Patent Publication Number: US-2013240479-A1

Title: Method for producing filtration filter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of PCT International Application No. PCT/JP2011/076129, filed Nov. 8, 2011, which is based upon and claims the benefit of priority from Japanese Application No. 2010-253080, filed Nov. 11, 2010. The entire contents of these applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for producing a filtration filter. 
     2. Description of Background Art 
     Filtration filters are often used to produce clean water from factory and household wastewater (sewage) by removing contaminants and foreign matter or to produce freshwater from seawater by removing salt content or the like. As for filtration filters, reverse osmosis membranes made of polymeric material such as polymeric membranes of methyl acetate are known. A reverse osmosis membrane has numerous penetrating holes with a diameter of a few nanometers. When pressure is added to sewage or seawater to make it flow through such penetrating holes, contaminant molecules the size of a few dozen nanometers and hydrated sodium ions surrounded by water molecules cannot pass through the penetrating holes, while water molecules each with an approximate diameter of 0.38 nm can pass though the penetrating holes. Accordingly, the reverse osmosis membrane produces clean water or freshwater from sewage or seawater by separating water molecules from contaminants or salt content. 
     However, when polluted water is purified using reverse osmosis membranes to provide clean water in developing countries and areas stricken by natural disasters, problems arise such as a notably reduced life span for reverse osmosis membranes due to bacteria contained in polluted water that cause decay in polymeric membranes. 
     Also, since salt and fine sand tend to be mixed into lubricating oil in windmill-type wind power generators located along shore lines, it is strongly required that salt and fine sand be removed from lubricating oil. However, if reverse osmosis membranes are used to remove salt and fine sand, ingredients of the lubricating oil may dissolve polymeric membranes, causing problems such as a notably short life span for the reverse osmosis membranes. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a method for producing a filtration filter includes using masking film formed on a surface of a rigid substrate and having a plurality of openings with a uniform size to expose portions of the surface, etching the portions of the substrate corresponding to the openings, and forming a plurality of holes or grooves in the substrate. 
     According to one aspect of the present invention, a method for producing a filtration filter includes laminating a plurality of rigid substrates by means of organic material to have a predetermined distance from each other, and removing the organic material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1A  is a view showing a step of a method for producing a filtration filter according to a first embodiment of the present invention; 
         FIG. 1B  is a view showing a step of a method for producing a filtration filter according to the first embodiment of the present invention; 
         FIG. 1C  is a view showing a step of a method for producing a filtration filter according to the first embodiment of the present invention; 
         FIG. 1D  is a view showing a step of a method for producing a filtration filter according to the first embodiment of the present invention; 
         FIG. 1E  is a view showing a step of a method for producing a filtration filter according to the first embodiment of the present invention; 
         FIG. 1F  is a view showing a step of a method for producing a filtration filter according to the first embodiment of the present invention; 
         FIG. 2A  is a view showing a step of a method for producing a filtration filter according to a second embodiment of the present invention; 
         FIG. 2B  is a view showing a step of a method for producing a filtration filter according to the second embodiment of the present invention; 
         FIG. 2C  is a view showing a step of a method for producing a filtration filter according to the second embodiment of the present invention; 
         FIG. 3A  is a view showing a step of a method for producing a filtration filter according to a third embodiment of the present invention; 
         FIG. 3B  is a view showing a step of a method for producing a filtration filter according to the third embodiment of the present invention; 
         FIG. 3C  is a view showing a step of a method for producing a filtration filter according to the third embodiment of the present invention; 
         FIG. 3D  is a view showing a step of a method for producing a filtration filter according to the third embodiment of the present invention; 
         FIG. 4A  is a view showing a step of a method for producing a filtration filter according to a fourth embodiment of the present invention; 
         FIG. 4B  is a view showing a step of a method for producing a filtration filter according to the fourth embodiment of the present invention; 
         FIG. 4C  is a view showing a step of a method for producing a filtration filter according to the fourth embodiment of the present invention; 
         FIG. 4D  is a view showing a step of a method for producing a filtration filter according to the fourth embodiment of the present invention; 
         FIG. 4E  is a view showing a step of a method for producing a filtration filter according to the fourth embodiment of the present invention; 
         FIG. 4F  is a view showing a step of a method for producing a filtration filter according to the fourth embodiment of the present invention; 
         FIG. 4G  is a view showing a step of a method for producing a filtration filter according to the fourth embodiment of the present invention; 
         FIG. 4H  is a view showing a step of a method for producing a filtration filter according to the fourth embodiment of the present invention; 
         FIG. 4I  is a view showing a step of a method for producing a filtration filter according to the fourth embodiment of the present invention; 
         FIG. 4J  is a view showing a step of a method for producing a filtration filter according to the fourth embodiment of the present invention; 
         FIG. 4K  is a view showing a step of a method for producing a filtration filter according to the fourth embodiment of the present invention; 
         FIG. 4L  is a view showing a step of a method for producing a filtration filter according to the fourth embodiment of the present invention; 
         FIG. 4M  is a view showing a step of a method for producing a filtration filter according to the fourth embodiment of the present invention; 
         FIG. 5A  is a view showing a step of a method for producing a filtration filter according to a fifth embodiment of the present invention; 
         FIG. 5B  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 5C  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 5D  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 5E  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 5F  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 5G  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 5H  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 5I  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 5J  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 5K  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 5L  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 5M  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 5N  is a view showing a step of a method for producing a filtration filter according to the fifth embodiment of the present invention; 
         FIG. 6A  is a view showing a step of a method for producing a filtration filter according to a sixth embodiment of the present invention; 
         FIG. 6B  is a view showing a step of a method for producing a filtration filter according to the sixth embodiment of the present invention; 
         FIG. 6C  is a view showing a step of a method for producing a filtration filter according to the sixth embodiment of the present invention; 
         FIG. 6D  is a view showing a step of a method for producing a filtration filter according to the sixth embodiment of the present invention; 
         FIG. 6E  is a view showing a step of a method for producing a filtration filter according to the sixth embodiment of the present invention; 
         FIG. 7A  is a view showing a step of a method for producing a filtration filter according to a modified example of the sixth embodiment of the present invention; 
         FIG. 7B  is a view showing a step of a method for producing a filtration filter according to the modified example of the sixth embodiment of the present invention; 
         FIG. 7C  is a view showing a step of a method for producing a filtration filter according to the modified example of the sixth embodiment of the present invention; 
         FIG. 7D  is a view showing a step of a method for producing a filtration filter according to the modified example of the sixth embodiment of the present invention; 
         FIG. 7E  is a view showing a step of a method for producing a filtration filter according to the modified example of the sixth embodiment of the present invention; 
         FIG. 7F  is a view showing a step of a method for producing a filtration filter according to the modified example of the sixth embodiment of the present invention; 
         FIG. 7G  is a view showing a step of a method for producing a filtration filter according to the modified example of the sixth embodiment of the present invention; 
         FIG. 8  is a cross-sectional view showing a modified example of a filtration filter produced by a method for producing a filtration filter according to the sixth embodiment of the present invention; 
         FIG. 9  is a view showing a step of a method for producing a filtration filter according to a seventh embodiment of the present invention; 
         FIG. 10A  is a view showing a step of a method for producing a filtration filter according to an eighth embodiment of the present invention; 
         FIG. 10B  is a view showing a step of a method for producing a filtration filter according to the eighth embodiment of the present invention; 
         FIG. 10C  is a view showing a step of a method for producing a filtration filter according to the eighth embodiment of the present invention; 
         FIG. 11A  is a view showing a step of a method for producing a filtration filter according to a first modified example of the eighth embodiment of the present invention; 
         FIG. 11B  is a view showing a step of a method for producing a filtration filter according to the first modified example of the eighth embodiment of the present invention; 
         FIG. 12A  is a view showing a step of a method for producing a filtration filter according to a second modified example of the eighth embodiment of the present invention; 
         FIG. 12B  is a view showing a step of a method for producing a filtration filter according to the second modified example of the eighth embodiment of the present invention; 
         FIG. 13  is a view showing a step of a method for producing a filtration filter according to a ninth embodiment of the present invention; 
         FIG. 14  is a view showing a step of a method for producing a filtration filter according to a tenth embodiment of the present invention; 
         FIG. 15  is a view showing a first modified example of a substrate to be used in a method for producing a filtration filter according to the present invention; 
         FIG. 16A  is a view showing a second modified example of a substrate to be used in a method for producing a filtration filter according to the present invention; and 
         FIG. 16B  is a view showing a third modified example of a substrate to be used in a method for producing a filtration filter according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings. 
     First, a method for producing a filtration filter is described according to a first embodiment of the present invention. 
       FIGS. 1A˜1F  are views showing steps of a method for producing a filtration filter according to the first embodiment of the present invention. 
     In  FIGS. 1A˜1F , plasma etching, for example, is conducted on silicon substrate  1  using masking film formed on a surface of substrate  1  and having numerous opening portions to expose portions of the surface. In the present embodiment, since each opening portion of the masking film is shaped to be a circle with an approximate diameter of 100 nm˜1 μm, numerous circular holes  2  with an approximate diameter of 100 nm˜1 μm are formed in substrate  1  ( FIG. 1A ). 
     Next, silica film  3  is deposited on the surface of substrate  1  and inner surfaces of circular holes  2  by CVD (Chemical Vapor Deposition) using thermal oxidation. During that time, more silica film  3  is deposited near the opening end than inside circular hole  2 , making the actual diameter of circular hole  2  the smallest near the opening end ( FIG. 1B ). In the present embodiment, CVD treatment duration is adjusted so that diameter (D 1 ) at minimum-diameter portion  4  near the opening end of circular hole  2  is reduced to be 1 nm˜100 nm by silica film  3 . 
     Next, silica films  3  of two substrates  1 , where the diameter of circular holes  2  is reduced by silica film  3 , make contact with each other, and the temperature of the ambient atmosphere is raised to 400° C.˜1000° C. so that silica films  3  are thermally bonded. At that time, two substrates  1  are laminated so that the position of each circular hole  2  in upper substrate  1  in the drawing is aligned with the position of each circular hole  2  in lower substrate  1  in the drawing ( FIG. 1C ). 
     Next, the lower surface of lower substrate  1  in the drawing is polished by CMP (chemical mechanical polishing) or the like to remove the silicon portion of that substrate  1  so that diameter-adjustment portion  5  made only of silica film  3  remains in lower substrate  1  in the drawing. At that time, the silicon portion is removed so that minimum-diameter portion  4  remains in diameter-adjustment portion  5  ( FIG. 1D ). 
     Next, lamination of substrates  1  shown in  FIG. 1C  and polishing of substrates shown in  FIG. 1D  are repeated until at least 10 or more layers of diameter-adjustment portion  5 , more preferably 100 or more layers, are laminated ( FIG. 1E ). Then, the lower surface of upper substrate  1  in the drawing is polished by CMP or the like to remove the silicon portion of that substrate  1  so that diameter-adjustment portion ( 5   a ) made only of silica film  3  remains in upper substrate  1  in the drawing. Accordingly, filtration filter  6  is formed as a reverse osmosis membrane, completing the present process ( FIG. 1F ). 
     In filtration filter  6 , flow channels  7  are formed by connecting minimum-diameter portions  4  of diameter-adjustment portions  5 , making the minimum diameter of flow channels  7  to be 1 nm˜100 nm. Accordingly, filtration filter  6  is used to remove vibrio cholerae and typhoid bacillus with a size of a few hundred nanometers by flowing sewage or seawater through flow channels  7 . Moreover, if the minimum diameter of flow channels  7  is controlled to be 1 nm˜5 nm, not only contaminants and salt content but also picornaviruses and parvoviruses with an approximate size of 20 nm are removed. 
     In the present embodiment, diameter-adjustment portions  5  are laminated downward by polishing the lower surface of lower substrate  1  in  FIG. 1D . However, diameter-adjustment portions  5  may also be laminated upward by polishing the lower surface of upper substrate  1 . 
     According to the method for producing a filtration filter of the present embodiment, the diameter of numerous circular holes formed in substrate  1  is directly controlled by adjusting the diameter of the opening portions of masking film. Accordingly, when forming circular holes  2  with a diameter of a few nanometers˜100 nm, irregularities are prevented in the diameter of circular holes  2 . As a result, by selecting the hole diameters, vibrio cholerae with a size of a few hundred nanometers, viruses with a size of a few dozen nanometers and contaminants are prevented from passing through substrate  1 , and a combined use of a distillation method or the like is not required when producing clean water or freshwater purified by filtration filter  6  formed by laminating substrates  1 . Thus, procedures for obtaining clean water or freshwater are simplified. Also, since filtration filter  6  is made of rigid silica film  3 , primary pressure applied on sewage or seawater can be increased, improving the purification efficiency of producing clean water or freshwater. 
     In the above-described method for producing a filtration filter according to the present embodiment, silica is deposited by CVD. Since the deposition amount is adjustable by adjusting the CVD treatment duration, it is easy to set the diameter of circular holes  2  at a required value. 
     Also, in the above-described method for producing a filtration filter according to the present embodiment, since the lower surface of substrate  1  is polished after numerous circular holes  2  are formed in substrate  1 , each circular hole will surely penetrate through substrate  1  by adjusting the polishing amount. 
     Moreover, in the above-described method for producing a filtration filter according to the present embodiment, since 10 or more layers of diameter-adjustment portions  5  are laminated, the strength of filtration filter  6  is enhanced. 
     In the above-described method for producing a filtration filter according to the present embodiment, silica film  3  is formed on a surface of substrate  1 , and silica film  3  on each substrate  1  is thermally bonded to each other when two substrates  1  are laminated. Thus, substrates  1  are firmly bonded to each other, further improving the strength of filtration filter  6 . 
     In the above-described method for producing a filtration filter according to the present embodiment, plasma etching is conducted on substrates. However, any other etching method may be employed as long as openings of masking film are accurately transcribed to a substrate. 
     In the above-described method for producing a filtration filter according to the present embodiment, silica film  3  is deposited on a surface of substrate  1  and inner surfaces of circular holes  2  by CVD. However, as long as it can be deposited by CVD, it may be any rigid film such as silicon-nitride film, polysilicon film or the like. Although silicon is used for substrate  1 , metal or metal oxide may also be used to form substrate  1  as long as it is a rigid material that can be etched. Also, CVD by thermal oxidation is used when depositing silica film  3 , but plasma CVD may also be used. 
     In the above-described method for producing a filtration filter according to the present embodiment, minimum-diameter portion  4  is included in all diameter-adjustment portions  5  of flow channel  7 . However, minimum-diameter portion  4  is not required for all diameter-adjustment portions  5 , and it is an option that only one diameter-adjustment portion  5  of flow channel  7  has minimum-diameter portion  4 . 
     Moreover, in the above-described method for producing a filtration filter according to the present embodiment, the entire silicon portion is removed from each substrate  1 . However, it is not always required to remove the entire silicon portion. It is sufficient if the silicon portion is removed to such a degree that at least circular holes  2  penetrate through substrate  1 . 
     In the above-described method for producing a filtration filter according to the present embodiment, circular holes  2  are formed in each substrate  1 . However, each opening portion of masking film may be formed as a slit so that grooves are formed in substrate  1  through etching by using such opening portions. In such a case, it is preferred to adjust the minimum width of grooves at 1 nm˜100 nm, more preferably at 1 nm˜5 nm, by depositing silica on the inner surfaces of the grooves. 
     Next, a method for producing a filtration filter is described according to a second embodiment of the present invention. 
       FIGS. 2A˜2C  are views showing steps of a method for producing a filtration filter according to a second embodiment of the present invention. 
     In  FIGS. 2A˜2C , first, using masking film formed on a surface of silicon substrate  8  and having numerous opening portions to expose portions of the surface, substrate  8  is etched so that numerous DTs (deep trenches)  9  are formed. In the present embodiment, plasma etching capable of high anisotropic etching is preferred so that DTs with a high aspect ratio are formed. 
     In the present embodiment, since each opening portion of the masking film is made into a slit shape with an approximate width of 20 nm˜40 nm, numerous DTs  9  with an approximate width of 20 nm˜40 nm are formed in substrate  8  ( FIG. 2A ). Usually, DTs with an aspect ratio of 10 or greater have narrowed tip portions. In DTs  9  of the present embodiment, the width of the tip portions is approximately 10 nm. 
     Next, silica film  10  is deposited on a surface of substrate  8  and on inner surfaces of DTs  9  through ALD (Atomic Layer Deposition), and only the silica  10  deposited on the surface of substrate  8  is further removed ( FIG. 2B ). In the present embodiment, the ALD treatment duration is adjusted so that minimum width (D 1 ) at tip portions of DTs  9  is set at 1 nm˜5 nm, preferably at 1 nm˜3 nm. 
     Next, a lower surface of substrate  8  is polished by CMP or the like, and such polishing is stopped when the tip portions of DTs  9  are exposed at the lower surface of substrate  8 . In doing so, filtration filter  11  is formed when each DT  9  penetrates through substrate  8  ( FIG. 2C ). Accordingly, the present process is completed. 
     In filtration filter  11 , minimum width (D 1 ) of DTs  9  penetrating through substrate  8  is set at 1 nm˜5 nm. Thus, when sewage or seawater flows through DTs  9  of filtration filter  11 , not only contaminants and salt content but also picornaviruses and parvoviruses with an approximate size of 20 nm are removed. 
     In the method for producing a filtration filter according to the present embodiment, silica film is deposited through ALD. Since ALD can deposit by a unit of one atom, minimum width (D 1 ) at tip portions of DTs  9  is adjusted precisely at a required value. 
     In the above-described method for producing a filtration filter according to the present embodiment, DTs  9  were formed in substrate  8 . However, each opening portion of masking film may be formed in a circular shape so that circular holes are formed in substrate  8  through etching by using such opening portions. In such a case, it is preferred to adjust the minimum diameter of circular holes to be 1 nm˜5 nm by depositing silica on the inner surfaces of circular holes. 
     When removing vibrio cholerae, typhoid bacillus and the like with a size of a few hundred nanometers, minimum width (D 1 ) of DTs  9  or the minimum diameter of penetrating holes may be set at 1 nm˜100 nm. Except that the opening size for forming such DTs is roughly 100 nm˜1 μm, the method is not different from that for forming DTs  9  with minimum width (D 1 ) of 1 nm˜5 nm. 
     The method for producing a filtration filter according to the present embodiment has the same effects as the method for producing a filtration filter according to the above-described first embodiment. 
     Next, a method for producing a filtration filter is described according to a third embodiment of the present invention. 
       FIGS. 3A˜3D  are views showing steps of a method for producing a filtration filter according to a third embodiment of the present invention. 
     In  FIGS. 3A˜3D , first, silicon substrate  12  is prepared ( FIG. 3A ), and amorphous carbon film  13  with a thickness of 1 nm˜100 nm is deposited on a surface of substrate  12  ( FIG. 3B ). 
     Next, multiple substrates  12  are laminated in such a way that amorphous carbon film  13  of one substrate  12  makes contact with a lower surface of another substrate  12 , their peripheral borders are secured by a frame (not shown) or the like ( FIG. 3C ), and each amorphous carbon film  13  is removed by ashing. Accordingly, filtration filter  14  is formed as a reverse osmosis membrane ( FIG. 3D ), completing the present process. 
     In filtration filter  14 , slit-shaped flow channel  15  is formed between two adjacent substrates  12  after each amorphous carbon film  13  is removed, and the width of flow channel  15  is 1 nm˜100 nm. When sewage or seawater flows through flow channels  15  in a direction indicated by arrows in the drawing, vibrio cholerae, typhoid bacillus and the like with a size of a few hundred nanometers are removed by filtration filter  14 . Moreover, by controlling the width of flow channels  15  to be 1 nm˜5 nm, not only contaminants and salt content but also picornaviruses and parvoviruses with an approximate size of 20 nm are removed. 
     According to the method for producing a filtration filter of the present embodiment, after multiple silicon substrates  12  are laminated by means of amorphous carbon film  13  to set their distance at 1 nm˜100 nm, each amorphous carbon film  13  is removed. Thus, the width of slit-shaped flow channel  15  formed between adjacent substrates  12  is directly controlled, preventing irregularities in the width of slit-shaped flow channels. 
     Also, in the above-described method for producing a filtration filter according to the present embodiment, since slit-shaped flow channels  15  with a width of 1 nm˜100 nm are used for filtration, a greater amount of sewage or seawater can flow through flow channels  15  than when using circular holes with a minimum diameter of 1 nm˜100 nm for filtration. As a result, the purification efficiency of producing clean water or freshwater is enhanced. 
     Moreover, in the above-described method for producing a filtration filter according to the present embodiment, the distance between adjacent substrates  12  is maintained when the peripheral borders of multiple substrates  12  are secured by a frame or the like. However, pillar-shaped distance retainers with a height of 1 nm˜100 nm may be placed between adjacent substrates  12  so that the distance is maintained between adjacent substrates  12 . 
     In the above-described method for producing a filtration filter according to the present embodiment, each amorphous carbon film  13  is removed by ashing. However, each amorphous carbon film  13  may be removed by wet etching using a supercritical chemical solution or the like. Since supercritical chemical solutions enter fine space smoothly, each amorphous carbon film  13  is surely removed. 
     The method for producing a filtration filter according to the present embodiment has the same effects as the method for producing a filtration filter according to the above-described first embodiment. 
     Next, a method for producing a filtration filter is described according to a fourth embodiment of the present invention. 
       FIGS. 4A˜4M  are views showing steps of a method for producing a filtration filter according to a fourth embodiment of the present invention. 
     In  FIGS. 4A˜4M , first, silicon substrate  17  with silicon-nitride film  16  formed on its surface is etched using masking film formed on a surface of substrate  17  and having opening portions to expose portions of the surface so that trenches  18  with an approximate width of 10 nm˜300 nm are formed in substrate  17  ( FIGS. 4A ,  4 B). Here,  FIG. 4A  is a plan view. 
     Next, amorphous carbon film  19  with a thickness of 1 nm-100 nm is deposited on a surface of substrate  17  and inner surfaces of trenches  18  ( FIG. 4C ). 
     Next, the surface of substrate  17  is made flat by depositing silica film  20  through CVD on inner surfaces of trenches  18  and the surface of substrate  17 , and photoresist film  22  having opening portions  21  is further formed on the flat surface of substrate  17  ( FIG. 4D ). Amorphous carbon film  19  in trenches  18  is covered accordingly by silica film  20  during that time. 
     Next, using photoresist film  22  as masking film, portions of silica film  20  and amorphous carbon film  19  are etched away to expose silicon-nitride film  16  ( FIG. 4E ), the entire surface of substrate  17  is covered by silicon-nitride film  23  through CVD (FIG.  4 F), and photoresist film  24  is further formed covering portions of the surface of substrate  17  ( FIG. 4G ). 
     Next, using photoresist film  24  as masking film, portions of silicon-nitride film  23  are etched away to expose silica film  20  ( FIG. 4H ), the entire surface of substrate  17  is covered by silicon-nitride film  25  through CVD ( FIG. 4I ), and photoresist film  26  is further formed covering portions of the surface of substrate  17  ( FIG. 4J ). 
     Next, using photoresist film  26  as masking film, portions of silicon-nitride film  25  and silica film  20  are etched away to expose portions of amorphous carbon film  19  ( FIG. 4K ), and the entire amorphous carbon film  19  is removed by ashing so that hollows  27  with a U-shaped cross section and with a width of 1 nm˜100 nm are formed in substrate  17  ( FIG. 4L ). 
     Next, the lower surface of substrate  17  is polished by CMP or the like, and such polishing is stopped when hollows  27  are exposed at the lower surface of substrate  17 . In doing so, flow channels  28  with a width of 1 nm˜100 nm are formed, penetrating through substrate  17  in a thickness direction ( FIG. 4M ). Accordingly, the present process is completed. 
     According to the method for producing a filtration filter of the present embodiment, amorphous carbon film  19  with a thickness of 1 nm˜100 nm is deposited on inner surfaces of trenches  18 , and amorphous carbon film  19  is removed after it is covered by silica film  20  so that flow channels  28  with a width of 1 nm˜100 nm are formed. When sewage or seawater flows through flow channels  28 , vibrio cholerae, typhoid bacillus and the like are removed. Also, by controlling the width of flow channels  28  at 1 nm˜5 nm, contaminants, salt content and even viruses are removed. Therefore, without a combined use of a distillation method or the like, clean water or freshwater is obtained. 
     In the above-described method for producing a filtration filter of the present embodiment, trenches  18  are formed in substrate  17 . However, circular holes may be formed in substrate  17 . In such a case, amorphous carbon film  19  is deposited on the inner surfaces of the circular holes, and such amorphous carbon film  19  is removed in a later step so that flow channels in a circular shape are formed. 
     The method for producing a filtration filter according to the present embodiment has the same effects as the method for producing a filtration filter according to the above-described first embodiment. 
     Next, a method for producing a filtration filter is described according to a fifth embodiment of the present invention. 
       FIGS. 5A˜5N  are views showing steps of a method for producing a filtration filter according to a fifth embodiment of the present invention.  FIGS. 5B ,  5 D,  5 F,  5 H,  5 J,  5 L and  5 N are plan views. 
     In  FIGS. 5A˜5N , first, silicon substrate  31  with silicon-nitride film  29  and silica film  30  formed on its surface is covered by photoresist film  33  having multiple circular opening portions  32  with an approximate diameter of 10 nm˜300 nm ( FIGS. 5A ,  5 B). Using photoresist film  33  as masking film, silicon-nitride film  29 , silica film  30  and substrate  31  are etched so that multiple circular holes  34  with an approximate diameter of 10 nm˜300 nm are formed in substrate  31  ( FIGS. 5C ,  5 D). 
     Next, silicon-nitride film  29  and silica film  30  are removed by etching or the like, and amorphous carbon film  35  with a thickness of 1 nm˜100 nm is deposited on the inner surfaces of circular holes  34  ( FIGS. 5E ,  5 F). Furthermore, photoresist film  37 , which covers part of circular holes  34  and amorphous carbon film  35  in a planar view and has slit-shaped opening portions  36 , is formed on a surface of substrate  31  ( FIGS. 5G ,  5 H), amorphous carbon film  35  exposed from photoresist film  37  as masking film is removed by ashing, and remaining photoresist film  37  is further removed by ashing or the like. Accordingly, amorphous film  35  shaped like a “C” in a planar view remains on the inner surfaces of circular holes  34  ( FIGS. 5I ,  5 J). 
     Next, circular holes  34  are filled with silica  38  through CVD ( FIGS. 5K ,  5 L). During that time, amorphous carbon film  35  in circular holes  34  is covered accordingly by silica  38 . Then, remaining amorphous carbon film  35  is removed by ashing. Accordingly, flow channels  39  sandwiched by substrate  31  and silica  38  and shaped like a “C” in a planar view are formed ( FIGS. 5M ,  5 N). The present process is completed. 
     According to the method for producing a filtration filter of the present embodiment, after amorphous carbon film  35  with a thickness of 1 nm˜100 nm is deposited on the inner surfaces of circular holes  34  and then covered by silica  38 , the amorphous carbon film  35  is removed. Thus, flow channels  39  with a width of 1 nm˜100 nm are formed. When sewage or seawater flows through flow channels  39  in a direction indicated by arrows in the drawing, vibrio cholera, typhoid bacillus, contaminants, salt content and viruses are removed. Accordingly, a combined use of a distillation method or the like is not required to produce clean water or freshwater. 
     The method for producing a filtration filter according to the present embodiment has the same effects as the method for producing a filtration filter according to the above-described first embodiment. 
     Next, a method for producing a filtration filter is described according to a sixth embodiment of the present invention. 
       FIGS. 6A˜6E  are views showing steps of a method for producing a filtration filter according to a sixth embodiment of the present invention. 
     In  FIGS. 6A˜6E , first, by etching silicon substrate  40  using masking film formed on a surface of substrate  40  and having numerous opening portions to expose portions of the surface, multiple penetrating holes  41  with a diameter of a few dozen nanometers to 300 nm are formed. Then, amorphous carbon film  42  is formed on a surface of substrate  40 . Amorphous carbon film  42  includes numerous distance retainers with a size of 1 nm˜100 nm, for example, micropillars  43  with a height of 1 nm˜100 nm ( FIG. 6A ). 
     Next, substrate  45  having multiple penetrating holes  44  with a diameter of a few dozen nanometers to 300 nm is formed by etching the same as substrate  40  using masking film. Then, substrate  45  is compressed and laminated to substrate  40  by means of amorphous carbon film  42 , and the substrates are further bonded. Although amorphous carbon film  42  is squeezed to be compressed in a thickness direction during that time, micropillars  43  are not compressed. Thus, the distance between substrate  40  and substrate  45  is maintained at 1 nm˜100 nm ( FIG. 6B ). Here, substrate  45  is laminated to substrate  40  in such a way that penetrating holes  44  do not align with penetrating holes  41  in a planar view. 
     Next, porous ceramic material  46  is fully filled in each penetrating hole  44  through PVD (physical vapor deposition) ( FIG. 6C ), and amorphous carbon film  42  is removed by ashing to form gap  47  between substrate  40  and substrate  45  ( FIG. 6D ). Here, since micropillars  43  exist between substrates  40  and  45  as described above, the thickness of gap  47  is the same as the height of micropillars  43 . 
     Next, after amorphous carbon film  42  is formed on a surface of substrate  45 , the above-described steps shown in  FIGS. 6B through 6D  along with a step for forming amorphous carbon film on the surface of the uppermost substrate are repeated so that substrates  48  and  49  having the same structure as substrate  45  are laminated in that order on substrate  45 . During such time, substrates  48  and  49  are laminated on substrate  45  in such a way that penetrating holes of adjacent substrates do not align in a planar view. In addition, each amorphous carbon film  42  between substrates is removed by ashing every time a substrate is laminated. Accordingly, filtration filter  50  is formed as a reverse osmosis membrane ( FIG. 6E ), completing the present process. 
     In filtration filter  50 , gaps  47  with a thickness of 1 nm˜100 nm, which are formed when each amorphous carbon film is removed, work as flow channels, and sewage or seawater flows through ceramic material  46  and gaps  47  in a direction indicated by an arrow in the drawing. Thus, vibrio cholerae, typhoid bacillus and the like with a size of a few hundred nanometers are removed by gaps  47 . Moreover, by controlling gaps  47  to be 1 nm˜5 nm, not only contaminants or salt content but also picornaviruses and parvoviruses with an approximate size of 20 nm are removed. 
     According to the method for producing a filtration filter of the present embodiment, since amorphous carbon film  42  includes micropillars  43  with a height of 1 nm˜100 nm, the thickness of gaps  47  is securely maintained at 1 nm˜100 nm by micropillars  43  even after amorphous carbon film  42  is removed. 
     In the above-described method for producing a filtration filter according to the present embodiment, substrates are laminated in such a way that the penetrating holes of each substrate do not align with each other in a planar view. Thus, ceramic material  46  of each substrate is prevented from aligning with each other to form penetrating holes made only of ceramic material  46 . Accordingly, vibrio cholerae and typhoid bacillus with a size of a few hundred nanometers, viruses with a size of a few dozen nanometers and contaminants are prevented from passing through filtration filter  50  in a thickness direction. 
       FIGS. 7A˜7G  are views showing a method for producing a filtration filter according to a modified example of the sixth embodiment of the present invention. 
     In  FIGS. 7A˜7G , first, by etching silicon substrate  51  using masking film formed on a surface of substrate  51  and having numerous opening portions to expose portions of the surface, multiple penetrating holes  52  with a diameter of a few dozen nanometers to 300 nm are formed. Then, amorphous carbon film  53  with a thickness of 1 nm˜100 nm is further formed on the surface of substrate  51  ( FIG. 7A ). 
     Next, silicon substrate  54  is laminated on substrate  51  and bonded by means of amorphous carbon film  53  ( FIG. 7B ), and by etching substrate  54  using masking film formed on a surface of substrate  54  and having numerous opening holes to expose portions of the surface, multiple penetrating holes  55  with a diameter of a few dozen nanometers to 300 nm are formed. At that time, penetrating holes  55  are formed not to align with penetrating holes  52  of substrate  51  in a planar view. Also, amorphous carbon film  53  is removed from the bottom of each penetrating hole  55  ( FIG. 7C ). 
     Next, a surface of substrate  54  is covered by porous ceramic material  56  through PVD and penetrating holes  55  are filled with ceramic material  56  ( FIG. 7D ). Ceramic material  56  in penetrating holes  55  are naturally bonded to substrates  51  and  54 . Then, ceramic material  56  deposited on the surface of substrate  54  during PVD is removed by polishing or the like ( FIG. 7E ). 
     Next, amorphous carbon film  53  is removed by ashing. Here, since ceramic material  56  in each penetrating hole  55  is bonded with substrates  51  and  54 , substrates  51  and  54  will not be separated from each other. Ceramic material  56  prevents substrates  51  and  54  from touching each other, and gap  57  with a thickness of 1 nm˜100 nm is formed between substrates  51  and  54  ( FIG. 7F ). 
     Next, after amorphous carbon film  53  is formed on a surface of substrate  54 , the above-described steps shown in  FIGS. 7B through 7F  along with a step for forming amorphous carbon film on the surface of the uppermost substrate are repeated so that substrates  58  and  59  having the same structure as substrate  54  are laminated in that order on substrate  54 . During that time, penetrating holes of each substrate are formed not to align with each other in a planar view. In addition, each amorphous carbon film  53  between substrates is removed by ashing every time a substrate is laminated. Accordingly, filtration filter  60  is formed as a reverse osmosis membrane ( FIG. 7G ), completing the present process. 
     In filtration filter  60 , porous ceramic material  56  and gaps  57  with a thickness of 1 nm˜100 nm, which are formed when each amorphous carbon film is removed, work as flow channels. Since sewage or seawater flows through ceramic material  56  and gaps  57  in a direction indicated by an arrow in the drawing, not only contaminants or salt content but also picornaviruses and parvoviruses with an approximate size of 20 nm are removed by gaps  57 . 
     In the method for producing a filtration filter according to the present modified example, penetrating holes of each substrate are also formed not to align with each other in a planar view. Thus, ceramic material  56  of each substrate is prevented from aligning with each other to form penetrating holes made only of ceramic material  56 . Accordingly, vibrio cholerae and typhoid bacillus with a size of a few hundred nanometers, viruses with a size of a few dozen nanometers and contaminants are prevented from passing through filtration filter  60  in a thickness direction. 
     In the above-described filtration filters  50  and  60 , multiple penetrating holes  61  are formed to go through each substrate ( 45 ,  48 ,  49  or  54 ,  58 ,  59 ) all at once, and a rigid member made of metal such as tungsten is inserted in each penetrating hole  61  to form pillars  62  which penetrate through filtration filter  50  or  60  in a thickness direction ( FIG. 8 ). Accordingly, the strength of filtration filter  50  or  60  is also enhanced. 
     The method for producing a filtration filter according to the present embodiment has the same effects as the method for producing a filtration filter according to the above-described first embodiment. 
       FIG. 9  is a view showing a step in a method for producing a filtration filter according to a seventh embodiment of the present invention. 
     In  FIG. 9 , first, using masking film formed on a surface of each substrate  63  and having numerous opening portions to expose portions of the surface, multiple penetrating holes  64  with a diameter of a few dozen nanometers to 300 nm are formed in multiple silicon substrates  63  through etching. Then, when multiple substrates  63  are laminated and bonded, penetrating holes  64  of each substrate  63  align with each other in a planar view so that penetrating flow channels  65  are formed to penetrate through all the substrates  63  in a thickness direction. At that time, the overlapping amount of penetrating holes  64  is adjusted so that maximum width (W 1 ) of penetrating flow channels  65  is 1 nm˜100 nm. Accordingly, filtration filter  66  is formed as a reverse osmosis membrane, completing the present process. 
     Since maximum width (W 1 ) of penetrating flow channels  65  is 1 nm˜100 nm in filtration filter  66 , vibrio cholerae and typhoid bacillus with a size of a few hundred nanometers are removed by flowing sewage or seawater through penetrating flow channels  65  of filtration filter  66  along a direction indicated by an arrow in the drawing. Moreover, by controlling minimum width (W 1 ) of penetrating flow channels  65  at 1 nm˜5 nm, not only contaminants and salt content but also picornaviruses and parvoviruses with an approximate size of 20 nm are removed. Accordingly, clean water or freshwater is obtained without a combined use of a distillation method or the like. 
     The method for producing a filtration filter according to the present embodiment has the same effects as the method for producing a filtration filter according to the above-described first embodiment. 
       FIGS. 10A˜10C  are views showing steps of a method for producing a filtration filter according to an eighth embodiment of the present invention. 
     In  FIGS. 10A˜10C , first, substrate  67  made of CF polymer or DLC (diamond-like carbon) is etched using masking film formed on a surface of substrate  67  and having numerous opening portions to expose portions of the surface so that multiple penetrating holes  68  with an approximate diameter of 20˜200 nm are formed. Substrate  67  is placed on base plate  69  made of titanium or diamond, and substrate  67  is further covered by cover  70  made of titanium or diamond ( FIG. 10A ). Depth (D 2 ) of cover  70  is set to be smaller than the thickness of substrate  67 . 
     Next, cover  70  is compressed against base plate  69 . At that time, substrate  67  is compressed in a thickness direction so as to be expanded in a horizontal direction. However, since its peripheral borders are covered by cover  70 , each inner wall of penetrating holes  68  protrudes, reducing the diameter of penetrating holes  68  accordingly ( FIG. 10B ). In the present embodiment, the amount to compress substrate  67  is adjusted to set the reduced diameter of penetrating holes  68  at 1 nm˜100 nm. 
     Next, base plate  69  and cover  70  are removed from substrate  67 , and filtration filter  71  is formed as a reverse osmosis membrane ( FIG. 10C ), completing the present process. 
     Since the diameter of penetrating holes  68  is 1 nm˜100 nm in filtration filter  71 , not only contaminants and salt content but also picornaviruses and parvoviruses with an approximate size of 20 nm are removed by flowing sewage or seawater through penetrating holes  68  of filtration filter  71 . 
     According to the method for producing a filtration filter of the present embodiment, the diameter of penetrating holes  68  is adjusted by compressing substrate  67  in a thickness direction so that penetrating holes  68  are deformed and the inner walls of penetrating holes  68  protrude. Thus, it is easy to produce filtration filter  71 . 
     In the above-described method for producing a filtration filter according to the present embodiment, as long as the diameter of each penetrating hole  68  is set at 1 nm˜100 nm at maximum, it is acceptable if some penetrating holes  68  are blocked. Thus, the amount to compress substrate  67  is preferred to be set relatively great. 
       FIGS. 11A and 11B  are views showing steps of a method for producing a filtration filter according to a first modified example of the eighth embodiment of the present invention. 
       FIGS. 11A and 11B , first, by etching long narrow base  72  made of CF polymer or DLC using masking film having numerous opening portions, multiple penetrating holes  73  with an approximate diameter of 20 nm˜200 nm are formed along a longitudinal direction of long narrow base  72  ( FIG. 11A ). 
     Next, long narrow base  72  is compressed sideways in a direction perpendicular to the direction of its length (directions indicated by arrows in the drawing). At that time, the inner wall of each penetrating hole  73  protrudes inside penetrating hole  73 , resulting in a reduced diameter of penetrating hole  73  ( FIG. 11B ). In the present embodiment, the amount to compress long narrow base  72  is adjusted to set the reduced diameter of penetrating hole  73  at 1 nm˜100 nm. Accordingly, filtration filter  74  is formed as a reverse osmosis membrane, completing the present process. 
     Since the diameter of penetrating holes  73  is at 1 nm˜100 nm in filtration filter  74 , vibrio cholerae and typhoid bacillus with a size of a few hundred nanometers are removed by flowing sewage or seawater through penetrating holes  73  of filtration filter  74 . Moreover, the diameter of the penetrating holes is controlled to be 1 nm˜5 nm so that not only contaminants or salt content but also picornaviruses and parvoviruses with an approximate size of 20 nm are removed. 
       FIGS. 12A and 12B  are views showing steps of a method for producing a filtration filter according to a second modified example of the eighth embodiment of the present invention. 
     In  FIGS. 12A˜12B , first, using masking film formed on a surface of base plate  69  and having numerous opening portions to expose portions of the surface, base plate  69  is etched so that multiple penetrating holes  75  with an approximate diameter of 20 nm˜200 nm are formed, and using masking film formed on a surface of cover  70  and having numerous opening portions to expose portions of the surface, cover  70  is etched so that multiple penetrating holes  76  with an approximate diameter of 20 nm˜200 nm are formed. The same as the producing method shown in  FIGS. 10A˜10C , substrate  67 , which is etched in advance using masking film to have multiple penetrating holes  68  with an approximate diameter of 20 nm˜200 nm, is placed on base plate  69  and then covered by cover  70  ( FIG. 12A ). At that time, positions of base plate  69 , substrate  67  and cover  70  are adjusted so that penetrating holes  75  of base plate  69 , penetrating holes  68  of substrate  67  and penetrating holes  76  of cover  70  align with each other in a planar view. 
     Next, cover  70  is compressed against base plate  69 . At that time, each inner wall of penetrating holes  68  protrudes, thus reducing the diameter of penetrating holes  68  accordingly ( FIG. 12B ). In the present embodiment as well, the amount to compress substrate  67  is adjusted so that the reduced diameter of penetrating holes  68  is set at 1 nm˜100 nm. Accordingly, filtration filter  77  is formed as a reverse filtration membrane without removing base plate  69  and cover  70  from substrate  67 , completing the present process. In filtration filter  77 , base plate  69  and cover  70  work as reinforcing material for substrate  67 . 
     The method for producing a filtration filter according to the above-described present embodiment has the same effects as the method for producing a filtration filter according to the above-described first embodiment. 
       FIG. 13  is a view showing a step of a method for producing a filtration filter according to a ninth embodiment of the present invention. 
     In  FIG. 13 , first, filtration filter  6  is formed by the method for producing a filtration filter shown in  FIGS. 1A˜1F , filtration filter  6  is sandwiched by two porous ceramic members ( 78 ,  79 ), and filtration filter  6  and two ceramic members ( 78 ,  79 ) are bonded together. Accordingly, complex filtration filter  80  is formed as a reverse osmosis membrane, completing the present process. 
     According to the method for producing a filtration filter of the present embodiment, since two ceramic members ( 78 ,  79 ) are bonded to filtration filter  6  where flow channels  7  with a minimum diameter of 1 nm˜100 nm are formed, the strength of resulting complex filtration filter  80  is enhanced. Also, since ceramic filters made of fine permeating holes are used as porous ceramic members ( 78 ,  79 ) in addition to filtration filter  6 , filtration is conducted at least twice in complex filtration filter  80 . Thus, contaminants, salt content, vibrio cholerae, typhoid bacillus, viruses and the like are surely removed. 
     In the method for producing a filtration filter of the present embodiment, filtration filter  6  is sandwiched by two ceramic members ( 78 ,  79 ). However, any one of the filtration filters obtained as shown in  FIGS. 2A through 12B  may be sandwiched by two ceramic members ( 78 ,  79 ). 
     Filtration filter  6  is sandwiched by two ceramic members ( 78 ,  79 ) in the present embodiment. However, it is an option for filtration filter  6  to be bonded to one ceramic member to form complex filtration filter  80 . 
       FIG. 14  is a view showing a step of a method for producing a filtration filter according to a 10th embodiment of the present invention. 
     In  FIG. 14 , first, filtration filter  6  is formed by the method for producing a filtration filter shown in  FIG. 1A˜1F . Then, using polymeric film of methyl acetate, reverse osmosis membrane  81  is formed on a surface of filtration filter  6 . Accordingly, complex filtration filter  82  is formed, completing the present process. 
     According to the method for producing a filtration filter of the present embodiment, since filtration filter  6  having flow channels  7  with a minimum diameter of 1 nm˜100 nm is bonded to reverse osmosis membrane  81  made of polymeric film, filtration is conducted twice when sewage or seawater flows through filtration filter  6  and reverse osmosis membrane  81 . Thus, contaminants, salt content or even viruses are surely removed. Also, since it is known that reverse osmosis membranes made of polymeric film are usually characterized by blocking ions by repelling or absorbing ions in water, filtration filter  6  can be used as ion blocking properties of reverse osmosis membrane  81 , thus surely removing sodium ions and chloride ions in seawater. 
     In the method for producing a filtration filter of the present embodiment, filtration filter  6  is bonded to reverse osmosis membrane  81 . However, any one of the filtration filters obtained as shown in  FIGS. 2A through 12B  may be bonded to reverse osmosis membrane  81 . 
     In a method for producing filtration filters according to each embodiment, slits or circular holes are formed in each filtration filter. However, flow channels may be formed in a filtration filter through etching by processing a substrate to have a pectinate shape in a planar view. In such a case, first, multiple grooves with a width of 1 nm˜5 nm are formed in a substrate, where the periphery of one end of the substrate is open in a planar view as shown in  FIG. 15 , then one end of each groove is covered by a plate member or part of a frame covering the substrate to form flow channels. In doing so, cross sections of flow channels are surely enlarged so that the amount of sewage or seawater flowing through the filter is increased, enhancing the purification efficiency of producing clean water or freshwater by the filtration filter. 
     The purification efficiency of producing clean water or freshwater by a filtration filter in each embodiment described above decreases after being used for providing purified clean water or freshwater due to clogs caused by trapped contaminants or salt content. Thus, filtration filters are restored by conducting etching or ashing again so that trapped contaminants or salt content are removed. Since filtration filters of each embodiment are made of relatively rigid material such as silicon, they do not sustain damage or deterioration even with another etching or ashing. Namely, filtration filters of each embodiment described above are reusable. Also, if the diameter of flow channels  7  or the like is enlarged due to another etching or ashing while restoring a filtration filter, the enlarged diameter of flow channels  7  or the like will be roughly a few dozen nanometers when the initial diameter of flow channels  7  is 1 nm˜5 nm, for example. Thus, the restored filtration filter may be used for filtering sewage containing filtration targets with a size of a few hundred nanometers or greater, or it may be used for dialysis treatments. Accordingly, waste that contains contaminants or the like can be prevented by methods for producing filtration filters according to the above-described embodiments. Also, by applying water pressure from a direction opposite the filtration direction of sewage or seawater, trapped contaminants or the like may be removed. In such a case as well, since filtration filters are made of rigid material, filtration filters are tolerant to relatively high pressures, allowing efficient removal of contaminants or the like. 
     In addition, when penetrating holes are formed in filtration filters of each embodiment, the opening at one end of a penetrating hole is set at a predetermined size effective for filtration and the diameter of the penetrating hole is set to increase from that end toward the other end as shown in  FIG. 16A . Alternatively, as shown in  FIG. 16B , the middle portion of a penetrating hole is set to be the same predetermined size as above and the diameter of the penetrating hole is set to increase from the middle point toward both of its ends. In doing so, portions of penetrating holes that require cleansing are reduced, making it easier to cleanse the penetrating holes of filtration filters. 
     Also, as described above, since filtration filters of each embodiment contain relatively rigid substrates such as silicon, sterilizing or antimicrobial metals such as silver may be coated through PVD or CVD, allowing filtration filters to produce purer clean water or freshwater. Here, filtration filters may also be coated with titania, and ultraviolet rays are irradiated during the purification process of producing clean water or freshwater so that a strong sterilization effect through photocatalysis is achieved. Thus, clean water or freshwater is surely sterilized. 
     Moreover, it is an option for substrates contained in filtration filters of each embodiment to be formed using conductive material or semiconductive material. Accordingly, electric power is provided to filtration filters, and clean water or freshwater is sterilized by electromagnetic waves generated by the electric power. 
     Electronic circuits with sensor functions may be built beforehand into substrates in filtration filters of each embodiment. For example, using electronic circuits with water quality sensors built into filtration filters, the degree of purification of clean water or freshwater can be monitored real time, thus preventing low-quality clean water or freshwater. 
     Also, when electronic circuits with flow sensors are built into filtration filters, the amount of purified clean water or freshwater is monitored, and the timing for replacing or restoring filtration filters can be determined properly. In addition, when electronic circuits with vibration sensors are built into filtration filters, vibrations in the substrates of filtration filters are directly monitored, and the timing for replacing filtration filters can be determined properly. When forming built-in electronic circuits with sensor functions, electronic circuits are directly formed on substrates if they are made of silicon. Forming filtration filters and forming electronic circuits are achieved in the same procedures, thus simplifying the formation of electronic circuits. Therefore, substrates are preferred to be made of silicon. 
     Since reverse osmosis membranes are mainly formed with polymeric membranes, their strength is low. Thus, when a load is applied to sewage or seawater by increasing pressure (primary pressure) for improving purification efficiency, problems such as damaged membranes may arise. 
     Reverse osmosis membranes of recent development are made of porous ceramics, which will not be decayed by bacteria nor be dissolved in lubricating oil and which are highly rigid (see Japanese Patent Publication No. 2007-526819, for example). The entire contents of this publication are incorporated herein by reference. 
     A method for producing a filtration filter according to embodiments of the present invention can simplify the process for providing clean water or freshwater. 
     According to a first embodiment of the present invention, a method for producing a filtration filter includes as follows: using masking film formed on a surface of a rigid substrate and having multiple openings with a uniform size to expose portions of the surface, the portions of the substrate corresponding to the openings are etched so that multiple holes or grooves are formed in the substrate. 
     In the first embodiment of the present invention, the etching is preferred to be plasma dry etching. 
     In the first embodiment of the present invention, the diameter of the holes or the width of the grooves is preferred to be adjusted to be 1 nm˜100 nm by depositing a predetermined substance on the inner surfaces of the holes or grooves. 
     In the first embodiment of the present invention, the diameter of the holes or the width of the grooves is preferred to be adjusted to be 1 nm˜5 nm. 
     In the first embodiment of the present invention, the predetermined substance is preferred to be deposited by CVD. 
     In the first embodiment of the present invention, the predetermined substance is preferred to be deposited by ALD. 
     In the first embodiment of the present invention, it is preferred that an organic film with a thickness of 1 nm˜100 nm be formed on the inner surfaces of the holes or the grooves, and that the organic film be removed after the organic film in the holes or the grooves is covered by another material. 
     In the first embodiment of the present invention, the thickness of the organic film is preferred to be 1 nm˜5 nm. 
     In the first embodiment of the present invention, it is preferred that the diameter of the holes or the width of the grooves be 10 nm˜100 nm, and that the diameter of the holes or the width of the grooves be adjusted to be 1 nm˜5 nm by compressing the substrate in a thickness direction so that the holes or the grooves are deformed, causing the inner walls of the holes or the grooves to protrude. 
     In the first embodiment of the present invention, it is preferred that the diameter of the holes or the width of the grooves be 10 nm˜1000 nm, and that the diameter of the holes or the width of the grooves be adjusted to be 1 nm˜100 nm by compressing the substrate in a thickness direction so that the holes or the grooves are deformed, causing the inner walls of the holes or the grooves to protrude. 
     In the first embodiment of the present invention, after multiple holes or grooves are formed in the substrate, the lower surface of the substrate is preferred to be polished so that the holes or the grooves penetrate through the substrate. 
     In the first embodiment of the present invention, multiple substrates having holes or grooves formed as above are preferred to be laminated. 
     In the first embodiment of the present invention, it is preferred that an oxide film be formed on at least either the upper or lower surface of the substrate, and that the oxide film of each substrate be thermally bonded to each other when laminating multiple substrates. 
     In the first embodiment of the present invention, multiple holes or grooves formed as above penetrate through the substrate, and when multiple substrates are laminated, it is preferred that the holes or grooves of each substrate are aligned in a planar view to form penetrating portions that go through all the multiple substrates, and that the width of such penetrating portions be adjusted to be 1 nm˜100 nm in a planar view. 
     In the first embodiment of the present invention, the width of the penetrating portions is preferred to be 1 nm˜5 nm. 
     In the first embodiment of the present invention, the rigid substrate is preferred to be made of silicon, a metal or a metal oxide. 
     In the first embodiment of the present invention, the substrate with multiple holes or grooves is preferred to be bonded to another substrate made of ceramic. 
     In the first embodiment of the present invention, a reverse osmosis membrane made of polymeric film is preferred to be bonded to the substrate having multiple holes or grooves. 
     In the first embodiment of the present invention, an electrical circuit with a sensor function is preferred to be built into the substrate having multiple holes or grooves. 
     A method for producing a filtration filter according to a second embodiment of the present invention includes laminating multiple rigid substrates by means of organic material to have a predetermined distance from each other, and removing the organic material. 
     In the second embodiment of the present invention, it is preferred that holes or grooves that penetrate through each substrate be formed, and that multiple substrates be laminated in such a way that the holes or grooves of each substrate do not align in a planar view. 
     In the second embodiment of the present invention, the organic material is preferred to contain distance retainer with a size of 1 nm˜100 nm. 
     In the second embodiment of the present invention, the distance retainer is preferred to have a size of 1 nm˜5 nm. 
     In the second embodiment of the present invention, it is preferred that multiple substrates be laminated, penetrating holes be formed to penetrate through the multiple substrates all at once, and pillars be formed by inserting rigid members into the penetrating holes. 
     In the second embodiment of the present invention, multiple laminated substrates are preferred to be bonded to another substrate made of ceramic. 
     In the second embodiment of the present invention, a reverse osmosis membrane made of polymeric film is preferred to be bonded to multiple laminated substrates. 
     In the second embodiment of the present invention, an electrical circuit with a sensor function is preferred to be built into at least one of the substrates. 
     According to an embodiment of the present invention, using etching technology capable of achieving highly accurate processing, especially using plasma dry etching, the size and shape of opening portions of masking film are adjusted so that the diameter of multiple holes or the width and shape of grooves formed in a substrate can be directly controlled. Accordingly, when holes or grooves are formed to have a required size of diameter or width, irregularities are prevented from occurring in the diameter of holes or the width of grooves to be formed. As a result, filtration targets such as viruses with a size of a few dozen nanometers, vibrio cholerae with a size of a few hundred nanometers and contaminants are prevented from passing through the substrate. When a filtration filter containing such a substrate is used to provide clean water or freshwater, a combined use of a distillation method or the like is not required, thus simplifying the purification process for obtaining clean water or freshwater. 
     Also, according to another embodiment of the present invention, after multiple rigid substrates are laminated by means of organic material to set the distance between substrates at a predetermined value, the organic material is removed. Thus, the width of slits formed between adjacent substrates is directly controlled. Accordingly, when slits with a width of a few nanometers or a few dozen to one hundred nanometers are formed, irregularities are prevented from occurring in the width of slits to be formed. As a result, depending on the width of the formed slits, viruses with a size of a few dozen nanometers, vibrio cholerae with a size of a few hundred nanometers and contaminants are prevented from passing through the slits. Accordingly, a combined use of a distillation method or the like is not required when a filtration filter made of the slits is used for purification to obtain clean water or freshwater, thus simplifying the purification process for obtaining clean water or freshwater. 
     Furthermore, according to another embodiment of the present invention, since a rigid substrate is used for a filtration filter, primary pressure applied to sewage or seawater can be increased. Thus, the purification efficiency of producing clean water or freshwater improves. 
     In addition, since the shape of holes or slits is accurately controlled, maintenance efficiency improves. By setting the shape of holes or slits to be suitable for the local situation, the purification efficiency of producing clean water or freshwater is enhanced. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.