance) as its drawback. On one hand, a chemical filter of the felt or sheet type is acceptably ventilative and shows the adsorption efficiency in adsorption of organic impurities which is not so different from that of the tower type chemical filter. However, there is a possibility that the filter of this type may act as a source of gaseous organic impurities. More specifically, the problem exists in various adhesives used for firmly fixing activated charcoal on the base filter media (e.g. nonwoven fabric) and/or sealing materials used for fixing the base filter media to the filter frame. For instance, neoprene resin, urethane resin, epoxy resin, silicone resin, etc. are typically used as adhesives, and neoprene rubber, silicone rubber, etc. are typically used as sealants. Thus, there is a possibility that gaseous organic impurities originating from these adhesives and sealing materials are included in the air passing through the chemical filter and may give an ill influence to the manufacture of semiconductor devices. In other words, the chemical filter of the felt or sheet type may remove the organic impurities of the ppb order contained in the clean room air, but it releases gaseous organic impurities by itself and again mixes them with the clean air once filtered by itself. In general, materials which release gaseous organic impurities as mentioned above are also included in a constituent of the particle filter for removing particulate impurities. This filter is typically arranged on the downstream side of the chemical filter for adsorbing gaseous organic impurities, so that the particle filter may undesirably act as a gaseous organic impurity source. A JP unexamined patent publication No. 61-103518 and a JP unexamined patent publication No. 3-98611 respectively disclose a filter which can be convertibly used as a chemical filter. The former patent publication describes a filter which is developed for removing offensive odor. This filter is formed by immersing a base material such as a urethane foam in an aqueous solution containing activated charcoal powder, adhesive of the emulsion type, and solid acid, and then drying it. In case of this filter, however, gaseous organic substances may be also released not only from the synthetic rubber latex and other organic adhesive dispersoid used as the adhesive of the emulsion type, but also from the urethane foam itself. On the other hand, the above latter patent publication describes a filter which is developed for removing harmful gas and offensive odor against human health. This filter requires combination use of the adsorbent and the organic binder as a necessary condition. However, polyethylene and other organic substances, which are described as preferable examples of the organic binder in the publication, may never fail to release undesirable gaseous organic substances by themselves. As a filter of this kind, a JP unexamined patent publication No. 63-310636 (JP Patent No. 2,579,767) and an unexamined international publication No. WO91/16971 (JP Patent No. 2,579,767) also describe a filter in which an adsorbent such as zeolite is fixed to the surface of a honeycomb matrix having perforations by using a water glass and other inorganic binder. Accordingly, an object of the invention is to provide a filter which is safe and reliable from the standpoint of disaster prevention, and neither disturbs the controlled humidity of the clean room air nor releases gaseous organic impurities by itself, and can remove even a very small quantity of gaseous organic impurities contained in the working space, thereby protecting the substrate surface from being contaminated with such impurities. Another object of the invention is to provide a method for manufacturing the filter as mentioned above. Still another object of the invention is to provide a clean room or the like equipped with the filter as mentioned above. DISCLOSURE OF THE INVENTION According to the invention, there is provided a clean room or the like having an air circulation mechanism which circulates the humidity controlled air through a predetermined air circulation path, wherein said air circulation path is provided with an air filter which has a hydrophobic zeolite layer formed on its base filter media surface and another air filter which is disposed on the downstream side of the filter having a hydrophobic zeolite layer and is used for removing particulate impurities contained in the circulating air. According to this clean room, gaseous organic impurities can be removed from the air circulating in the clean room without lowering the humidity of the circulating air. Since this clean room includes no activated charcoal which is inflammable, the apparatus is excellent from the standpoint of disaster prevention, and the filter of the invention can be installed with safety in the ceiling portion of the clean room along with a particle filter. Furthermore, it is preferable for the filter to be constituted only with materials which generate no gaseous organic impurities. In the filter as mentioned above, the base filter media is preferably formed in a honeycomb shape, and includes a number of tiny vent pores therein, and hydrophobic zeolite layer is firmly fixed to the surface of the honeycomb shaped base filter media (referred to as `honeycomb structure` hereinafter). In this case, this hydrophobic zeolite layer is formed by immersing the honeycomb structure in a suspension in which hydrophobic zeolite powder is dispersed, and then drying it. Also, the hydrophobic zeolite layer may be firmly fixed to the surface of the honeycomb structure by means of an adhesive. The honeycomb structure is preferably formed by using incombustible materials such as inorganic fiber (indispensable component) including either clay minerals or calcium silicate. Hydrophobic zeolite preferably has an effective pore diameter of 7 angstroms or more. Furthermore, the particle filter should be formed of only materials which generate no gaseous organic impurities. According to the invention, there is provided a filter having an adsorption layer which is formed by firmly fixing synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms for adsorbing gaseous organic impurities contained in the working space to the surface of a supporter (i.e. base filter media) by using inorganic substance as a binder. This inorganic substance can adsorb gaseous organic impurities and has an effective pore diameter larger than that of zeolite. Also, the invention provides a filter in which there are provided the first adsorption layer which is formed by firmly fixing synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms for adsorbing gaseous organic impurities contained in the working space to the surface of the supporter with the help of an inorganic substance as a binder, and the second adsorption layer which is formed by firmly fixing an inorganic substance to the surface of the first adsorption layer, the inorganic substance having ability to adsorb gaseous organic impurities and an effective pore diameter larger than that of zeolite. In this case, the inorganic substance used as a binder in the step of the formation of the first absorption layer may have an effective pore diameter larger than that of zeolite. Furthermore, the invention provides an air filter in which synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms for adsorbing gaseous organic impurities contained in the air of the working space is pelletized by using an inorganic substance as a binder, and the resultant zeolite pellets are firmly fixed to the surface of the supporter. In this case, the inorganic substance used as a binder can adsorb gaseous organic impurities and has an effective pore diameter larger than that of zeolite. Still further, the invention provides an air filter in which synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms for adsorbing gaseous organic impurities contained in the air of the working space is pelletized using an inorganic substance as a binder, and the resultant zeolite pellets are coated with an inorganic substance which can adsorb gaseous organic impurities and has an effective pore diameter larger than that of zeolite, and are firmly fixed to the surface of the supporter. In these filters as just mentioned above, if the filter according to the invention utilizes zeolite having an effective pore diameter of 8 angstroms, the filter can adsorb gaseous organic impurities such as DOP and DBP of which the molecular size is less than 8 angstroms. However, when it comes to gaseous organic impurities such as BHT and siloxane of which the molecular size of more than 8 angstroms, the filter might fail to adsorb larger gaseous organic impurities as such. However, such impurities can be adsorbed by the filter according to the invention, because the filter additionally employs an inorganic substance having an effective pore diameter of more than 8 angstroms. The above-mentioned `binder` can make not only the zeolite-supporter bond, but also the zeolite--zeolite bond. In the filter according to the invention, its supporter can be formed into the honeycomb structure. In this case, it is preferable that inorganic fiber is used as an indispensable component of the honeycomb structure as the supporter. If the constituents of the filter, not only the supporter but also an adsorbent such as zeolite, a binder for fixing the adsorbent to the supporter surface, etc., are selected from proper inorganic substances, there is no chance for the constituents of the filter to become a gaseous impurity source. The shape of the supporter is not limited to be of honeycomb structure. It may be of other shape, of which the cross section is in the form of a grid or a wave, for instance. Furthermore, the supporter can be of 3-dimensional mesh structure like a rock wool including a great number of air vent pores. In case of this 3-dimensional mesh structure, as will be described later, zeolite as the adsorbent can be firmly fixed not only to its surface but also to the inside thereof along its depth (thickness). Adhesion of the adsorbent to the supporter surface is achieved, for instance, by immersing the supporter in a suspension in which at least an adsorbent is dispersed and then drying it, or by blowing adsorbent pellets against the supporter surface which is made adherent. These adsorbent pellets are pelletized from the mixture of adsorbent and inorganic binder. Furthermore, the invention provides an air filter having a casing filled up with synthetic hydrophobic zeolite pellets, having an effective pore diameter of not less than 7 angstroms which are pelletized from zeolite capable of adsorbing gaseous organic impurities contained in the air of the working space by using the inorganic substance as a binder which has ability to adsorb gaseous organic impurities and has an effective pore diameter larger than that of zeolite. Still further, the invention provides an air filter having a casing filled up with synthetic hydrophobic zeolite pellets, having an effective pore diameter of not less than 7 angstroms which are pelletized from zeolite capable of adsorbing gaseous organic impurities contained in the air of the working space by using the inorganic substance as a binder, and which are further coated with an inorganic substance having ability to adsorb gaseous organic impurities and an effective pore diameter larger than that of zeolite. Similarly, in these air filters as mentioned just in the above, if the filter uses zeolite having an effective pore diameter of 8 angstroms, the filter can adsorb gaseous organic impurities such as DOP and DBP of which the molecular size is less than 8 angstroms. However, when it comes to gaseous organic impurities such as BHT and siloxane of which the molecular size of more than 8 angstroms, the filter might fail to adsorb such larger gaseous organic impurities. However, such impurities can be adsorbed by the filter according to the invention, because it additionally uses an inorganic substance having an effective pore diameter of more than 8 angstroms. The shape and dimension of the casing and the amount of adsorbent pellets to be charged therewith can be determined based on the shape of air flow path, effective filtration area, and the installation condition of the filter. This would give considerable flexibility to the design of the filter. Zeolite used as an adsorbent is either hydrophobic or A hydrophilic. The moisture amount adsorbed by hydrophobic zeolite is smaller than that adsorbed by hydrophilic zeolite, so that the porous structure of hydrophobic zeolite is less occupied with moisture than that of hydrophilic zeolite. This means that hydrophobic zeolite can more effectively use its porous structure for adsorbing the object impurities than hydrophilic zeolite. Therefore, the filter using hydrophobic zeolite has a longer life than that using hydrophilic zeolite. On one hand, if both of zeolite are compared to each other in terms of the cost thereof, hydrophilic zeolite still gains an advantage over hydrophobic zeolite. When the above zeolite has an effective pore diameter of 7 angstrom or more and the inorganic substance adsorbing gaseous organic impurities has an effective pore diameter larger than that of zeolite, it is preferable that a total volume of the pore distributing in the range of 15 to 300 angstroms is more than 0.2 cc per unit weight (g) of the inorganic substance, or that a specific surface of the pore of the inorganic substance is more than 100 m.sup.2 /g. A main component of the inorganic substance which adsorbs gaseous organic impurities and has an effective pore diameter larger than that of zeolite, is preferably either one of porous viscous clay mineral, diatom earth, silica, alumina, mixture of silica and alumina, activated alumina, aluminum silicate and porous glass, or is mixture thereof. The porous viscous clay mineral is either one of hydrated magnesium silicate clay mineral such as sepiolite, palygorskite, etc., activated clay, acid clay, activated bentonite, and a complex of micro-crystal of aluminosilicate and microscopic particles of silica, or is mixture thereof. In any event, the inorganic substance which is used as a binder, adsorbs gaseous organic impurities and has an effective pore diameter larger than zeolite, fills various roles, for instance mechanically supporting zeolite powder on the surface of the support, serving as an adhesion assisting agent when pelletizing zeolite powder, and adsorbing gaseous organic impurities which have a molecular size larger than the effective pore diameter, thus not being adsorbed by zeolite. It is preferable that the above-mentioned filter is assembled by using only materials which neither generate any gaseous organic impurities nor burst into flame. The above-mentioned inorganic substance may include an inorganic adhesion assisting agent, which includes at least one of sodium silicate, silica, and alumina. In addition, according to the invention, there is provided a method for manufacturing an air filter, which includes the steps of: preparing a suspension in which there are dispersed powder of synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms and powder of an inorganic substance which absorbs gaseous impurities and has an effective pore diameter larger than that of zeolite and serves as a binder; and immersing the supporter in the suspension and then drying it, thereby forming an absorption layer on the surface of the support. Furthermore, the invention provide a method for manufacturing an air filter which includes the steps of: immersing a supporter in a suspension in which there are dispersed powder of synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms and powder of an inorganic substance as a binder and then drying it, thereby forming the first adsorption layer on the surface of the supporter; and immersing the supporter already having the first adsorption layer, in a suspension in which there is dispersed an inorganic substance which has ability to adsorb gaseous organic impurities and an effective pore diameter larger than that of zeolite and then drying it, thereby forming the second adsorption layer on the surface of the first adsorption layer. According to these filter manufacturing methods, the filter can be assembled substantially only with component materials which neither generate any gaseous organic impurities nor burst into flame. In these methods, it is possible that an inorganic adhesion assisting agent of the sol state may be mixed with the suspension which is used for forming the first adsorbing layer and the second adsorbing layer thereon, so that the above-mentioned inorganic substance comes to include the inorganic adhesion assisting agent. Furthermore, according to the invention, there is provided a clean room or the like having an air circulation mechanism which circulates the humidity controlled air through a predetermined air circulation path, wherein the air circulation path is provided with any one of air filter according to the invention and a particle filter which removes particulate impurities contained in the circulating air and is arranged on the downstream side of the air filter. According to this clean room, it becomes possible to remove gaseous and particulate impurities contained in the circulating air without disturbing the controlled humidity of the circulating air. Since this clean room includes no activated charcoal which is inflammable, the clean room is excellent from the standpoint of disaster prevention. Accordingly, the filters of the invention and particle filters can be installed with safety in the ceiling portion of the clean room. Still further, according to the invention, there is provided an air filter which includes the first filter portion including synthetic hydrophobic zeolite having an effective pore diameter of not less than 7 angstroms as an adsorbent, and the second filter portion which is disposed on the upstream or downstream side of the above first filter portion so as to be adjacent thereto, the second filter portion including an inorganic adsorbent which adsorbs gaseous organic impurities and has an effective pore diameter larger than synthetic zeolite. Since this air filter includes no activated charcoal which is inflammable, it is excellent from the standpoint of disaster prevention. According to this filter, gaseous organic impurities contained in the air circulating in the clean room can be removed without disturbing the controlled humidity of the circulating air. In this filter, the first filter portion includes a supporter to the surface of which synthetic zeolite is firmly fixed, and the second filter portion also includes a supporter to the surface of which an inorganic adsorbent is firmly fixed. In this case, the supporter of the first filter portion is immersing in a suspension in which synthetic zeolite is dispersed, and then dried, thereby synthetic zeolite being firmly fixed to the surface of the supporter, and the supporter of the second filter portion is immersing in a suspension in which an inorganic adsorbent is dispersed, and then dried, thereby the inorganic adsorbent being firmly fixed to the surface of the supporter. Furthermore, the first filter portion may be formed such that pellets formed by pelletizing synthetic zeolite powder are firmly fixed to the supporter surface of the first filter portion, and the second filter portion may be formed such that pellets formed by pelletizing the powder of an inorganic adsorbent are firmly fixed to the supporter surface of the second filter portion. The supporter may be divided into two regions by an imaginary boundary plane intersecting the air flow passing through the supporter at right angles, and pellets formed by pelletizing synthetic zeolite or powder thereof are firmly fixed to the surface of one of these two regions, thereby forming the first filter portion, and pellets formed by pelletizing the inorganic adsorbent or powder thereof are firmly fixed to the surface of the remaining region, thereby forming the second filter portion. When firmly fixing pellets formed by pelletizing synthetic zeolite or powder thereof to the surface of the supporter and/or when firmly fixing pellets formed by pelletizing an inorganic adsorbent or powder thereof to the surface of the supporter, there is used an adhesion assisting agent which consists of at least one selected from talc, kaolin mineral, bentonite, sodium silicate, silica, and alumina. The inorganic adsorbent as used in the second filter portion consists of at least one selected from diatom earth, silica, alumina, mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, hydrated magnesium silicate clay mineral, activated clay, and activated bentonite. In the inorganic adsorbent of the second filter portion, a total volume of the pore distributing in the range of 15 to 300 angstroms is more than 0.2 cc per unit weight (g) of inorganic adsorbent, or the specific surface of the pore of the inorganic adsorbent is more than 100 m.sup.2 /g. The above-mentioned supporter may be of honeycomb structure. In this case, the honeycomb structure is preferably formed of inorganic fiber as its indispensable component. The first filter portion may be made in the form of a cylindrical casing which is filled with the synthetic zeolite pellets while the second filter portion may be made in the form of a separate cylindrical casing which is filled with in organic adsorbent pellets. Further, it is preferable that said synthetic zeolite is hydrophobic. |
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