Method and apparatus for supplying ozonated ultrapure water

Abstract: An ozonated ultrapure water supply apparatus that prevents reduction of the dissolved ozone concentration in the supply pipe system and maintains the dissolved ozone at points of use along the supply system at desired concentrations. This permits the system to supply ozonated water on long pipes. An apparatus for supplying ozonated ultrapure water includes a sequential arrangement of a main pipe for supplying ultrapure water (3), an ozonated gas supplying device (4), a branch pipe (8) having a gas/liquid separation device (9) and an ultimate point of use (7). The flow distance along the main pipe from the ozonated gas supplying device to the branch pipe is selected to achieve a desired level of ozonation.

Description: BACKGROUND OF THE INVENTION The present invention relates to a novel system for providing a relatively constant concentration of ozonated ultrapure water to remote points of use. The present invention may be used in applications requiring a supply of ozonated ultrapure water such as for use in cleaning semiconductor substrates. In the prior art, silicon substrates for semiconductors and glass substrates for liquid crystals are cleaned using what is called RCA cleaning. RCA cleaning is a high temperature cleaning method that uses concentrated hydrogen peroxide-based chemicals. Examples of these chemicals include a water solution of hydrogen chloride and aqueous hydrogen peroxide, or a water solution of aqueous ammonia and aqueous hydrogen peroxide. This method is expensive due to the costs of purchasing chemicals, obtaining purified water for rinsing and for waste disposal. In addition, air pollution devices are required to decontaminate exhaust and steam produced from the process. This method is also costly in terms of its impact on the environment. The method uses a great deal of water, discharges liquid chemical waste and releases waste gases. In recent years, in order to reduce these drawbacks, wet cleaning processes have been reassessed. For example, Japanese Laid-Open Patent Number 7-14817 discloses a cleaning method and device for washing silicon wafers. The method reduces chemical concentrations, shortens cleaning times, reduces the number of chemicals, makes recovery of wastes easier and reduces investment in equipment. The method involves placing the object to be washed horizontally inside a cleaning tank. A chemical cleaning step provides a continuous chemical flow from above. The object to be cleaned is rotated under the flow. A purified water cleaning step supplies purified water. The chemical cleaning step and the purified water cleaning step are performed sequentially in the same cleaning tank. Ozonated ultrapure water is proposed as the first chemical solution. Furthermore, in Japanese Laid-Open Patent Number 8-316187, the present inventors have proposed a method for the efficient removal of metal pollutants and organic pollutants from the top of a semiconductor substrate. The method does not use chemicals such as highly concentrated hydrochloric acid or hydrogen peroxide. Rather, a cleaning method is proposed wherein the cleaning step adds ozone to an acidic aqueous solution containing chloride compounds. This makes processing the waste solution generated by the cleaning process easier. Ozone dissolved in ultrapure water has a very strong oxidizing effect even at concentrations of only a few ppm. It is highly effective in removing organic substances and metals. It would be desirable to generate a source of ultrapure water containing a high concentration of dissolved ozone. However, the solubility of ozone in water is relatively small. In addition, autolysis of ozone into oxygen also occurs. These limitations prevent the preparation of a high concentration of dissolved ozone in water that can then be diluted to an appropriate concentration. The realities of the cleaning process are such that the demand for ozonated ultrapure water is not always constant. It is difficult to supply ozonated purified water at a constant concentration that is independent of demand. Because of this problem, the prior art method continually supplies ozonated ultrapure water at a constant flow and constant concentration, even when the demand is low. The present inventors have previously developed a closed loop system, wherein the makeup water of ultrapure water and the excess ozonated ultrapure water are collected in a tank. Ozone is dissolved in this water to compensate both for the effect of dilution by the makeup water and for the effect of autolysis. Ozone dissolved in ultrapure water is liable to autolyse even at normal temperatures. In this system, if the loop length is too long, the amount of supplemental ozone required to compensate for dilution and autolysis becomes large. In addition, a dissolved ozone concentration gradient develops between the upstream and downstream portions of the pipes. This uneven concentration of dissolved ozone is not desirable. Therefore, when long piping is used, this system is not adequate. In the water supply system for ozonated ultrapure water previously developed by the present inventors, the ozone dissolving area and the ozonated purified water supply piping system are set up separately. In other words, upon increasing the dissolved ozone concentration to a specified concentration, gas/liquid separation is first conducted in the ozone dissolving area so that no undissolved ozone gas remains in the ultrapure water supply pipe. Alternatively, ozone is dissolved while the gas and liquid are separated using a gas permeable membrane. Ozonated ultrapurified water is then sent to the supply system. This method is adequate if the ozone dissolving area is placed near the point of use. This method is also adequate if ozonated ultrapurified water is transported from the ozone dissolving area to the point of use via a short piping system. However, when using a long piping system, the dissolved ozone concentration is constantly being reduced through autolysis. Thus, it is not possible, using this system, to have points of use remotely located from the ozone dissolving device. OBJECTS AND SUMMARY OF THE INVENTION In light of the above, it is an object of the present invention to overcome the limitations of the prior art. The present inventors have discovered through intensive study, that using the ultrapure water supply pipe as the ozone dissolving area, allows for the dissolved ozone concentration to be maintained at a fairly constant concentration. The fairly constant concentration can be maintained even as autolysis occurs. The present invention was developed based on this finding. It is an object of the present invention to provide an apparatus for supplying ozonated ultrapure water which can suppress the reduction of dissolved ozone concentration within a supply piping system and which can maintain a relatively constant ozone concentration within the supply system. This apparatus may be applied to ultrapure water supply/circulating systems having any length of piping. It is another object of the present invention to provide an apparatus for supplying ozonated ultrapure water, including, in sequence, a main ultrapure water supply pipe for supplying ultrapure water, an ozonated gas supply device, a branch pipe off of the main ultrapure water supply pipe and a gas/liquid separation device just before the ultimate point of use. It is another object of the present invention to provide ozonated ultrapure water to a plurality of points of use while only using a single ozonated gas supply device. It is another object of the present invention to provide an economical way to provide a source of ozonated ultrapure water to a series of points of use having only a single ozone supply device. It is another object of the present invention to provide a closed loop system for providing ozonated ultrapure water that conserves unused ozonated ultrapure water. Briefly stated, the present invention provides an ozonated ultrapure water supply apparatus that prevents reduction of the dissolved ozone concentration in the supply pipe system and maintains the dissolved ozone at points of use along the supply pipe system at desired concentrations. This permits the system to supply ozonated water on long pipes. An apparatus for supplying ozonated ultrapure water includes sequential arrangement of an ultrapure water supply pipe for supplying ultrapure water, an ozonated gas supply device, a branch pipe feeding into a gas/liquid separation device and an ultimate point of use. The flow distance along the main pipe from the ozonated gas supply device to the branch pipe is selected to achieve a desired level of ozonation. According to an embodiment of the invention, there is provided an ultrapure ozonated water supply apparatus comprising: a main ultrapure water supply pipe for conveying ultrapure water in a loop originating from and returning to an ultrapure water storage means; an ozone gas supply means for supplying ozone to the said ultrapure water; at least one gas/liquid separator means for separating undissolved ozone gas from the said ultrapure water; characterised in that the said ozone gas supply means is connected by at least one ozone delivery pipe to at least one respective ozone gas delivery point on the said main supply pipe; in that the or each separator means is connected to the said ultrapure water pipe, downstream of a respective ozone gas delivery point, on a respective portion of the said ultrapure water pipe in which an equilibrium is established, in use, between loss of ozone from the ultrapure water due to autolysis and ozone gas dissolving into the ultrapure water such that the dissolved ozone concentration of the said ultrapure water in the said portion is substantially constant; and in that the or each said gas/liquid separator means includes an outlet for delivering ozonated ultrapure water to a respective point of use. The apparatus may further comprise means for increasing dissolving of ozone, connected to the main ultrapure water supply pipe at a point downstream of each gas/liquid separation device. For this purpose the apparatus may further include an in-line mixing device placed downstream from the ozonated gas supply device. The device may be, for example, a static mixer, a line mixer, a filler or a convoluted section of pipe. In one embodiment the ozonated gas is supplied to the main ultrapure water supply pipe by gas pressure, suctioning by an ejector or suctioning on the intake side of a pump. The main pipe for supplying ultrapure water may form a closed loop originating at an ultrapure water tank and terminating at a different point on the water tank. An ozone gas supply pipe, leading from the ozone containing gas supply device, may provide ozonated gas to multiple locations along the main ultrapure water supply pipe via branched pipe leading from the ozonated gas supply pipe. Accordingly there may be provided a plurality of ozone delivery pipes each having at least one respective gas/liquid separation device leading ozonated ultrapure water to respective points of use. A method of supplying ozonated ultrapure water, the said method comprising the steps of: conveying ultrapure water in a main ultrapure water supply pipe (3) in a loop originating from and returning to an ultrapure water storage device (1); introducing ozone gas into the ultrapure water; separating undissolved ozone gas from the ultrapure water before supplying ozonated ultrapure water to a point of use (7); characterised in that, the said ozone gas is introduced into the said ultrapure water at one or more ozone gas delivery points along the said loop; and in that the said undissolved gas is separated by at least one separator (9) downstream of a respective ozone gas delivery point on a portion of the pipe (3) in which; an equilibrium is established between loss of ozone from the ultrapure water due to autolysis and ozone gas dissolving into the ultrapure water, thereby to maintain the dissolved ozone concentration of the said ultrapure water in the or each portion is substantially constant. The method may further comprise the step of mixing the ozone gas and the ultrapure water in the main ultrapure water supply pipe whereby to increase dissolving of ozone. The ozonated gas may be introduced into the main ultrapure water supply pipe at a plurality of points each supplying at least one respective point of use. The dissolved ozone concentrations within the portion supplying each respective point of use may be substantially equal. Alternatively, different concentrations may be supplied to different respective points of use. The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals and letters designate the same elements. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows an embodiment of an ozonated ultrapure water supply apparatus according to an embodiment of the present invention. Fig. 2(a) is a schematic diagram showing one embodiment of a gas/liquid separation device of the present invention. Fig. 2(b) is a schematic diagram showing a second embodiment of a gas/liquid separation device of the present invention. Fig. 3 is a curve showing the relationship between pipe length and dissolved oxygen concentration in the ozonated ultrapure water supply apparatus of the present invention. Fig. 4 shows an embodiment of an ozonated ultrapure water supply apparatus of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The ozonated ultrapure water supply apparatus consists of a main pipe for supplying ultrapure water and an ozonated gas supply device connected to the main ultrapure water supply pipe. A branch pipe or pipes lead from the main pipe. The branch pipe or pipes connect to the ultimate point or points of use. Each branch pipe is equipped with a gas/liquid separation device placed prior to the ultimate point of use. Referring to Fig. 1, makeup ultrapure water is transported from an ultrapure water producing device (not shown) to collection in an ultrapure water tank 1. Ultrapure water is transferred out of ultrapure water tank 1 to a main ultrapure water supply pipe 3 by a pump 2. In this embodiment, the ultrapure water supply source can be the ultrapure water tank. Alternatively, a subsystem for providing ultrapure water can be used as the source of ultrapure water. An apparatus for supplying ozone-containing gas 4 is connected through a gas supply pipe 5 to the main ultrapure water supply pipe 3. There are no particular limitations to the ozone containing gas supply device. For example, ozone generators which produce ozone through the electrolysis of water, or through silent discharge in air or oxygen can be used. Methods for supplying ozonated gas to the main ultrapure supply pipe can include gas pressure, suction provided by an ejector and suction provided by a pump. When the distance between the ozonated gas supply point and the point of use is comparatively long, the supplied ozone dissolves in the ultrapure water as the water flows through the main pipe. However, if the distance between the ozonated gas supply point and the point of use is comparatively short, an inline mixing mechanism is preferred in order to efficiently accelerate the dissolution of ozone. An inline mixing mechanism 6 can be set up as needed downstream from the ozonated gas supply point on the ultrapure water supply main pipe. There are no particular limitations to the inline mixing mechanism. For example, static mixers or line mixers can be used. Alternatively, packing material can be placed in the main pipe to accelerate the gas/liquid contact or the pipe can be bent or coiled, as shown, to lengthen the retention time. The main ultrapure water supply pipe 3 extends to one or more points of use 7 to supply ozonated ultrapure water to these points. In the present embodiment, the ultrapure water supply main pipe 3 is formed in a loop. The ultrapure water that is not used at the points of use 7 is returned to the ultrapure water tank 1. Alternatively, the end of the ultrapure water supply main pipe 3 may be connected to the ultrapure water producing device via a residual ozone removal device (not shown). The ultrapure water which is not used at the points of use, is thus returned to the ultrapure water producing device, after any residual ozone is removed. In addition to dissolved ozone, the ozonated ultrapure water also contains undissolved ozone in a carrier gas. This undissolved gas on its way to points of use 7 passes through a branch pipe 8 which branches off from the ultrapure water supply pipe 3. Undissolved gas is not desirable for precise and uniform cleaning. This undissolved gas is removed from the ultrapure water by a gas/liquid separation device 9 in the branch pipe 8. There are no particular limitations for the gas/liquid separation device 9 as long as it can separate the undissolved gas component from the ultrapure water flow. After this, the ultrapure water is supplied to the point(s) of use 7. Referring now to Fig. 2(a), one embodiment of the gas/liquid separation device 9 has a gas phase area 10 on top, and a water phase area 11 on the bottom. The velocity of the ozonated ultrapure water is reduced when it enters the larger cross section of the gas/liquid separation device 9. The undissolved ozone gas is released in bubbles that rise to the surface of the water and enter the gas phase area 10. The ozonated ultrapure water, with the undissolved ozone removed, is supplied to the point of use 7 from the bottom water phase area 11 of the gas/liquid separation device 9. The undissolved gas in the gas phase area 10 is released via a valve 12. In Fig. 2(b), the gas/liquid separation device 9 of Fig. 2(a) is equipped with a multiporous plate 13 in the water phase area 11 of the device. This further reduces the flow velocity of the water. Using a multiporous plate 13 or fillers in the water phase area 11, improves separation of the undissolved gas from the water stream. Referring now to Fig. 3, a concentration gradient chart for the ozonated ultrapure water supply apparatus of the present invention is represented with the horizontal axis representing the length of the piping. The vertical axis represents the dissolved ozone concentration. When ozonated gas is supplied to ultrapure water at ozonated gas supply point "A" of the main pipe (see also Fig. 1), the ozone dissolves in the ultrapure water and the dissolved ozone concentration increases rapidly. However, as the concentration of ozone dissolved in the ultrapure water increases, the loss of ozone through autolysis becomes prominent. This effect is offset by the fact that gaseous ozone is more stable than ozone dissolved in water. The undissolved ozone gas available in the ultrapure water then dissolves in the ultrapure water. As a result, at point "B," there is an equilibrium where the loss of ozone in the ultrapure water due to autolysis is balanced by the amount of ozone gas dissolving into the ultrapure water. The concentration of dissolved ozone in the ultrapure water is thus maintained at an approximately constant concentration. When the ozonated ultrapure water flows further downstream along the main pipe, the amount of undissolved ozone available in the accompanying gas is reduced. At point "C," it becomes difficult to maintain the equilibrium between the amount of ozone lost due to autolysis and the amount of ozone available to be dissolved in the ultrapure water. At this point, the concentration of dissolved ozone decreases. Zone I is defined as the zone between points "A" and "B." Zone II is defined as the zone between points "B" and "C." Zone III is defined as the zone downstream of point "C." The device of the present invention uses the ultrapure water in zone II, where the dissolved ozone concentration remains relatively constant. In other words, the branch pipes 8 connected to the points of use 7 are placed in the portion of the main ultrapure water supply pipe 3 corresponding to zone II. The ozonated ultrapure water treated with a gas containing undissolved ozone is supplied to one or more points of use 7 via branch pipes 8. The undissolved ozone is separated from the water stream by gas/liquid separation devices 9. In this way, ultrapure water containing a relatively constant concentration of dissolved ozone is supplied. Thus, the device of the present invention uses the main ultrapure water supply pipe 3 as the ozone dissolving area. Even if the piping system is long, a supply of ozonated ultrapure water having a relatively constant concentration of dissolved ozone is provided. Thus, in the apparatus of the present invention, the area of zone II is used as the ozonated ultrapure water supply pipe system. In zone II, there is a portion after point "B" where the dissolved ozone concentration rises slowly, and after passing the maximum ozone concentration value, the dissolved ozone concentration decreases slowly. In light of this, it is possible to specify zone II at the manufacturing site, based on the allowable lower limit and upper limit of the dissolved ozone concentration for the particular application. For example, if some amount of variation in concentration is permitted, as in a dissolved ozone concentration of 2 ppm or greater, branch pipes 8 can all be simply connected in zone II. On the other hand, if the dissolved ozone concentration must be strictly maintained, ozone in excess of the specified ozone concentration is dissolved in the ultrapure water flowing in the main ultrapure water supply pipe 3. Each branch pipe 8 is equipped with a mechanism for monitoring ozone concentration and a control mechanism for regulating ozone concentration (not shown). The ozone concentration is adjusted at this point, by adding ultrapure water which does not contain ozone, so that the specified ozone concentration is supplied. Referring now to Fig. 4, another embodiment of an ozonated ultrapure water supply apparatus of the present invention includes branches in the ozonated gas supply pipe 5. The illustrated embodiment includes five ozone containing gas supply pipes 5 connected to spaced-apart points on the main ultrapure water supply pipe 3. In this design, new ozonated gas is supplied to the ultrapure water flow before the amount of ozone gas available for dissolving in the ultrapure water becomes less than the amount of autolysed ozone. In this way, the concentration of dissolved ozone gas is maintained at the zone II level all along the length of the main ultrapure water supply pipe 3. Thus, zone II, where the dissolved ozone concentration in the ultrapure water is approximately constant, is lengthened. It becomes possible to supply ozonated ultrapure water to multiple points of use. Because gaseous ozone is more stable compared to ozone dissolved in water, it is easier to provide ozonated gas at a specified concentration from a series of ozonated gas supply points to the main ultrapure water supply pipe 3. This system obviates the need for a plurality of ozonated gas supply devices. By using this method of supplying ozonated gas along the way, the limitation on the length of the main ultrapure water supply pipe 3 is eliminated. In addition, a gas/liquid separation device 9 can be placed on the individual branch pipe leading to every point of use 7. Alternatively, a manifold gas/liquid separation device 14 can be shared by a number of points of use 7. Thus, branch pipes 8 can be placed after the gas/liquid separation device 14 to supply ozonated ultrapure water to a plurality of points of use 7 from a single gas/liquid separation device 14. According to the ozonated ultrapure water supply apparatus of the present invention, dissolved ozone is maintained at a relatively constant concentration. The length of the main ultrapure water supply pipe 3 is no longer a limiting factor, as it was in the prior art. In addition, ozonated ultrapure water can be supplied to numerous points of use 7. Furthermore, by using a method of supplying ozonated gas to numerous supply points on the main pipe, there is no limitation on the length of the main ultrapure water supply pipe 3. As a result, use of ozonated ultrapure water for wet cleaning processes becomes more economical.

Claims: An ultrapure ozonated water supply apparatus comprising: a main ultrapure water supply pipe (3) for conveying ultrapure water in a loop originating from and returning to an ultrapure water storage means (1); an ozone gas supply means (4) for supplying ozone to the said ultrapure water; at least one gas/liquid separator means (9) for separating undissolved ozone gas from the said ultrapure water; characterised in that the said ozone gas supply means is connected by at least one ozone delivery pipe (5) to at least one respective ozone gas delivery point on the said main supply pipe (3); in that the or each separator means (9) is connected to the said ultrapure water pipe (3), downstream of a respective ozone gas delivery point, on a respective portion of the said ultrapure water pipe in which an equilibrium is established, in use, between loss of ozone from the ultrapure water due to autolysis and ozone gas dissolving into the ultrapure water such that the dissolved ozone concentration of the said ultrapure water in the said portion is substantially constant; and in that the or each said gas/liquid separator means (9) includes an outlet for delivering ozonated ultrapure water to a respective point of use (7). Apparatus according to Claim 1, further comprising means (6) for increasing the dissolved ozone concentration, connected to said main ultrapure water supply pipe at a point downstream of said ozone delivery pipe and upstream of the or each said gas/liquid separation device. Apparatus according to Claim 2, in which said means (6) for increasing the dissolved ozone concentration comprises an in-line mixing device. Apparatus according to Claim 3, in which the said in-line mixing device (6) is a static mixer, a line mixer, filler or a convoluted section of pipe. Apparatus according to any preceding claim, in which said main ultrapure water supply pipe (3) is in a closed loop originating in said storage device (1) and terminating at a separate point on said storage device (1). Apparatus according to any preceding claim, in which the said storage device (1) is an ultrapure water tank for storing ultrapure water that is interposed between an ultrapure water source and said main ultrapure water pipe (3). Apparatus according to any preceding claim, in which there are provided a plurality of said ozone delivery pipes (5) each having at least one respective gas/liquid separation device (9) leading ozonated ultrapure water to respective points of use (7). A method of supplying ozonated ultrapure water, the said method comprising the steps of: conveying ultrapure water in a main ultrapure water supply pipe (3) in a loop originating from and returning to an ultrapure water storage device (I); introducing ozone gas into the ultrapure water; separating undissolved ozone gas from the ultrapure water before supplying ozonated ultrapure water to a point of use (7); characterised in that , the said ozone gas is introduced into the said ultrapure water at one or more ozone gas delivery points along the said loop; and in that the said undissolved gas is separated by at least one separator (9) downstream of a respective ozone gas delivery point on a portion of the pipe (3) in which an equilibrium is established between loss of ozone from the ultrapure water due to autolysis and ozone gas dissolving into the ultrapure water, thereby to maintain the dissolved ozone concentration of the said ultrapure water in the or each portion is substantially constant. A method according to Claim 8, further comprising the step of mixing the said ozone gas and the said ultrapure water in a main ultrapure water supply pipe whereby to increase dissolved ozone concentration. A method according to Claim 8 or Claim 9, in which ozone gas is introduced into the main ultrapure water supply pipe (3) at a plurality of points for supplying at least one respective point of use (7). A method according to Claim 10, in which the said constant dissolved ozone concentrations supplied to each said respective point of use (7) are substantially equal. A method according to Claim 10, in which the said constant dissolved ozone concentrations supplied to each said respective point of use (7) are substantially different.

Inventors: IDA JUNICHI, MIZUNIWA TETSUO, MORITA HIROSHI, IDA, JUNICHI, MIZUNIWA, TETSUO, MORITA, HIROSHI, IDA, JUNICHI, C/O KURITA WATER IND. LTD., Mizuniwa, Tetsuo, c/o Kurita Water Ind. Ltd., Morita, Hiroshi, c/o Kurita Water Ind. Ltd.
Applicants: KURITA WATER IND LTD, KURITA WATER INDUSTRIES LTD.
Application year: 1998
Main classification: C02F1
IPC codes: H01L21, C02F1