PROCESS FOR PRODUCING ULTRA-PURE WATER AND PROCESS FOR USING SAID ULTRA-PURE WATER

Abstract: The present invention relates to a process for producing ultra-pure water, an apparatus for producing said ultra-pure water and a process for using the ultra-­ pure water produced according to said process. More particularly, the present invention relates to a process for producing ultra-pure water which comprises boiling a raw water to vaporize off the volatile components from the raw water, subsequently generating steam from the water, contacting the steam with a hydrophobic, porous, gas-­ permeable and liquid-impermeable membrane to make the steam permeate the membrane, and then condensing the steam, as well as to an apparatus for producing said ultra-pure water and a process for using said ultra-pure water.

Description: The invention relates to a process for producing superultra-pure water from raw water in the form of service water or reverse osmosis water or the like, containing volatile and non-volatile components and pre-treated in order to prevent deposition of scale. Such volatile substances are mostly carbondioxide gas components in the form of H₂CO₃, HCO₃⁻, CO₃²⁻, organic substances and the like. The non-volatile substances are usually fine particles, microorganisms and the like. In Kankyo Gijutsu, Vol. 14, No. 4 (1985), pp. 353-358, Figure 12 illustrates a prior art apparatus for producing ultra-pure water. The raw water is subjected to pre-treatments such as aggregation and precipitation, filtration, microfiltration, etc., after which it is sent to a step of reverse osmosis (RO) where the major part of soluble organic components and 90 to 99% of the inorganic salts present in the raw water are removed. The water having permeated the RO membrane is then decarbonated in the deaerating tower, after which it is sent to the ion exchange resin step. The ion exchange resin tower is of two bed/mixed bed regeneration type. In this step, salts are completely removed, and usually a primary pure water having a specific resistance of 10 MΩ·cm or above is obtained in this step and stored in a pure water tank. The pure water is further treated with mixed bed type polishing ion exchange resin to remove impurities more completely. Then, it is made free from microorganisms in the ultra-violet germicidal step and then made free from the residual fine particles and dead microorganisms in the ultrafiltration (UF) step to give an ultra-pure water. Japanese Patent Application Kokai (Laid-Open) No. 230,703/86 discloses a pure-water producing apparatus, an inner space of which is partitioned by a hydrophobic porous membrane which is permeable to vapor of liquids and impermeable to liquids themselves, and one side is used as the path of raw water, while the other side is provided with a cooling plate. The space between the cooling plate and the hydrophobic porous membrane is used as a condensing chamber, and outside of the cooling plate is used as the path of cooling water. The apparatus at least has a separating membrane unit for condensing the vapor supplied from the raw water path and having passed the porous membrane in the condensing chamber and a heat tank having a heater. First, a raw water (e.g. service water or the like) is led to the cooling water path of the separating membrane unit, and the raw water having passed the cooling water path is further led to the heat tank, and the raw water having been heated in the heat tank to a high temperature is led to the raw water path of the separating membrane unit, by pipings. The above-mentioned prior art involves many elementary instruments such as filter membranes, ion exchange resin, germicidal lamp and the like, and improvement in the water quality of the resulting ultra-pure water necessarily requires to improve the quality of the elementary instruments. In the existing membrane module systems, there are a number of requirements as follows. Thus, the surface of the membrane having a separating performance must be free from defect, and the water having permeated the membrane must be perfectly sealed from raw water. Further, the module must be so constructed that it involves no part allowing the residence of liquid and all the parts of module must be thoroughly washable to prevent the growth of bacteria. Further, the quantity of substances dissolving out of the module must be as small as possible. Thus, a special grade of membrane module is used for production of ultra-pure water. However, a periodic washing by flushing and chemical sterilization are necessary to the prevention of bacterial growth in module. Further, in the desalting process using ion exchange resin, contamination by external impurities must be prevented by some means at the time of regeneration. Further, as the polishing ion exchange resin of the final stage, non-regeneration type of high purity resin which has been thoroughly washed previously must be used. As above, the prior techniques for producing ultra-pure water have been disadvantageous in that they require considerably many maintenance works for keeping the water quality and the many elementary instruments used therein cause dissolution of contaminative substances to lower water quality, and further the existence of residence part causes a deterioration of water quality. Further, in the prior apparatuses for producing pure water using a hydrophobic membrane, there have been provided no particular decarbonating apparatus nor de-"TOC" (Total Organic Carbon) apparatus for removing carbon dioxide and organic substances from raw water in the fore stage, so that they have been unable to give a high water quality enough for use in the washing of LSI. US-A-3 418 214 discloses a flash evaporator comprising a vessel in the inlet section of which an impure liquid like salt water is flash evaporated thereby releasing air and non-condensable gases from the salt water. The salt water drops down in said first section in contact with a flow of steam for further heating and driving out air. The salt water arriving at the bottom of the vessel is mixed with preheated water. Because of pressure difference steam is produced, part of which flows into the first section as mentioned above and part of which flows through a demister for retaining droplets into a condensing section, from which pure water is discharged. In said first section of said vessel a substantial portion of the air and non-condensable gases are vented. The described system does not disclose a hydrophobic porous membrane, only a demister. Further, all the equipment is mounted in one and the same vessel, the first section being in connection with the second section by the inlet for the steam from the second section for contacting the salt water droplets in said first section. With this device volatile substances are transferred into the condensing section and into the condensed water. US-A-3 726 234 describes a steam generator provided with a heat means and a discharge tube in its upper part opening to a receiver. In this receiver, the tube is surrounded on part of its length by condensed liquid kept near the boiling point. Included vaporizable substances will thereby be sufficiently vaporized from said liquid. The steam flows from the steam generator into the tube through a mist separator means made of porous materials such as sintered metal, porcelaine and others. The mist separator is constructed so as to prevent boiling splashes or mist from entering said receiver. The receiver opens into a condenser. The liquid in the receiver is passed through a gas remover into a storage for the distillate. Even if this apparatus is used for producing highly purified water, there is no hydrophobic porous membrane. Further, the mist separation means is disposed in the first vessel, which is the steam generator. It is the object of the invention to provide a process for producing water having so high a purity as unknown hitherto, a so-called "superultra-pure water" with a simple apparatus having a small number of elementary instruments. This object is achieved by a process comprising the steps of heating and boiling the raw water in a first tower, thereby vaporizing off the volatile components; discharging the vaporized volatile components together with steam from said first tower; transferring the raw water free of volatile components to a second tower; heating the raw water free of volatile components in said second tower such that steam is generated; contacting the steam generated in said second tower with a hydrophobic porous gas-permeable and liquid-impermeable membrane to make the steam permeate the membrane and to separate the metallic components containing mists of tiny droplets accompanying the steam; and condensing the steam to superultra-pure water having a specific resistance of 18 MΩ cm or above, a total organic carbon content of not more than 20 x 10⁻³ ppm, 12 or less fine particles per ml with a size of 0.1 µm or above and alive microorganisms undetectable. Preferably, the steam having permeated the membrane is lead into an atmosphere of saturated steam or of inert gas in which atmosphere the steam is condensed. Advantageously, the raw water is kept under a reduced pressure in said first tower during its boiling for vaporizing off the volatile components. Conveniently, only the steam is contacted with the hydrophobic porous gas-permeable and liquid-impermeable membrane in said second tower. It is preferred that the raw water from which the volatile components were vaporized off in the first tower is heated in said second tower in heat exchange with the steam discharged together with the vaporized volatile components from said first tower. It is advantageous that the volatile components are removed from said raw water in said first tower by flash evaporation of the raw water. The apparatus for producing superultra-pure water comprises a first volatile component-removing tower equipped with means for heating and boiling the raw water containing volatile and non-volatile components, with means for discharging the vaporized volatile components formed by said heating and with means for transferring the raw water, from which the volatile components are vaporized off, from said first tower to at least one second distillation tower equipped with a heat source for heating the transferred raw water such that steam is generated, a hydrophobic porous gas-permeable and liquid-impermeable membrane associated with each distillation tower with which membrane the steam generated in said second tower is contacted, cooling means for condensing the steam having permeated said membrane to the superultra-pure water and discharge means for the condensed superultra-pure water. Preferably, the hydrophobic porous gas-permeable and liquid-impermeable membrane is mounted inside of the distillation tower. Conveniently, the cooling means for condensing the steam having permeated said membrane is mounted inside of the distillation tower. Advantageously, said means for discharging the vaporized volatile components formed by said heating in the first volatile component removing tower are connected with means for heat exchange in said distillation tower to act at least partly as the heat source for generating steam in said distillation tower. It is preferred that said means for heating and boiling raw water containing volatile and non-volatile components are a heater and a spray nozzle for flash evaporation. It is convenient that a gas supply means is provided for making the atmosphere for condensing the steam in the distillation tower an atmosphere of saturated steam or of inert gas. The said volatile component removing tower can be equipped with a pressure reducing means. Preferably, the superultra-pure water produced according to the invention is used for washing semiconductor integrated circuits, for washing medical tools, for producing medical drugs or for oxidizing the silicon surface of a semiconductor wafer by contacting said silicon surface with the superultra-pure water. In the latter case, the thickness of the oxide film is controlled by changing the temperature of the superultra-pure water and/or the contact time with the superultra-pure water. Further, the surface contacted with superultra-pure water can be dried by evaporating the adherent superultra-pure water with a controlled vaporizing speed. According to the present invention, a superultra-pure water heated to a temperature higher than ordinary temperature is obtained. Accordingly, the superultra-pure water of the invention is advantageously usable for washing LSI as with increasing temperature water decreases its surface tension and increases affinity to substrate and undergoes a smaller influence by static electricity, in its nature. The water obtained according to the invention is quite low in the content of impurities, so that it can relax the problems in the washing of LSI, such as difficulty in formation of desired LSI pattern, increase in leakage current or pn junction, deterioration in pressure resistance of gate oxide film, etc. and thereby it enables to improve the yield and characteristics of LSI. Further, according to the present invention, there is obtained a high temperature superultra-pure water containing microorganisms at all or hardly containing microorganisms. Even if it contains microorganisms, their growth can be suppressed to a marked extent by maintaining the water at a high temperature. Thus, the superultra-pure water of the invention is quite effectively usable for washing medical tools and producing medical drugs. Usually, service water and reverse osmosis-treated water contain various inorganic substances, ions, organic substances and microorganisms in a large amount. According to the invention, only the volatile substances which cannot be removed by distillation (carbon dioxide components, volatile organic and inorganic components, ammonia, SO₃, etc.) are removed in the fore stage by means of heating, reduction of pressure, ozone oxidation, etc. The raw water from which the volatile components have been removed are further heated to generate steam. Since volatile components have already been removed, this steam has a considerably high purity. The steam is further passed through hydrophobic porous membrane, whereby the accompanying water droplets (mists) are removed, and there is obtained a high purity steam containing no impurities other than water. Thereafter, it is condensed to give the superultra-pure water having a high purity and containing no impurities such as ions, organic substances, fine particules and alive microorganisms. Further, by condensing the steam having passed the hydrophobic porous membrane in an atmosphere of saturated steam or inert gas, pollution from the air can be prevented. As said hydrophobic porous membrane, membranes of polyolefins (e.g. polyethylene, polypropylene and the like), polysulfone, polyether sulfone, silicone resin, fluororesin and the like can be used, for example, as mentioned in Japanese Patent Application Kokai (Laid-Open) No. 118,284/85. Preferably, the hydrophobic porous membrane should satisfy the conditions mentioned in Japanese Patent Application Kokai (Laid-Open) No. 118,284/85. Generally saying, Si surface is much richer than SiO₂ surface in activity, so that it takes various polluting components into itself from media contacted with it. If an Si substrate having taken-in the polluting components is oxidized to form SiO₂ film on its surface, the SiO₂ film is bad in film quality, so that insulating resistance of the wafer is deteriorated. The taking-in of the pollution occurs when the wafer is exposed to the air in clean room in the course of transporting the wafer from the drying step (the final step of washing process) to the oxidation process. As a measure for preventing it, the wafer is oxidized in the superultra-pure water whereby, a very thin oxide film of high quality can be formed. The water quality can be evaluated by dropping a predetermined quality of test water onto Si wafer, drying up the water and observing the residue on the wafer (called "water mark"). When 0.097 g of superultra-pure water and 0.101 g of prior-art ultra-pure water are dropped and dried at ordinary temperature (19.5°C), there is no significant difference between their water mark quantities. However, when 0.102 g of superultra-pure water and 0.089 g of prior-art ultra-pure water are dried up at high temperature (103°C), an outstanding difference is observed between their water mark quantities. Thus, the impurities in the superultra-pure water are completely lost by vaporization together with water when dried at a high temperature of 103°C, while the impurities in the prior-art ultra-pure water almost completely remain as understandable by comparing the water mark at ordinary temperature with that at high temperature. XMA analysis has revealed that these water marks were mainly composed of organic substance. However, the organic substance in superultra-pure after is evidently different from that in prior-art ultra-pure water. Figure 17 illustrates the infrared absorption spectrum of the water mark of superultra-pure water dried at ordinary temperature, where all the peaks demonstrate that the water mark is a hydrocarbon type substance except for the OH peak due to the water absorbed into KBr at the time of sample preparation and the SiO peak derived from the Si substrate (a part of substrate surface is scraped off when water mark is taken from Si substrate). Making a contrast to the impurities in prior-art ultra-pure water which are a mixture of various types of substances, the impurities in superultra-pure water belong to almost one series of substances having neither hydrophilic group nor hydrophobic group. From the viewpoint of adsorption, this exercises a good influence on the film quality at the time of forming SiO₂ film. Since water mark has the above-mentioned properties, water mark is difficult to observe by SEM etc. where the sample is exposed to vacuum or high temperature. Another important difference between superultra-pure water and prior-art ultra-pure water is that prior-art ultra-pure water contains no detectable silica (JIS K 0101). In water, silica dissociates to a very slight extent and combines with the Si of surface having dangling bond, due to which the quality of formed oxide film is deteriorated. In the recent years, the degree of integration of semiconductors is markedly increasing and D-RAM of megabit class are mass produced largely. Further, the production is advancing into 4M bits and 16M bits of products. In the phase of design rule, 0.8 µm, smaller than 1.0 µm, is adopted in megabit class, so that even a very slight quantity of impurities has become unallowable. At the same time, the oxide film is becoming thinner from 200 Å to 100 Å. In the consciencious washing a semiconductor wafer, it is necessary that no impurities (for example, fine particles and dissolved substance present in solution and water) remain on the surface of the washed product. This can be achieved by washing a semiconductor wafer with a chemical solution, subsequently washing the semiconductor wafer with prior-art ultra-pure water, subsequently forming on oxide film on the surface of semiconductor wafer in superultra-pure water produced according to the invention drying the semiconductor wafer, and subsequently oxidizing the semiconductor wafer. In the first step, the fatty substances adhering to the surface of semiconductor wafer are washed away with a solvent and metallic impurities are washed away with acid and alkali, successively. In the second step, the chemical solution adhering to the surface of semiconductor wafer is washed away with prior-art ultra-pure water. In the third step, a hydrophilic thin film of oxide is formed on the surface of semiconductor wafer with the dissolved oxygen in water in a superultra-pure water containing no metallic ions exercising adverse influence on the semiconductor which has been prepared by making a steam permeate a hydrophobic porous membrane to remove the mists containing metallic components and then condensing it. In the fourth step, the water adhering to the thin oxide film formed on the surface of semiconductor wafer is dehydrated and dried. In the fifth step, an oxide film is additionally formed on the thin oxide film on the semiconductor wafer in an oxidation furnace. In the production processes of semiconductors and various optical and electronic parts, the thin film-forming technique is largely adopted. For example, in the production of semiconductor wafer, SiO₂ film is formed on Si substrate by oxidation and a thin film of insulating material or metal such as aluminum is formed by vapor deposition. In the production of optical disks, a film of metal such as nickel is formed on substrate by plating or a film of photosensitive material is formed by coating. In any of these techniques, the substrate surface can be washed with the superultra-pure water according to the invention and then brought into dryness as a pre-treatment. In heating a substrate to be dried which substrate was washed with superultra-pure water produced according to the invention, heat is transferred by using a heating element placed in a clean atmosphere as the heat transfer surface to maintain a temperature not causing any violent boiling at the interface between substrate surface and adherent water and not bringing about dryness. That is, overheating can be prevented by controlling the temperature so as not to exceed the saturation temperature of water. If the temperature of heating is too low, the vaporization progresses very slowly and dissolved substances having a boiling point higher than that temperature are deposited. Accordingly, it is an indispensable condition to maintain an appropriate velocity of vaporization. This vaporization velocity is proportional to the temperature of substrate (i.e. heat transfer surface)/or the temperature difference at the boundary layer. Accordingly, the substrate temperature should be adjusted to a temperature close to the uppermost boiling point of dissolved organic substances and boiling point of water to keep an appropriate velocity of vaporization. As understandable from the examples mentioned later, the vaporization velocity is preferably about 4 g/hour or above. Next examples of the invention will be illustrated with reference to the accompanying drawings, in which Figure 1 is an outlined diagram illustrating the construction of one example of the apparatus for producing the superultra-pure water of the invention; Figure 2 is a characteristic graph illustrating the water quality of ultra-pure water obtained by the production process of comparative example; Figure 3 is a characteristic graph illustrating the water quality of a superultra-pure water obtained according to one example of the invention; Figures 4 to 7 are outlined diagrams illustrating the constructions of other examples of the apparatus for producing superultra-pure water of the invention; Figure 8 is an outlined flow diagram of the present invention; Figure 9 is an outlined diagram illustrating the construction of steam generater; Figure 10 is an outlined diagram illustrating the construction of condenser; Figure 11 is an outlined diagram illustrating the construction of heat recovery type condenser; Figure 12 is an outlined diagram illustrating the prior-art production process of ultra-pure water; Figure 13 is a schematic view of a device for oxidizing wafers in superultra-pure water Figure 14 is a flow diagram illustrating the oxidation of Si wafer; Figure 15 is a graph illustrating the result of breakdown voltage test of oxidized wafer; Figure 16 is a graph illustrating the IR analysis of water mark remaining in the ordinary temperature-drying of superultra-pure wafer; Figure 17 is a first process block diagram Figure 18 is a second process block diagram Figure 19 is a sectional view of the in-water oxidation film-forming tank Figure 20 (a), (b) and (c) are voltage-withstand characteristic graphs of the oxide films of silicon wafers produced in different ways Figure 21 is a third process block diagram Figure 22 is a forth process block diagram Figure 23 shows schematically a checking device and Figure 24 is a graph illustrating the percentage of residue on substrate surface after practice of the drying process using superultra-pure water. The present invention is by no means limited by these examples. Example 1 Figure 1 illustrates a fundamental example of the present invention. This apparatus is constructed of volatile component-removing tower 2 having raw water heater 110 and vapor discharging hole 112, membrane distillation tower 1 having raw water heater 108, hydrophobic porous membrane 101 and condensing surface 113, raw water piping 114 connecting between the volatile component removing tower and the membrane distillation tower, and pump 106 for sending raw water provided in the middle of said piping. Raw water 111 to be treated is introduced into the volatile component removing tower 2 and heated and boiled by raw water heater 110, whereby the carbon dioxide gas and volatile organic substances in the raw water are vaporized and discharged from vapor outlet 112 out of the system together with steam. The raw water from which the dissolved carbon dioxide gas and volatile organic substances have been removed is then sent into membrane distillation tower 1 by water supply pump 106. The raw water having entered membrane distillation tower 1 is again heated by raw water heater 108 and vaporized. The steam (arrow 15) generated by the vaporization is filtered with hydrophobic porous membrane 101, condensed on the condensing surface 113 cooled by the flowing cooling water 103, i.e. on the surface of cooling water piping, and taken out as formed superultra-pure water 104. In this example, the cooling means is constructed of cooling water 103 and condensing surface 113. Although the major part of the steam passing hydrophobic porous membrane is made into superultra-pure water, its minor part is blown out from valve 102 in order to remove the uncondensable gases in the system. In order to prevent the scaling in raw water heaters 108 and 110, the raw water is also blown out from respective drain valve 107 and 109, preferably. Since in this apparatus the carbon dioxide gas components and volatile organic substances present in raw water can be removed in the fore stage of membrane distillation tower, a superultra-pure water of high purity can be produced. Next, the properties of the superultra-pure water produced with this apparatus will be illustrated with reference to Figures 2 and 3. Figure 2 illustrates specific resistance and pH of formed water produced by feeding a raw water (reverse osmosis-treated water) into membrane distillation tower, for comparison with example of the present invention. Figure 3 illustrates those in this example. In both Figures 2 and 3, (1) means the point at which feeding of water into membrane distillation tower is started, and (2) means the point at which the feeding of raw water is stopped. In Figure 2 where the raw water does not pass volatile component removing tower, specific resistance of formed water descends to a few MΩ·cm simultaneously with the start of water feed. Since pH also decreases from 6.7 to about 5.8 at this time, it is considered that carbondioxide gas enters the formed water to form carbonate ion and bicarbonate ion lowering the specific resistance. On the other hand, in Figure 3 where a water having passed volatile component removing tower is fed, specific resistance and pH of formed water are not changed by starting water feed, demonstrating that a formed water of high purity is continuously obtained. According to one analysis on the water quality of the superultra-pure water obtained according to this example, no microorganisms were detected at all. The number of fine particles having a particle size of 0.1 µm or above was 10 or less per 1 mm³, and the total organic carbon content (TOC) was 10 ppb. Using the superultra-pure water obtained in this example and a ultra-pure water prepared according to the prior art shown in Figure 12, water marks were prepared on silicon wafer, and they were compared with each other. The term "water mark" means a residue formed by dropping one drop of water on a silicon wafer and drying it up. When water marks formed by drying drops at ordinary temperature (19.5°C) were compared, the formed water of the apparatus of the invention gave a considerably smaller quantity of residue, demonstrating its high purity. When the drops were dried at 103°C, the merit of the apparatus of the invention was exhibited more evidently. Thus, when the drying was carried out at high temperature, the water mark appearing at ordinary temperature completely disappeared and nothing was observable on the silicon wafer. On the other hand, in case of the formed water prepared by prior art, a considerable quantity of impurity was observed, even though its quantity was a little smaller than that at ordinary temperature. This result indicates that, if the water prepared with the apparatus of the invention contains a quantity of impurity, the impurity is exclusively composed of volatile substances which completely vaporize to leave nothing on the wafer water dried at high temperature. It is apparent from this fact that the formed water of the present invention is quite effectively usable in the production of LSI. Example 2 Figure 4 illustrates another example of the present invention. This apparatus is constructed of volatile component removing tower 2 having raw water heater 110, heat exchanger for heating raw water 202, membrane distillation tower 3 having hydrophobic porous membrane 101 and condensing surface 113, and water feeding pump 106 for transporting raw water between them. The raw water to be treated 111 is introduced into volatile component removing tower 2, where it is heated and boiled by raw water heater 110 to vaporize the carbon dioxide gas and volatile organic substances present in it and discharges them out of the system from vapor outlet 203. The raw water from which the dissolved carbondioxide gas components and volatile organic substances have been removed is sent to membrane distillation tower 3 by feeding pump 106. The raw water having entered the membrane distillation tower 3 is again heated and vaporized by the heat exchanger for heating raw water 202. The heating medium in the heat exchanger for heating raw water 202 is the steam generated in volatile component removing tower 2. After releasing its latent heat, the resulting liquid water is discharged out of the system from outlet 201 of heat exchanger. The steam (arrow 15) generated by the heating action of heat exchanger for heating raw water 202 passes hydrophobic porous membrane 101 where the accompanying mists are removed, after which the steam is condensed on the condensing surface 113 and taken out of the system as formed water 104, i.e. as a superultra-pure water. With this apparatus, a superultra-pure water having a high purity similarly to that of Example 1 is obtained. Further, since the heat consumed in volatile component removing tower 2 is recovered, the consumption of energy can be saved. When the heat exchanger for heating raw water 202 is insufficient in capacity in itself alone as heat source for heating raw water in the membrane distillation tower, raw water heater 108 shown in Figure 1 may be provided in combination with it. Example 3 Referring to Figure 5, an example using flash evaporation as a means for removing volatile components from raw water will be illustrated. This apparatus is constructed of volatile component removing tower 4 having spray pump 301, raw water heater 302 and spray nozzle 303 and membrane distillation tower 3. The raw water to be treated 111 is introduced into the suction side of spray pump 301 and pressurized by the pump, after which it is heated by raw water heater 302 to a temperature a few degrees (e.g. 5-10°C) higher than the saturated temperature in the volatile component removing tower and then discharged into the system from spray nozzle 303. Here, the intermediate part from pump 301 to spray nozzle 303 must be pressurized up to a pressure not lower than the saturated pressure at the temperature to which the raw water heater 302 has elevated, and vaporization (boiling) in piping must be prevented. The raw water sprayed from spray nozzle 303 is rapidly heated to the inner temperature of volatile component removing tower 4 and vaporized to form steam. At this time, the carbon dioxide gas components and low-boiling organic components dissolved in the raw water are also gasified simultaneously and released into gas phase. The liquid staying in the volatile component removing tower 4 without vaporization is again led to spray pump 301, where it is subjected to flash evaporation. The steam generated herein, carbon dioxide gas components and low-boiling organic components are taken out of the system from steam outlet 203, sent into heat exchanger for heating raw water 202 of membrane distillation tower 3 and utilized as a heat source of membrane distillation. The raw water treated in volatile component removing tower 4 is sent into membrane distillation tower 3 by feeding pump 106. Thereafter, it is treated in the same manner as in Figure 4 to give a formed water of high purity. In the present example, raw water is fed into the suction side of spray pump 301, and it is overheated above its boiling point by raw water heater 302 before being fed into volatile component removing tower 4 and flash-evaporated from spray nozzle 303. Accordingly, the raw water passes the step of flash evaporation at least once before being introduced into feeding pump 106 leading to membrane distillation tower 3, so that feeding of untreated raw water into membrane distillation tower 3 is prevented. Example 4 Figure 6 illustrates one example of multi-state process devised with consideration of the cost of formed water. The raw water which has been made free from carbon dioxide gas components and volatile TOC components by flash evaporation in volatile component removing tower 4 is sent into the first membrane distillation tower 5. Since the first membrane distillation tower 5 is in a saturated state at a lower pressure and a lower temperature than in volatile component removing tower 4, the raw water can be sent into the first membrane distillation tower 5 by merely operating valve 414. Here, the latent heat of steam 410 generated in volatile component removing tower 4 is utilized for heating the raw water in the first membrane distillation tower 5. The steam 411 generated in the first membrane distillation tower is separated from mist when it passes through hydrophobic porous membrane 105, after which it passes piping 411 and is utilized for heating the raw water in the second membrane distillation tower. The steam having released its latent heat for heating the second membrane distillation tower returns to liquid water, which is discharged out of the system from heat exchanger outlet 413. The final step of this multi-stage process is provided with a simple condenser 7 where the generated high purity steam is condensed to give an superultra-pure water. Since the final stage is worked in the neighborhood of atmospheric pressure, the withdrawal of the formed water is prevented from contamination by dust formation in sliding parts such as pumps and the like. If a high-performance dust-free pump 401 shown in Figure 7 is developed, however, it will become possible to sent the formed water into outside of the system by providing a small-sized formed water tank 402. In this case, the final stage can be worked under a considerably reduced pressure and air can be drawn from formed water tank 402. Example of Prior Art Figure 12 illustrates an outlined view of prior apparatus for producing ultra-pure water. The raw water is subjected to pre-treatments such as coagulation-precipitation, filtration, microfiltration, etc. and then sent to the step of reverse osmosis (RO) where the major part of dissolved organic components and about 90% of inorganic salts present in the raw water are removed. Then, the reverse osmosis water is decarbonated in deaerating tower and sent to ion exchange resin step. Although vacuum deaerating tower is usually used as the deaerating apparatus, a deaerating module using hydrophobic porous membrane is also devised for this purpose (for example, see: Japanese Patent Application Kokai (Laid-Open) No. 118,284/85). The ion exchange tower is of two bed/mixed bed regeneration type. In this step, salts are completely removed, and usually a primary pure-water having a specific resistance of 10 MΩ·cm or above is obtained. The primary pure water is once stored in a pure water tank. Then, it is treated in a mixed bed type polishing ion exchange resin tower to remove the impurities further completely, after which alive microorganisms are eliminated in a ultraviolet germicidal step and then the residual fine particles and dead microorganisms are removed by ultrafiltration (UF) to give an ultra-pure water. According to this prior-art process, an ultra-pure water having an ordinary temperature (about 25°C) is obtained. In the best case, water quality of the ultra-pure water produced by the use of this prior-art apparatus is as follows: number of microorganism 0.05/ml; number of fine particles having a size of 0.1 µm or above 50 to 100 particles/ml; TOC 100 ppb; electrical specific resistance 18 mΩ·cm. Example 5 Figure 8 is the fundamental flow diagram of the present invention. A raw water such as service water is pre-treated by means of RO or the like in order to prevent the scaling in steam generater. The pre-treated water is sent into a steam generater where the volatile substances are removed. Then, the steam free from volatile substances is sent in the state of steam into a clean room via a clean piping and enters a condenser placed in the neighborhood of use point. After removing the mists in the steam by means of hydrophobic porous membrane placed in the condenser, the steam is condensed and formed into an superultra-pure water. Figure 9 illustrates one example of the outlined view of the steam generater. The pre-treated raw water 20 enters de-CO₂ de-TOC part 21 where it is boiled with heater 23 to remove CO₂ and TOC components together with steam. Though a direct heating is adopted in this example, other methods such as flash evaporation and the like are also usable. This steam 24 serves as the heat source for steam generation in the subsequent steam generating part 22, and the condensed water thereof is discharged out of the system as drain 27. The raw water having been made free from CO₂ and TOC components in the de-CO₂ de-TOC part 21 then passes flow control valve 26 and enters steam generating part 22 where the raw water is converted to steam. The steam is sent via piping 25 into clean room. For controlling heat quantity, an auxiliary heater 28 is provided. Figure 10 illustrates the outlined view of condenser. The steam sent from piping 25 passes membrane module 31 having a hydrophobic porous membrane 33 where it is separated from the accompanying mist. The elementary material of said hydrophobic porous membrane 33 is polytetrafluorethylene, polypropylene polyethylene, or the like. The steam separated from mist is led into condenser 32 where it is condensed on the cooling surface 34 cooled with cooling water 33' and then taken out an superultra-pure water 35. According to the present example, a superultra-pure water of high purity containing a very small quantity of impurities other than water can be taken out in a high temperature state. Characteristics of the water having passed the membrane module 31 are as follows: specific resistance 18 MΩ·cm or above; number of fine particles having a size of 0.1 µm or above: 100 particles/liter or below; number of microorganisms 10/liter or below; organic substances 10 ppb or below. Figure 11 illustrates the outlined view of heat recovery type condenser. Since the above-mentioned structure of condenser is insufficient in the point of heat recovery, the condenser of Figure 11 is made into multistage structure for the sake of heat recovery. The steam sent from piping 25 is filtered by membrane module 31 end then sent into the first tower 42 where it is cooled with raw water 41 to give an ultra-pure water. The raw water 41 is sent from the steam generater placed outside the clean room. At the same time, the raw water in the first tower 42 vaporizes, and the steam is fitered by hydrophobic porous membrane 46 and then condensed in the second tower 43 with cooling water 44 to give superultra-pure water 45. According to this process, the cost of the superultra-pure water can be reduced because heat can be recovered. Though this example refers to the case of two stage condensation, 3-stage and 4-stage condensation are also adoptable. In Figure 8, the steam is returned to water by means of a condenser and the water is used as superultra-pure water. However, when the use of steam is more effective than the use of water for the washing of LSI, the cooling surface 34 (Figure 10) may be subtracted from the present process. By this, a clean or superultra-pure steam can be obtained. Since the formed water has a high temperature, it is directly usable as a high temperature superultra-pure water more effectively usable for washing wafers and the like. Example 6 Figure 13 illustrates one example of the in-water oxidation process according to the present invention. A desired quantity of a superultra-pure water 2' having a desired temperature is introduced into a quarz glass container 1. Si wafer 3' is oxidized by dipping it in the superultra-pure water 2' for a predetermined period of time. Thickness of the oxide film can be controlled arbitrarity at least by changing temperature of the superultra-pure water and time period of dipping. Since this process is easier to control than the oxidation process using furnace, it can form a very thin oxide film of high quality uniformly. For example, if Si wafer is contacted with a superultra-pure water having a temperature of 45°C for 20 minutes, an oxidation film of 10 Å can be formed. If Si wafer is contacted with a superultra-pure water having a temperature of 80°C for 30 minutes, an oxide film of 40 to 50 Å can be formed. Figure 14 illustrates the process for preparing a wafer for evaluating the quality of oxide film. The process of Run 1 is a process according to the present invention. Thus, after treating Si wafer with HF, it is washed with ultra-pure water and the water on the wafer is removed with spin drier, after which wafer is oxidized by dipping it in a superultra-pure water having a temperature of 80°C for 30 minutes (this is one example of the present invention). Then, the water on the wafer is removed with spin drier and the wafer is oxidized in furnace. The process of Run 3 is different from that of Run 1 in that oxidation is carried out in an ultra-pure water instead of superultra-pure water, though the conditions of the oxidation are identical (dipping at 80°C for 30 minutes). The process of Run 2 corresponds to the existing process which lacks the step of oxidation in water. In all these three processes, the condition of HF treatment is "dipping in 10% HF for 30 seconds", and the condition of washing with ultra-pure water is "15 minutes with flowing water", and the time of drying with spin drier is 4 minutes, and the conditions of oxidation in furnace are "at 850°C for 33 minutes", which are all identical throughout the processes. Figure 15 illustrates the result of breakdown voltage performance test on the wafers having oxide films formed by Runs 1 to 3 of Figure 14. The measurement was carried out on approximately 300 points, and thickness of oxide film was 200 Å. The result demonstrates that preparation of wafer according to Run 1 is best, because it produced no defective sample. Example 7 Figure 17 is a flow diagram illustrating the washing of silicon wafer and formation of oxide film according to the present invention. First, a silicon wafer substrate is washed with chemical solutions in order to remove the surface pollutions and natural oxide film. Then, the chemical solutions used above are washed away with an ultra-pure water, after which the wafer is oxidized with a fresh superultra-pure water in water. After forming an oxide film having a thickness of about 10 to 40 Å, the wafer is dried by the method of spin drying, IPA vapor drying or steam drying and then oxidized in a furnace. In this process, the surface of silicon wafer keeps hydrophilic during the period from the washing step to the drying step and further to the oxidation step, so that its pollution can be prevented to a considerable extent as compared with the case of prior process where the wafer surface was hydrophobic. Since this process is accompanied by a phase change, the cost of water thus produced is necessarily high. In order to reduce the cost of process, there may be another process shown in Figure 21 where prior-art inexpensive ultra-pure water is used for removing the chemical solutions and the superultra-pure water is used in the subsequent steps of washing and oxidation in water. Example 8 Figure 18 is another example of the process for washing silicon wafer and forming oxide film according to the present invention. According to this process, the impurities on the wafer surface are removed with chemical solutions and the chemical solutions are washed with ultra-pure water, after which an oxide film necessary for production of semiconductor is formed in a superultra-pure water, so that oxidation furnace is unnecessary unlike the prior processes. According to the present process, the steps of washing and oxidation can be carried out simultaneously, owing to which transportation between steps is unnecessary and pollution is prevented. Further, the velocity of oxidation is not high, so that thickness of film can be controlled easily. Example 9 Figure 19 illustrates one example of the in-water oxidation film-forming tank according to the present invention. Oxide tank 31' is composed of an insoluble material such as quarz glass, and superultra-pure water 34 is sent from the bottom of oxidation tank and made to overflow. Oxidation tank 31 is equipped with heater 33' for the sake of water temperature control, and silicon wafer 32' is completely dipped. Example 10 In Figure 20, the voltage-withstand performance of the oxide film having a thickness of about 200 Å, prepared according to the process of Figure 17, is compared with that of a prior sample prepared without in-water oxidation and another prior sample prepared by in-water oxidation using prior-art ultra-pure water. The sample prepared by the process of the invention is comparable in the performance to the other samples prepared by prior-art methods. This demonstrates the high quality of the oxide film of the invention. Example 11 Figure 22 illustrates one example which is a further advanced example of the system of the invention. After the washing with chemical solutions, the silicon wafer is washed with ultra-pure water and then washed and oxidized in superultra-pure water. Subsequently, it is dried by the use of a steam having passed hydrophobic porous membrane. According to this process, washing and oxidation can be carried out in one tank, so that external pollutions can be prevented. Further, owing to the drying process using steam, drying can be carried out without contamination by external impurities. Next, water quality of the superultra-pure water will be explained below. Table 1 illustrates the water qualities of a superultra-pure water prepared by the process according to the invention and a prior-art ultra-pure water. The hydrophobic porous membrane used herein is a porous PTFE membrane having a pore size of 0.2 µm, and the boiling is carried out under ordinary pressure (at 100°C) Water qualities of superultra-pure water and ultra-pure water Item Superultra-pure water Ultra-pure water Specific resistance (MΩ·cm) 18 or above 18 or above TOC (ppb) 20 50 or below Fine particles (number/ml) 12 (0.1 µm or above) 50 (0.1 µm or above) Alive microorganisms (number/ml) Undetectable 0.25 As the raw water in the production of the superultra-pure water, a reverse osmotic membrane-treated water was used. It is apparent from Table 1 that the superultra-pure water has a considerably good water quality. Then, one drop of the superultra-pure water was dropped onto silicon wafer and dried, and the residue (water mark) was examined. Particularly when the wafer was dried at 100°C, no water mark was observed, demonstrating that nonvolatile impurities (metal ions, etc.) had been removed completely, and the water was a water mark-free superultra-pure water. When a semiconductor wafer is oxidized with the superultra-pure water, the formed oxide film contains no metallic components and has a high quality. Example 12 Figure 23 is a diagram illustrating the construction of one example of the invention, wherein substrate 1'' is placed on heat transfer plate 2'' provided in container 3'', and said heat transfer plate 2'' is heated by heating element 5''. Container 3'' is provided with an exhaust hole 4 on its one terminal. The inner atmosphere of the container is kept clean. By heating heat transfer plate 2'', heat is transferred to substrate 1 which is just after being washed with superultra-pure water and has adherent water drops, whereby the water is vaporized and dried up. Here, the surface temperature of heating element 5'' is detected, and it is controlled so as to fulfil the above-mentioned temperature condition. In accordance with the temperature condition, a sucking apparatus or a throttling mechanism is connected to the exhaust hole 4'' in order to keep the inner pressure of container constant. In the experiment of Figure 24, superultra-pure water was dropped onto semiconductor wafer and dried according to the construction of practice of the invention shown in Figure 24, and Figure 25 illustrates the relation between vaporization velocity of adhering water and the percentage of residual deposit held on the wafer surface. Thus, about 0.1 g of superultra-pure water (TOC 10 ppb) was dropped onto wafer surface under atmospheric pressure and the water was subjected to a vaporization treatment, taking the case of no heating as a standard. Though in the case of no heating the vaporization velocity was somewhat enhanced by the use of a drying material, it corresponded to a case of natural drying because the temperature was ordinary temperature. The results shown in the figure demonstrate that residual deposit was successively lost on the surface as vaporization velocity was increased by elevating the temperature of heating. The microscopic photographs revealed the following facts. Thus, in the case corresponding to natural drying, the existence of residual substance was clearly observed on the substrate. In the cases when the vaporization velocity was kept at about 0.6 g/hour and about 0.3 g/hour, a slight quantity of residual substance was observed. When the vaporization velocity was 4 g/hour or above, no residual substance was observed at all, and the wafer surface could be dried without pollution. In the last case, the temperature was about 100°C. This means that the dissolved substance in the superultra-pure water is a mixture mainly composed of organic substances of which boiling points are in the neighborhood of 100°C. Based on this result, a substrate can be washed with such a superultra-pure water and dried and then sent into the subsequent step according to the present invention.

Claims: A process for producing superultra-pure water from raw water in the form of service water or reverse osmosis water or the like, containing volatile and non-volatile components, and pre-treated in order to prevent deposition of scale comprising the steps of heating and boiling the raw water in a first tower (2, 4, 21) thereby vaporizing off the volatile components, discharging the vaporized volatile components together with steam from said first tower (2, 4, 21), transferring the raw water free of volatile components to a second tower (1, 3, 5, 6; 22, 32, 42), heating the raw water free of volatile components in said second tower (1, 3, 5, 6; 22, 32, 42) such that steam is generated, contacting the steam generated in said second tower (1, 3, 5, 6; 22, 32, 42) with a hydrophobic porous gas-permeable and liquid-impermeable membrane (101, 105, 31, 46) to make the steam permeate the membrane (111, 105, 31, 46) and to separate the metallic components containing mists of tiny droplets accompanying the steam, and condensing the steam to superultra-pure water having -- a specific resistance of 18 MΩ cm or above, -- a total organic carbon content of not more than 20 x 10⁻³ ppm, -- 12 or less fine particles per ml with a size of 0.1 µm or above and -- alive microorganisms undetectable. A process according to claim 1, wherein the steam having permeated the membrane (101, 105, 31, 46) is lead into an atmosphere of saturated steam or of inert gas in which atmosphere the steam is condensed. A process according to claim 1 or 2, wherein the raw water is kept under a reduced pressure in said first tower (2, 4, 21) during its boiling for vaporizing off the volatile components. A process according to one of the claims 1 to 3, wherein only the steam is contacted with the hydrophobic porous gas-permeable and liquid-impermeable membrane (101, 105, 31, 46) in said second tower (1, 3, 5, 6; 22, 32, 42). Process according to one of the preceding claims, wherein the raw water from which the volatile components were vaporized off in the first tower (2, 4, 21) is heated in said second tower (1, 3, 5, 6; 22, 32, 42) in heat exchange with the steam discharged together with the vaporized volatile components from said first tower (2, 4, 21). Process according to one of the preceding claims, wherein the volatile components are removed from said raw water in said first tower (2, 4, 21) by flash evaporation of the raw water. Apparatus for producing superultra-pure water, particularly according to one of the preceding claims, comprising a first volatile component-removing tower (2, 4, 21) equipped -- with means (110; 302, 303, 23) for heating and boiling the raw water containing volatile and non-volatile components, -- with means (112, 203, 410, 24) for discharging the vaporized volatile components formed by said heating and -- with means (106, 114; 440, 26, 41) for transferring the raw water from which the volatile components are vaporized off, from said first tower (2, 4, 21) to at least one second distillation tower (1; 3, 5, 6; 22, 32, 42) equipped with a heat source (108, 202, 28) for heating the transferred raw water such that steam is generated, a hydrophobic porous gas-permeable and liquid-impermeable membrane (101, 105, 31, 46) associated with each distillation tower (1; 3, 5, 6; 22, 32, 42) with which membrane (101, 105, 31, 46) the steam generated in said second tower (1; 3, 5, 6; 22, 32, 42) is contacted, cooling means (113; 7; 32; 43) for condensing the steam having permeated said membrane (101, 105, 31, 46) to the superultra-pure water and discharge means (104, 413, 402, 401, 35, 45) for the condensed superultra-pure water. Apparatus according to claim 7, wherein the hydrophobic porous gas-permeable and liquid-impermeable membrane (101, 105, 46) is mounted inside of the distillation tower (1, 3, 5, 42). Apparatus according to claim 8, wherein the cooling means (113) for condensing the steam having permeated said membrane (101, 105, 46) is mounted inside of the distillation tower (1, 3, 5, 42). Apparatus according to one of the claims 7 to 9, wherein said means (203, 410, 24) for discharging the vaporized volatile components formed by said heating in the first volatile component removing tower (2, 4, 21) are connected with means (202) for heat exchange in said distillation tower (3, 5, 22, 42) to act at least partly as the heat source for generating steam in said distillation tower (3, 5, 22, 42). Apparatus according to one of the claims 7 to 10, wherein said means (302, 303) for heating and boiling raw water containing volatile and non-volatile components are a heater (302) and a spray nozzle (303) for flash evaporation. Apparatus according to one of the claims 8 to 11, wherein a gas supply means is provided for making the atmosphere for condensing the steam in the distillation tower (1, 3, 5, 6, 22, 32, 42) an atmosphere of saturated steam or of inert gas. Apparatus according to one of the claims 7 to 12, wherein said volatile component removing tower (2, 4, 21) is equipped with a pressure reducing means. Use of superultra-pure water produced according to the process according to one of the claims 1 to 6 and/or with an apparatus according to one of the claims 7 to 13 for washing semiconductor integrated circuits. Use of superultra-pure water produced according to the process according to one of the claims 1 to 6 and/or with an apparatus according to one of the claims 7 to 13 for washing medical tools. Use of superultra-pure water produced according to the process according to one of the claims 1 to 6 and/or with an apparatus according to one of the claims 7 to 13 for producing medical drugs. Use of superultra-pure water produced according to the process according to one of the claims 1 to 6 and/or with an apparatus according to one of the claims 7 to 13 for oxidizing the silicon surface of a semiconductor wafer by contacting said silicon surface with the superultra-pure water. Use according to claim 17, wherein the thickness of the oxide film is controlled by changing the temperature of the superultra-pure water and/or the contact time with the superultra-pure water. Use according to claim 17 or 18, wherein the surface contacted with superultra-pure water is dried by evaporating the adherent superultra-pure water with a controlled vaporizing speed.

Inventors: EBARA, KATSUYA, HAYASHI, NOBUATSU, HISHINUMA, YUKIO, KOSEKI, YASUO, KUROKAWA, HIDEAKI, MATSUZAKI, HARUMI, MOMMA, NAOHIRO, OKOUCHI, ISAO, TAKAHASHI, SANKICHI, YAMADA, AKIRA, YODA, HIROAKI
Applicants: HITACHI, LTD.
Application year: 1988
Main classification: C02F1
IPC codes: B01D3, C02F1