Abstract:
Provided are a system and method capable of providing a sulfuric acid solution containing peroxosulfuric acid for use in cleaning even in an installation-limited space such as a clean room while suppressing the fluid pressure in a heating part such as a rapid heating heater. 
     The system comprises: an electrolytic part for electrolyzing a sulfuric acid solution to produce peroxosulfuric acid; a first reservoir for storing the sulfuric acid solution; a circulation line for circulating the sulfuric acid solution between the electrolytic part and the first reservoir; a cleaning apparatus for cleaning a cleaning object by use of the sulfuric acid solution containing peroxosulfuric acid; a supply line for sending the sulfuric acid solution electrolyzed in the electrolytic part to the cleaning apparatus; a heating part for heating the sulfuric acid solution to be used in the cleaning apparatus, the heating part being interposed in the supply line on the upstream side of the cleaning apparatus; and a second reservoir which is interposed in the supply line on the upstream side of the heating part. Since the second reservoir and the heating parts are positioned spatially above the level of the first reservoir, the electrolytic part and the circulation line, the device can be disposed with good space efficiency while preventing application of a high fluid pressure to the heating part or the like.

Description:
TECHNICAL FIELD 
     The present invention relates to a cleaning system and a cleaning method, which can be suitably used for cleaning of a resist adhered to an electronic material such as a silicon wafer by supplying a sulfuric acid solution containing peroxosulfuric acid, which is obtained by electrolyzing a sulfuric acid solution, to a cleaning apparatus for performing the cleaning of the resist or the like. 
     BACKGROUND ART 
     In a resist stripping step in semiconductor manufacturing, a sulfuric acid electrolytic method is known, in which cleaning is performed using a peroxosulfuric acid solution as cleaning liquid by electrolyzing a sulfuric acid solution to produce peroxosulfuric acid (peroxodisulfuric acid and peroxomonosulfuric acid; molecular peroxosulfuric acid and ionic peroxosulfuric acid). In the resist stripping step, the resist stripping is promoted more efficiently as the temperature of the cleaning liquid is higher. This is considered to be attributable to that when the cleaning liquid produced by the sulfuric acid electrolytic method reaches a predetermined high temperature, the peroxosulfuric acid in the cleaning liquid self-decomposes to generate a sulfuric radical with extremely high oxidative power, contributing to the cleaning. 
     However, since the radical has a short life-span, the cleaning liquid is consumed without contributing to the cleaning, when the temperature of the cleaning liquid is raised in an early stage, due to premature self-decomposition of the peroxosulfuric acid contained in the cleaning liquid. When the cleaning liquid is heated slowly over a long time (e.g., about several minutes), a problem in which the concentration of peroxosulfuric acid has been reduced at the moment the cleaning liquid reaches the high temperature arises since the self-decomposition of the peroxosulfuric acid and decomposition of the sulfuric radial associated therewith progress in the middle of heating. 
     The cleaning of an electronic material substrate or the like is performed by a single-wafer type as well as a batch type. In the single-wafer type, for example, a cleaning object is fixed to a rotary table, and cleaned by means of spraying of a chemical, flushing with the chemical, or the like while rotating the table. In a single-wafer cleaning apparatus, the cleanliness of the electronic material substrate such as a wafer can be kept high, compared with batch type cleaning. Further, an unnecessary resist can be efficiently stripped from an electronic material such as a silicon wafer with relatively little usage of the chemical. However, the chemical to be used in the single-wafer cleaning apparatus is required to have characteristics conditioned more stringently than those of an electrolytic sulfuric acid solution to be used in a batch type cleaning apparatus. In stripping of a resist which is ion-implanted at a high concentration of 1×10 15  atoms/cm 2  or more, particularly, a cleaning liquid having a further high peroxosulfuric acid concentration and a further high liquid temperature is needed. 
     In the above-mentioned viewpoint, the present inventors proposed a cleaning system provided with a rapid heating heater (refer to Patent Literature 1) in consideration of the need to perform the temperature rise of cleaning liquid in a given short time just before cleaning. In this cleaning system, a storage tank storing an electrolyte is connected to the rapid heating heater through a supply line, and the outlet side of the rapid heating heater is further joined to a supply line to a cleaning apparatus by a flange or the like. A sulfuric acid solution is sent from the storage tank to the rapid heating heater by a pump, heated by the rapid heating heater, and supplied to the cleaning apparatus as it is at the pressure at the time of solution sending. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1; Japanese Patent Application Laid-Open No. 2010-60147 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, quartz which is glassy and brittle or the like is often used for a flow passage of the above-mentioned rapid heating heater, and an increased feeding pressure can cause breakage of the rapid heating heater. 
     Further, although the cleaning of the electronic material substrate is generally performed in a clean room, the cleaning system cannot be freely installed due to many restrictions on installation since the installation space in the clean room is limited, and it is difficult to install the cleaning system in consideration of the feeding pressure. 
     The present invention is achieved with the above-mentioned conditions as a background, and has an object to provide a cleaning system and a cleaning method, which facilitate the installation in a clean room with a limited installation space or the like while minimizing the feeding pressure for rapid heating. 
     Solution to Problem 
     Namely, the cleaning system of the present invention includes; an electrolytic part for electrolyzing a sulfuric acid solution to produce peroxosulfuric acid; a first reservoir for storing the electrolyzed sulfuric acid solution; a circulation line for circulating the sulfuric acid solution between the electrolytic part and the first reservoir; a cleaning apparatus for cleaning a cleaning object by use of the sulfuric acid solution containing peroxosulfuric acid; a supply line for sending the sulfuric acid solution electrolyzed in the electrolytic part to the cleaning apparatus; a heating part for heating the sulfuric acid solution to be used in the cleaning apparatus, the heating part being interposed in the supply line on the upstream side of the cleaning apparatus; and a second reservoir which is interposed in the supply line on the upstream side of the heating part, wherein the second reservoir and the heating part are positioned spatially above the level of the first reservoir, the electrolytic part and the circulation line. 
     The cleaning method of the present invention for cleaning a cleaning object with a sulfuric acid solution containing peroxosulfuric acid comprises: circulating a sulfuric acid solution while electrolyzing it in an electrolytic part; extracting a part of the circulating sulfuric acid solution; storing the extracted circulating sulfuric acid solution at a position spatially above the level where the circulation is performed; and then rapidly heating the stored sulfuric acid solution to be served for the cleaning. 
     In the present invention, the sulfuric acid solution is circulated between the electrolytic part and the first reservoir by the circulation line, whereby the peroxosulfuric acid can be continuously produced by electrolysis. 
     In the supply line, the circulating sulfuric acid solution which has been electrolyzed is extracted. The extraction position can be set to any one of the delivery side of the electrolytic part, the circulation line, and the first reservoir. 
     The sulfuric acid solution sent through the supply line is temporarily stored in the second reservoir, and then sent to the heating part. Therefore, the feeding pressure on the upstream side of the second reservoir is never transmitted to the downstream side as it is. Since the second reservoir and the heating part are positioned above the level where the circulation is performed, the sulfuric acid solution can be sent to the cleaning apparatus through the heating part at a further small feeding pressure on and after the second reservoir. 
     In the electrolytic part, the peroxosulfuric acid which enhances the cleaning effect is produced by electrolyzing the sulfuric acid solution as described above. In this electrolysis, the lower the solution temperature is, the higher the production efficiency of peroxosulfuric acid is. Therefore, the electrolysis temperature in the production of peroxosulfuric acid is preferably set to 80° C. or lower. A temperature exceeding this temperature range causes noticeable deterioration of electrolytic efficiency. On the other hand, an excessively low temperature causes serious wear of electrodes. Therefore, the above-mentioned temperature is preferably set to 40° C. or higher. 
     In the above-mentioned electrolytic part, the electrolysis is performed with an anode and a cathode being paired. The materials of these electrodes are never limited to specific ones in the present invention. However, when platinum which is widely used as electrodes is used as the anode of the electrolytic part in the present invention, there are problems that the peroxosulfuric acid cannot be efficiently produced and platinum is eluted. In contrast, a conductive diamond electrode allows efficient production of peroxosulfuric acid with minimized wear of the electrode. Therefore, it is preferred that at least the anode at which the peroxosulfuric acid is produced, of the electrodes in the electrolytic part, is constituted by the conductive diamond electrode, and it is further preferred that both the anode and the cathode are constituted by the conductive diamond electrode. As the conductive diamond electrode, a conductive diamond thin film synthesized on a surface of a semiconductor material such as a silicon wafer as a substrate, or self-stand conductive polycrystalline diamond precipitated and synthesized thereon in a plate-like shape can be adopted. A diamond thin film laminated on a metal substrate such as Nb, W, or Ti can be also used. The conductive diamond thin film is obtained by giving conductivity to a diamond thin film by doping with a predetermined amount of boron or nitrogen in the synthesis of the diamond thin film, and the diamond thin film is generally doped with boron. The doping amount is suitably in the range of 50 to 20,000 ppm relative to the amount of carbon in the diamond thin film since an excessively small amount cannot develop a technical value, and an excessively large amount saturates the doping effect. 
     The temperature of the sulfuric acid solution in the first reservoir is preferably set to 50 to 90° C. (particularly, 60 to 70° C.). If the temperature is high, the sulfuric acid solution in the first reservoir to be sent to the electrolytic part must be cooled in preparation for electrolysis, resulting in an increased cooling load. Therefore, the temperature is preferably 90° C. or lower. Further, since a excessively lower temperature can cause wear of the electrodes in the electrolytic part, the temperature of the sulfuric acid solution in the reservoir for electrolysis solution is preferably 50° C. or higher. 
     In the second reservoir, it is preferred to store the sulfuric acid solution which has been preheated. The preheating may be performed in the supply line or may be performed in the second reservoir. The preheating further facilitates the rapid heating with minimized heating load in the heating part on the downstream side. The temperature of the preheating is preferably set within the range of 90 to 120° C. At a temperature below 90° C., the effect of reducing the heating load in the heating part is insufficient. At a temperature exceeding 120° C., sufficient oxidation performance cannot be secured in the cleaning since the self-decomposition of peroxosulfuric acid is promoted. 
     In the heating part, the sulfuric acid solution is preferably heated so that the sulfuric acid solution has a temperature of 150 to 220° C. at the time of cleaning. When the heating temperature is below 150° C., the oxidation performance by self-decomposition of peroxosulfuric acid cannot be sufficiently secured. On the other hand, since a too high temperature of the sulfuric acid solution causes deterioration of cleaning performance due to a too high decomposition rate of peroxosulfuric acid, the temperature is preferably 220° C. or lower. 
     The sulfuric acid solution to be used in the above-mentioned cleaning system preferably has a sulfuric acid concentration of 85 mass % or more. When the sulfuric acid concentration is below 85 mass %, the resist stripping performance in the cleaning apparatus is deteriorated even if the peroxosulfuric acid concentration is high. On the other hand, since the current efficiency in the electrolytic process is deteriorated at a sulfuric acid concentration exceeding 96 mass %, the concentration is preferably 96 mass % or lower. 
     In the cleaning apparatus, a sulfuric acid solution having a relatively high temperature is discharged after cleaning a cleaning object such as an electronic material substrate. In the present invention, a reflux line for refluxing this sulfuric acid solution to the circulation line side can be provided. The reflux line is connected to at least one of the first reservoir, the electrolytic part and the circulation line, whereby the sulfuric acid solution can be refluxed. 
     In the reflux line, a cooling part is preferably interposed for maintaining the liquid temperature in the first reservoir or the liquid temperature at the inlet of the electrolytic part at a predetermined temperature. Further, when the cooling part is provided in the reflux line, a decomposition part for retaining the sulfuric acid solution received from the cleaning apparatus using side and performing decomposition of a residual organic material such as a resist which is stripped from the electronic material substrate and is contained in the sulfuric acid solution can be provided on the upstream side of the cooling part. An oxidizing substance such as peroxosulfuric acid is left in the sulfuric acid solution, and the resist or the like in the sulfuric acid drainage retained in the decomposition part is oxidized, decomposed and cleared away by use of the remaining heat of the sulfuric acid solution by the effect of the oxidizing substance. This oxidation decomposition can be more effectively performed as the temperature is higher. 
     In the layout of the above mentioned cleaning system, the cleaning apparatus, the second reservoir and the heating part can be installed within the clean room, electrolytic part, while the first reservoir and the circulation line can be installed outside the clean room. According to this layout, the cleaning system can be efficiently arranged inside and outside of the clean room with limited space. 
     The present invention is suitable for use in cleaning treatment on an electronic material substrate such as a silicon wafer, a glass substrate for liquid crystal or a photomask substrate, although cleaning can be performed on various cleaning objects. More specifically, the present invention can be used for a stripping process of an organic compound such as a resist residue adhered onto a semiconductor substrate. It can be used also for a removing process of foreign matter such as fine particles, metal or the like adhered to the semiconductor substrate. 
     Furthermore, the present invention can be used also for a process for cleaning and stripping a contaminant adhered on a substrate such as a silicon wafer with a high-concentration sulfuric acid solution. The present invention can be desirably used as a system which does not need addition of a chemical such as hydrogen peroxide or ozone from the outside by producing a peroxosulfuric acid solution on site by an electrolytic part for enhancing the resist stripping and oxidation effects without a pretreatment such as ashing process, and using the sulfuric acid solution in a repetitive manner. 
     Advantageous Effects of Invention 
     As described so far, according to the present invention, in electrolysis of the sulfuric acid solution with circulation, the solution sending, for example, is performed in two stages such that the sulfuric acid solution is not directly sent from the first reservoir to the heating part, but is primarily sent from the first reservoir side to the second reservoir and then sent from the second reservoir to the heating part, whereby the rise of liquid pressure in the heating part can be suppressed to secure the safety. 
     The preheating of the sulfuric acid solution allows reduction in load of the heating part. When the heating part uses a halogen lamp, an extended life of the halogen lamp can be attained in addition to reduction in energy cost. 
     A part of the cleaning system is located outside and below the clean room (e.g., downstairs) as described above, whereby the cleaning system can be used even if the installation space in the clean room is limited. Since the sulfuric acid solution after rapid heating must be served for cleaning in a short period of time, it is particularly desired to install the heating part near the cleaning apparatus in the clean room. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart showing a cleaning system in one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
     One embodiment in a functional solution supply system of the present invention will be described based on  FIG. 1 . 
     An electrolytic device  1  which corresponds to the electrolytic part of the present invention is a non-diaphragm type, in which an anode and a cathode (not shown) constituted by diamond electrodes are arranged inside without being separated from each other through a diaphragm, and a DC power source  2  is connected to both the electrodes. In the present invention, the electrolytic device can be constituted as a diaphragm type. 
     A first storage tank  20  which corresponds to the first reservoir of the present invention is connected to the electrolytic device  1  through a circulation line  11  so as to be capable of circulating and passing through solution. A gas-liquid separation tank  10  is interposed in the circulation line on the returning side. As the gas-liquid separation tank  10  for receiving a gas-containing sulfuric acid solution, and separating and discharging the gas in the sulfuric acid solution to the outside, a known one can be used, and the configuration thereof is not particularly limited in the present invention as long as the separation of gas and liquid can be performed. 
     A circulation pump  12  for circulating the sulfuric acid solution and a cooler  13  for cooling the sulfuric acid solution are interposed in the circulation line  11  on the feeding side. The cooler  13  corresponds to the first cooling part of the present invention and can preferably cool the sulfuric acid solution so that it can be electrolyzed at a liquid temperature of 40 to 80° C. The configuration of the first cooling part is not particularly limited in the present invention. 
     An upstream supply line  22  is connected to the first storage tank  20  through a first supply pump  21  which corresponds to the first pump of the present invention. 
     An electrolytic unit A is constituted by the above-mentioned electrolytic device  1 , DC power source  2 , circulation line  11 , circulation pump  12 , cooler  13 , gas-liquid separation tank  10  and first storage tank  20  and a cooler  53  to be described later. 
     Although the gas-liquid separation tank  10  and the first storage tank  20  are separately provided in the above-mentioned system, the first storage tank may be used also as the gas-liquid separator. 
     A preheating tank  30  which corresponds to the second reservoir of the present invention is connected to the feeding side of the upstream supply line  22 . The preheating tank  30  is provided with a preheating heater  31  that is a preheating part for heating the sulfuric acid solution stored in the preheating tank  30  to 90 to 120° C. In the present invention, the preheating part may be provided in the upstream supply line  22 . 
     Further, a downstream supply line  32  for sending the sulfuric acid solution in the preheating tank  30  is connected to the preheating tank  30 . A second supply pump  33  which corresponds to the second pump of the present invention and a rapid heating heater  34  are interposed in the downstream supply line  32 , and a single-wafer cleaning apparatus  40  is connected to the feeding end side of the downstream supply line  32 . The above-mentioned upstream supply line  22  and downstream supply line  32  constitute the supply line of the present invention. 
     The above-mentioned rapid heating heater  34  which corresponds to the heating part of the present invention has a quartz-made conduit, and rapidly heats the sulfuric acid solution in one pass manner by a near infrared heater so that a liquid temperature of 150 to 220° C. can be obtained at the inlet of the cleaning apparatus  40 . 
     On the downstream side of the rapid heating heater  34 , a liquid temperature measuring instrument  35  for measuring temperature of the sulfuric acid solution to be sent is provided in the downstream supply line  32 , and the measurement result of the liquid temperature measuring instrument  35  is outputted to a current-carrying control unit  36  including the DC power source. The current-carrying control unit  36  for applying current to the second pump  33  and the rapid heating heater  34  at a predetermined energization quantity controls the energization quantity to the second pump  33  and the rapid heating heater  34 . The current-carrying control unit  36  controls the energization quantity to the heater  34  based on the measurement result of the liquid temperature measuring instrument  35  so that the liquid temperature becomes a predetermined temperature. The pump flow rate of the second pump  33  controlled by the current-carrying control unit  36  is changed in output according to a request of the cleaning apparatus. 
     For example, if the liquid temperature is lower than the predetermined temperature, the energization quantity to the rapid heating heater  34  is increased. If the liquid temperature is higher than the predetermined temperature, the energization quantity to the rapid heating heater  34  is decreased. A controlled variable thereof is preliminarily set in association with the liquid temperature or the like, whereby the control can be performed based on the set value of the controlled variable. 
     The above-mentioned preheating tank  30 , preheating heater  31 , second pump  33 , rapid heating heater  34 , liquid temperature measuring instrument  35  and current-carrying control unit  36  constitute a rapid heating unit B. 
     The above-mentioned single-wafer cleaning apparatus  40  includes a nozzle  41  turned to an electronic material substrate  100  that is a cleaning object being carried in, and a rotary table  42  for placing and rotating the electronic material substrate  100  on which the sulfuric acid solution is sprayed or dripped little by little by the nozzle  41  as cleaning liquid. Further, the cleaning apparatus  40  includes a sulfuric acid solution recovery part  43  for recovering droplets of the sulfuric acid solution used for the cleaning, and an upstream reflux line  45  with a first reflux pump  44  interposed therein is connected to the sulfuric acid solution recovery part  43 . 
     The above-mentioned cleaning apparatus  40 , nozzle  41 , rotary table  42 , and sulfuric acid solution recovery part  43  constitute a cleaning unit C. 
     Although the single-wafer cleaning apparatus is described in this embodiment, the type of the cleaning apparatus is not limited thereto, and a cleaning apparatus of batch type or the like may be adopted. 
     The upstream reflux line  45  is connected to a decomposition tank  50  to temporarily store the sulfuric acid solution used for cleaning. The decomposition tank  50  corresponds to the decomposition part of the present invention. A downstream reflux line  52  is connected to the decomposition tank  50  through a second reflux pump  51 , and a cooler  53  which corresponds to the second cooling part of the present invention is interposed in the downstream reflux line  52 . The feeding end of the downstream reflux line  52  is connected to the first storage tank  20 . 
     The above-mentioned upstream reflux line  45  and downstream reflux line  52  constitute the reflux line of the present invention. 
     The above-mentioned rapid heating unit B and cleaning unit C are installed inside a clean room  200  for treating the electronic material substrate  100  prior to cleaning, and the electrolytic unit A and the decomposition tank  50  are installed outside the clean room  200  and downstairs below the clean room  200 . This allows the cleaning treatment in the clean room  20  with a limited space. 
     The operation of the cleaning system having the above-mentioned configuration will be then described. 
     In the first storage tank  20 , a sulfuric acid solution having a sulfuric acid concentration of 85 to 96 mass % is stored. The sulfuric acid solution is sent by the circulation pump  12 , adjusted to a suitable temperature (40 to 80° C.) for electrolysis by the cooler  13 , and introduced to the inlet side of the electrolytic device  1 . In the electrolytic device  1 , a current is carried to between the anode and cathode by the DC power source  2  to electrolyze the sulfuric acid solution introduced to the electrolytic device  1 . In the electrolytic device  1 , oxidizing substance is produced on the anode side including peroxosulfuric acid oxygen in addition to gas generation, while hydrogen gas is generated on the cathode side. These oxidizing substance and gases are sent to the gas-liquid separation tank  10  through the reflux line  11  in a state where they are mixed with the sulfuric acid solution to separate the gases. The separated gases are discharged out of this system and safely treated by a catalyst device (not shown) or the like. 
     The sulfuric acid solution subjected to the gas separation in the gas-liquid separation tank  10 , which contains peroxosulfuric acid, is further repeatedly sent to the electrolytic device  1 , after returned to the first storage tank  20  through the circulation line  11 , whereby the concentration of the peroxosulfuric acid is enhanced by electrolysis. When a proper concentration of peroxosulfuric acid is attained, a part of the sulfuric acid solution in the first storage tank  20  is fed to the preheating tank  30  by the first pump  21  through the upstream supply line  22 . In the preheating tank  30 , the sulfuric acid solution stored therein is heated to and maintained at a temperature of 90 to 120° C. by the preheating heater  31 . The sulfuric acid solution in the preheating tank  30  is then fed to the cleaning apparatus  40  by the second supply pump  33  through the downstream supply line  32  and further through the rapid heating heater  34 . At that time, since the preheating tank  30 , the rapid heating heater  34  and the cleaning apparatus  40  are positioned above the level of the first storage tank  20 , and the feeding pressure by the first supply pump  21  is relaxed in the preheating tank  30 , the feeding pressure to the rapid heating heater  34  is significantly reduced. Accordingly, the second reservoir should not have a structure such that the feeding pressure of the first reservoir is transmitted to the downstream side of the second reservoir as it is. 
     In the rapid heating heater  34 , the sulfuric acid solution contained the peroxosulfuric acid is rapidly heated by the near infrared heater while being passed through the flow passage so that it has a liquid temperature within the range of 150 to 220° C. when it is supplied to the cleaning apparatus  40 . The heating temperature can be substantially equalized to the temperature in use by disposing the rapid heating heater  34  in the vicinity of the cleaning apparatus  40 . 
     The heated sulfuric acid solution contained peroxosulfuric acid is supplied to the single-wafer cleaning apparatus  40  through the downstream supply line  32 , and used for cleaning of the electronic material substrate  100 . At that time, the flow rate of the sulfuric acid solution is desirably adjusted so that the solution passing time from the inlet of the rapid heating heater  34  to the use in the cleaning apparatus  40  is less than 1 minute. An appropriate flow rate in the single-wafer cleaning apparatus is 500 to 2000 mL/min, and the flow passage length and flow passage sectional area in the rapid heating heater  34  and the line length and flow passage sectional area in the downstream supply line  32  on the downstream side of the rapid heating heater  34 , or the like are set so that the solution passing time is less than 1 minute at this flow rate. 
     In the cleaning apparatus  40 , for example, the electronic material substrate  100  such as a silicon wafer provided with a resist ion-implanted at a high concentration of 1×10 15  atoms/cm 2  or more is taken as a cleaning object, and the above-mentioned sulfuric acid solution is brought into contact with the electronic material substrate  100  while rotating it on the rotary table  42 , whereby the resist is efficiently stripped and removed. 
     The sulfuric acid solution used for cleaning is recovered by a sulfuric acid solution recovery part  43 , then discharged out of the cleaning apparatus  40 , and sent to and stored in the decomposition tank  50  through the upstream reflux line  45  by the first reflux pump  44 . The sulfuric acid solution includes a residual organic substance such as the resist cleaned by the cleaning apparatus  40 , and the residual organic substance is oxidation-decomposed by the oxidizing substance included in the sulfuric acid solution while it is stored in the decomposition tank  50 . The retention time of the sulfuric acid solution in the decomposition tank  50  can be optionally adjusted depending on the content of the residual organic substance or the like. The decomposition tank  50  is configured so as to be capable of retaining heat, whereby the oxidation decomposition using the remaining heat of the sulfuric acid solution can be secured. Further, a heating device can be provided in the decomposition tank  50  as needed. 
     The sulfuric acid drainage in which the residual organic substance contained therein is oxidation-decomposed in the decomposition tank  50  is refluxed to the first storage tank  20  by the second reflux pump  51  through a cooler  53  interposed in the reflux line  52 . A filter may be interposed on the downstream side of the decomposition tank  50  on the upstream side of the cooler  53 . According to this, SS in the sulfuric acid solution which cannot be treated in the decomposition tank  50  is trapped and removed by the filter. 
     Since reflux of a high-temperature sulfuric acid solution to the first storage tank  20  promotes the decomposition of the peroxosulfuric acid in the sulfuric acid solution stored in the first storage tank  20 , the sulfuric acid solution is introduced to the first storage tank  20  after it is cooled to a proper temperature by the cooler  53 . The sulfuric acid drainage introduced into the first storage tank  20  is fed to the electrolytic device  1  as a sulfuric acid solution through the circulation line  11  to produce peroxosulfuric acid by electrolysis, and then circulated again to the first storage tank  20  through the circulation line  11 . 
     A high temperature cleaning liquid containing peroxosulfuric acid at high concentration can be continuously supplied to the cleaning apparatus  40  that is the using side by the operation of this system. 
     A discharge line, which has not been described in the above, may be branched and connected to the upstream reflux line  45  on the upstream side of the decomposition tank  50  so that the sulfuric acid solution can be properly discharged to the outside without being sent to the decomposition tank  50 . 
     The discharge line allows a control such that when the resist stripping quantity in the sulfuric acid solution used for cleaning is remarkably large just after starting of cleaning or the like, the sulfuric acid solution is discharged out of the system to reduce the load of the decomposition tank  50 , and the above-mentioned sulfuric acid drainage is sent to the decomposition tank  50  in a stage where the resist stripping quantity is reduced. This control can be performed by opening/closing control of an on-off valve provided in the reflux line or the discharge line. 
     The present invention h as been described based on the above-mentioned embodiment. However, the scope of present invention is never limited to the content of the above-mentioned embodiment, and can be properly changed without departing from the gist of the present invention. 
     DESCRIPTION OF THE REFERENCE NUMERAL 
     
         
           1  Electrolytic device 
           2  DC power source 
           10  Gas-liquid separator 
           11  Circulation line 
           13  Cooler 
           20  First storage tank 
           21  First supply pump 
           22  Upstream supply line 
           30  Preheating tank 
           31  Preheating heater 
           32  Downstream supply line 
           33  Second supply pump 
           34  Rapid heating heater 
           40  Cleaning apparatus 
           50  Decomposition tank 
         A Electrolytic unit 
         B Rapid heating unit 
         C Cleaning unit