Patent Publication Number: US-9415356-B1

Title: Chemical-liquid mixing method and chemical-liquid mixing apparatus

Description:
FIELD OF THE INVENTION 
     The present invention relates to a chemical-liquid mixing method for mixing a sulfuric acid and a hydrogen peroxide solution, and a chemical-liquid mixing apparatus therefor. In particular, it pertains to a chemical-liquid mixing method and chemical-liquid mixing apparatus capable of generating an enough peroxomonosulfuric acid that is effective in removing a resist from a substrate, when a sulfuric acid and a hydrogen peroxide solution are mixed with each other. 
     BACKGROUND ART 
     There has been conventionally known a method of cleaning substrates such as semiconductor wafers (also referred to as “wafer” below) with the use of a mixture liquid of a sulfuric acid and a hydrogen peroxide solution as a cleaning liquid (see, JP5-166780A, for example). To be specific, a wafer is cleaned by fully decomposing a resist adhered to the wafer, by means of a peroxomonosulfuric acid (H 2 SO 5 ) which has been generated by mixing a sulfuric acid and a hydrogen peroxide solution (this theory will be described below). 
     With reference to  FIG. 6 , a chemical-liquid mixing apparatus for producing a mixture liquid of a sulfuric acid and a hydrogen peroxide solution is described.  FIG. 6  is a schematic structural view showing a structure of a general chemical-liquid mixing apparatus. 
     As shown in  FIG. 6 , the general chemical-liquid mixing apparatus includes an inner tank  10  in which a wafer is accommodated to be cleaned, and an outer tank  12  disposed around a circumference of the inner tank  10 , into which a liquid overflowing from the inner tank  10  flows. There is disposed a return pipe  14  for returning a liquid in the outer tank  12  into the inner tank  10 . In the return pipe  14 , there are serially arranged a return pump  16  for sending a liquid in the outer tank  12  to the inner tank  10 , a dumper  18  for reducing vibrations or the like of the return pipe  14 , a heater  20  for heating a liquid passing through the return pipe  14 , and a filter  22  for filtering a liquid passing through the return pipe  14 . The chemical-liquid mixing apparatus also includes a sulfuric-acid storage tank  24  for storing a sulfuric acid (H 2 SO 4 ), and a hydrogen-peroxide-solution storage tank  30  for storing a hydrogen peroxide (H 2 O 2 ) solution. The sulfuric acid and the hydrogen peroxide solution stored in the respective storage tanks  24  and  30  are supplied into the inner tank  10  by a sulfuric-acid supply pipe  28  and a hydrogen-peroxide-solution supply pipe  34 , respectively. Supply operations of the respective chemical liquids can be adjusted by a sulfuric-acid supply valve  26  and a hydrogen-peroxide-solution supply valve  32 , respectively. 
     Next, a method of producing a mixture liquid by such a chemical-liquid mixing apparatus will be described below. The inner tank  10  and the outer tank  12  are empty in their initial states. 
     At first, the sulfuric-acid supply valve  26  and the hydrogen-peroxide-solution supply valve  32  are opened, while the return pump  16  and the heater  20  are left OFF, so as to simultaneously supply a sulfuric acid and a hydrogen peroxide solution from the sulfuric-acid storage tank  24  and the hydrogen-peroxide-solution storage tank  30  into the inner tank  10 . For example, a supply ratio between the sulfuric acid and the hydrogen peroxide solution is 5:1. Namely, a supply rate of the sulfuric acid is, e.g., 25 liters/min, and a supply rate of the hydrogen peroxide solution is, e.g., 5 liters/min. The reason why the supply rate of the sulfuric acid is considerably larger than that of the hydrogen peroxide solution is described below. The supply of the sulfuric acid and the hydrogen peroxide solution is continued until the inner tank  10  becomes full and the liquid overflows to the outer tank  12 . 
     By supplying the sulfuric acid and the hydrogen peroxide solution into the inner tank  10 , the sulfuric acid and the hydrogen peroxide solution are mixed with each other. 
     Mixture of the sulfuric acid and the hydrogen peroxide solution is classified into the following two patterns. 
     The first pattern is the following chemical reaction.
 
H 2 SO 4 +H 2 O 2 →H 2 SO 4 +H 2 O+O*  Formula (1)
 
     The reaction represented by Formula (1) generates an active oxygen (O*). This active oxygen is a strong oxidizer. 
     The second pattern is the following chemical reaction.
 
H 2 SO 4 +H 2 O 2 →H 2 SO 5 +H 2 O  Formula (2)
 
     The reaction represented by Formula (2) generates a peroxomonosulfuric acid (H 2 SO 5 ). Similar to the active oxygen, the peroxomonosulfuric acid is also a strong oxidizer. However, the peroxomonosulfuric acid is more effective than the active oxygen in decomposing an organic matter such as a resist adhering to a wafer. That is to say, by mixing a sulfuric acid and a hydrogen peroxide solution to generate a peroxomonosulfuric acid, it is possible to fully remove a resist adhering to a wafer therefrom. 
       FIG. 7  is a graph in which a horizontal axis shows a ratio (molar ratio) of a sulfuric acid relative to a hydrogen peroxide solution, and a vertical axis shows a generation ratio of peroxomonosulfuric acid. As shown in  FIG. 7 , when the ratio (molar ratio) of a sulfuric acid relative to a hydrogen peroxide solution is raised, the generation ratio of peroxomonosulfuric acid is correspondingly increased. Thus, a resist adhering to a wafer can be more sufficiently removed. For this reason, a ratio between supply rates of a sulfuric acid and a hydrogen peroxide solution to be supplied into the inner tank  10  is set at, for example, 5:1. 
     After the supply of the sulfuric acid and the hydrogen peroxide solution into the inner tank  10  is finished, the return pump  16  is activated to return the liquid in the outer tank  12  into the inner tank  10  through the return pipe  14 . Again, the liquid overflows from the inner tank  10  to the outer tank  12 . In this manner, the liquid is circulated through the combination unit of the inner tank  10  and the outer tank  12 . At the same time, the heater  20  is activated to heat the liquid passing through the return pipe  14 . Thus, a temperature of the liquid contained in the inner tank  10  is heated to a temperature suitable for cleaning a wafer (e.g., 100° C. to 150° C.). 
     After the return pipe  16  and the heater  20  are kept in ON state until a temperature of the liquid in the inner tank  10  reaches a certain temperature, the return pipe  16  and the heater  20  are again switched off. Thereafter, a plurality of wafers are all together immersed into the inner tank  10 . Thus, a resist adhering to each wafer is decomposed by the mixture liquid of the sulfuric acid and the hydrogen peroxide solution, more specifically, a peroxomonosulfuric acid generated by mixing the sulfuric acid and the hydrogen peroxide solution, to thereby remove the resist from the wafer. In this manner, a series of steps for cleaning the wafer is completed. 
     DISCLOSURE OF THE INVENTION 
     However, there is case in which a sufficient supply amount of a sulfuric acid into the inner tank  10  cannot be practically ensured. That is to say, the chemical-liquid mixing apparatus as shown in  FIG. 6  is generally installed in a wafer manufacturing factory where a sulfuric acid is directly supplied into the inner tank  10  of the chemical-liquid mixing apparatus from a sulfuric-acid storage tank for collectively storing a sulfuric acid to be used in the factory or a sulfuric acid generator for collectively generating a sulfuric acid to be used in the factory. Since this structure elongates a supply line for a sulfuric acid, there is a possibility that a supply of a sulfuric acid cannot be smoothly carried out because of a high specific gravity and density thereof. Specifically, when the chemical-liquid mixing apparatus is installed in a wafer manufacturing factory, and a sulfuric acid is supplied from a sulfuric-acid storage tank in the factory into the inner tank  10  of the chemical-liquid mixing apparatus, it may occur that a supply rate of a hydrogen peroxide solution is 5 liters/min, while a supply rate of a sulfuric acid is only 1 to 2 liters/min. 
     Under this condition, since a ratio of the sulfuric acid relative to the hydrogen peroxide solution is quite low, a generation ratio of peroxomonosulfuric acid, which is to be generated when a sulfuric acid and a hydrogen peroxide solution are mixed with each other, becomes undesirably low, as shown in  FIG. 7 . In this case, the chemical reaction represented by Formula (1) takes place more actively than the chemical reaction represented by Formula (2), and a sufficient amount of peroxomonosulfuric acid cannot be generated. Thus, a resist cannot be fully removed from a wafer. 
     Another example of a chemical-liquid mixing method is shown in  FIG. 8 .  FIG. 8( a )  to  FIG. 8( h )  sequentially show steps of another general chemical-liquid mixing method. 
     The chemical-liquid mixing apparatus used in the chemical liquid mixing steps shown in  FIG. 8  is identical to the chemical-liquid mixing apparatus shown in  FIG. 6 , excluding that a hydrogen peroxide solution is not supplied into an inner tank but into an outer tank. 
     As shown in  FIG. 8( a ) , the inner tank and the outer tank are empty in their initial states. A return pump and a heater are OFF. 
     At first, as shown in  FIG. 8( b ) , a sulfuric-acid supply valve is opened, and a sulfuric acid is supplied into the inner tank so as to fill the inner tank. Then, as shown in  FIG. 8( c ) , the sulfuric-acid supply valve is closed, and a hydrogen-peroxide-solution supply valve is opened to supply a hydrogen peroxide solution into the outer tank. Thereafter, as shown in  FIG. 8( d ) , the hydrogen-peroxide-solution supply valve is closed, and the sulfuric-acid supply valve is again opened to further supply a sulfuric acid into the inner tank. Since the inner tank has been already filled up with the sulfuric acid, the sulfuric acid overflows to the outer tank so that the sulfuric acid and the hydrogen peroxide solution are mixed with each other in the outer tank. 
     Subsequently, as shown in  FIG. 8( e ) , the sulfuric-acid supply valve is again closed, and the return pump is switched on. Thus, the liquid in the outer tank is returned to the inner tank, and the liquid in the inner tank overflows to the outer tank. Namely, the liquid is circulated through the combination unit of the inner tank and the outer tank. After a passage of a certain period of time, as shown in  FIG. 8( f ) , the return pump is temporarily made OFF, and the sulfuric-acid supply valve is opened to supply a certain amount of a sulfuric acid into the inner tank. Then, as shown in  FIG. 8( g ) , the sulfuric-acid supply valve is again closed, and simultaneously therewith the return pump is switched on so as to circulate the liquid again. Finally, after a previously set time period has passed, as shown in  FIG. 8( h ) , the heater is switched on to heat the liquid passing through the return pipe. Thus, a temperature of the liquid in the inner tank is raised to a temperature suitable for cleaning wafers (e.g., 100° C. to 150° C.). 
     However, even in the chemical-liquid mixing method shown in  FIG. 8 , a ratio of the sulfuric acid relative to the hydrogen peroxide solution is low in the outer tank, a peroxomonosulfuric acid cannot be sufficiently generated. In other words, there still occurs the problem in that a resist cannot be fully removed from a wafer. 
     The present invention has been made in view of the above. The object of the present invention is to provide a chemical-liquid mixing method and a chemical-liquid mixing apparatus capable of generating an enough peroxomonosulfuric acid which is effective in removing a resist from a substrate, when a sulfuric acid and a hydrogen peroxide solution are mixed with each other. 
     The chemical-liquid mixing method according to the present invention is a chemical-liquid mixing method for mixing a sulfuric acid and a hydrogen peroxide solution, the method comprising the steps of: preparing respectively an inner tank; an outer tank disposed around the inner tank, into which outer tank a liquid overflowing from the inner tank flows; a return pipe configured to return the liquid in the outer tank to the inner tank; and a return pump provided on the return pipe for sending the liquid in the outer tank to the inner tank; supplying a sulfuric acid into the inner tank for filling the inner tank with the sulfuric acid, and allowing the sulfuric acid overflowing from the inner tank to flow into the outer tank; supplying a hydrogen peroxide solution into the inner tank or the outer tank, after the sulfuric-acid supplying step, and allowing the hydrogen peroxide solution to flow into the outer tank to store in the outer tank the two kinds of liquids of the hydrogen peroxide solution and the sulfuric acid; and activating the return pump simultaneously with the start of the hydrogen-peroxide-solution supplying step to simultaneously send to the inner tank the two kinds of liquids of the hydrogen peroxide solution and the sulfuric acid in the outer tank, while mixing the hydrogen peroxide solution and the sulfuric acid with each other. 
     In addition, the chemical-liquid mixing apparatus according to the present invention is a chemical-liquid mixing apparatus for mixing a sulfuric acid and a hydrogen peroxide solution, the apparatus comprising: an inner tank; an outer tank disposed around the inner tank, into which outer tank a liquid overflowing from the inner tank flows; a return pipe configured to return the liquid in the outer tank to the inner tank; and a return pump provided on the return pipe for sending the liquid in the outer tank to the inner tank; a sulfuric-acid supply unit configured to supply a sulfuric acid into the inner tank; a hydrogen-peroxide-solution supply unit configured to supply a hydrogen peroxide solution to the inner tank or the outer tank; and a controller configured to control the sulfuric-acid supply unit, the hydrogen-peroxide-solution supply unit, and the return pump, the controller controlling at first the sulfuric-acid supply unit to fill the inner tank with a sulfuric acid, allowing the sulfuric acid overflowing from the inner tank to flow into the outer tank, then the controller controlling the hydrogen-peroxide-solution supply unit to supply a hydrogen peroxide solution into the inner tank or the outer tank, allowing the hydrogen peroxide solution to flow into the outer tank whereby the two kinds of liquids of the hydrogen peroxide solution and the sulfuric acid are stored in the outer tank, and the controller activating the return pump simultaneously with the start of the activation of the hydrogen-peroxide-solution supply unit. 
     According to the chemical-liquid mixing method and the chemical-liquid mixing apparatus, since the hydrogen peroxide solution is added to the sulfuric acid which has been already stored in the outer tank, a ratio of the sulfuric acid relative to the hydrogen peroxide solution is increased in the outer tank. Therefore, a generation ratio of a peroxomonosulfuric acid can be elevated. Further, since the return pump is activated simultaneously when the hydrogen peroxide solution flows into the outer tank, there is performed a so-called stirring of the sulfuric acid and the hydrogen peroxide solution in the return pipe. During the stirring operation, since an amount of the hydrogen peroxide solution is relatively small, generation of a peroxomonosulfuric acid is further promoted in the return pipe. Therefore, a ratio of the amount of peroxomonosulfuric acid contained in a finally produced mixture liquid of the sulfuric acid and the hydrogen peroxide solution is increased. As a result, a mixture liquid that is effective in removing a resist from a substrate can be provided. 
     In the chemical-liquid mixing method according to the present invention, it is preferable that a heater is provided on the return pipe, and that a heater activating step for activating the heater is performed after a preset period of time has passed from the start of the return-pump activating step. 
     In the chemical-liquid mixing apparatus according to the present invention, it is preferable that a heater is provided on the return pipe, and that the controller controls the heater such that the heater is activated after a preset period of time has passed from the activation of the return pump. 
     In generating a peroxomonosulfuric acid by mixing a sulfuric acid and a hydrogen peroxide solution with each other, if temperatures of the liquids which have not been mixed yet are excessively high, a peroxomonosulfuric acid cannot be sufficiently generated. However, since there is an enough period of time in which the return pump is operated while the heater is not operated, in other words, since there is a period of time in which only a circulation of the liquids is performed, an enough amount of peroxomonosulfuric acid can be generated. 
     Preferably, the chemical-liquid mixing method according to the present invention further comprises a hydrogen-peroxide-solution replenishing step for replenishing a hydrogen peroxide solution by a hydrogen-peroxide-solution replenishing pipe that is communicated with the return pipe. 
     In the chemical-liquid mixing apparatus according to the present invention, it is preferable that a hydrogen-peroxide-solution replenishing pipe for replenishing a hydrogen peroxide solution is arranged in communication with the return pipe. 
     With this structure, when a hydrogen peroxide solution is replenished, the hydrogen peroxide solution is directly sent to the return pipe. Thus, a so-called stirring of the replenished hydrogen peroxide solution and the sulfuric acid is performed in the return pipe. During the stirring operation, since an amount of the hydrogen peroxide solution is relatively small, an enough peroxomonosulfuric acid can be generated in the return pipe. Therefore, a ratio of the amount of a peroxomonosulfuric acid generated by the replenishment of the hydrogen peroxide solution is increased. As a result, a resist removing efficiency of the mixture liquid can be maintained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic structural view showing a structure of a chemical-liquid mixing apparatus in one embodiment of the present invention; 
         FIG. 2  is a top view showing an inner tank and an outer tank in the chemical-liquid mixing apparatus shown in  FIG. 1 ; 
         FIG. 3  shows views (a) to (f) illustrating sequential steps of a chemical-liquid mixing method carried out by the chemical-liquid mixing apparatus shown in  FIG. 1 ; 
         FIG. 4  is a top view showing an inner tank and an outer tank in another embodiment of the chemical-liquid mixing apparatus shown in  FIG. 1 ; 
         FIG. 5  is a structural view showing still another embodiment of the chemical-liquid mixing apparatus shown in  FIG. 1 ; 
         FIG. 6  is a schematic structural view showing a structure of a conventional chemical-liquid mixing apparatus; 
         FIG. 7  is a graph in which a horizontal axis shows a ratio (molar ratio) of a sulfuric acid relative to a hydrogen peroxide solution, and a vertical axis shows a generation ratio of a peroxomonosulfuric acid; 
         FIG. 8  shows views (a) to (h) illustrating sequential steps of a conventional chemical-liquid mixing method; and 
         FIG. 9  shows a view (a) illustrating a condition of residue of a resist on a cleaned wafer of Present Example, and a view (b) illustrating a condition of residues of a resist on a cleaned wafer of Comparative Example. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention will be described below with reference to the accompanying drawings.  FIGS. 1 to 3  are views showing a chemical-liquid mixing apparatus in one embodiment according to the present invention.  FIG. 1  is a schematic structural view showing a structure of the chemical-liquid mixing apparatus in this embodiment.  FIG. 2  is a top view showing an inner tank and an outer tank in the chemical-liquid mixing apparatus shown in  FIG. 1 .  FIG. 3  shows views (a) to (f) illustrating sequential steps of a chemical-liquid mixing method carried out by the chemical-liquid mixing apparatus shown in  FIG. 1 . 
     In  FIGS. 1 to 3 , the same members as those in the conventional chemical-liquid mixing apparatus shown in  FIG. 6  are depicted by the same reference numbers as those in  FIG. 6 . 
     As shown in  FIG. 1 , the chemical-liquid mixing apparatus in this embodiment includes an inner tank  10  in which a wafer W is accommodated, and an outer tank  12  disposed around the inner tank  10 , into which a liquid overflowing from the inner tank  10  flows. There is disposed a return pipe  14  for returning the liquid in the outer tank  12  into the inner tank  10 . In the return pipe  14 , there are serially arranged a return pump  16  for sending a liquid in the outer tank  12  to the inner tank  10 , a dumper  18  for reducing vibrations or the like of the return pipe  14 , a heater  20  for heating a liquid passing through the return pipe  14 , and a filter  22  for filtering a liquid passing through the return pipe  14 . The chemical-liquid mixing apparatus also includes a sulfuric-acid storage tank  24  for storing a sulfuric acid (H 2 SO 4 ), and a hydrogen-peroxide-solution storage tank  30  for storing a hydrogen peroxide (H 2 O 2 ) solution. The sulfuric acid and the hydrogen peroxide solution stored in the respective storage tanks  24  and  30  are supplied into the inner tank  10  by a sulfuric-acid supply pipe  28  and a hydrogen-peroxide-solution supply pipe  34 , respectively. Supply operations of the respective chemical liquids are adjusted by a sulfuric-acid supply valve  26  and a hydrogen-peroxide-solution supply valve  32 , respectively. Further, the chemical-liquid mixing apparatus includes a controller  40  that controls the return pump  16 , the heater  20 , the sulfuric-acid supply valve  26 , and the hydrogen-peroxide-solution supply valve  32 . 
     Herebelow, the respective components of the chemical-liquid mixing apparatus are described in detail with reference to  FIGS. 1 and 2 . 
     As shown in  FIG. 2 , the inner tank  10  is of substantially a rectangular solid shape. The outer tank  12  is also of substantially a rectangular solid shape which is larger than the inner tank  10 . The outer tank  12  is disposed to surround the circumference of the inner tank  10 . As described above, a liquid overflowing from the inner tank  10  is sent to the outer tank  12 . The return pipe  14  is diverged into two parts whose ends are connected to a bottom of the inner tank  10 . Thus, a liquid sent from the return pipe  14  can enter the inner tank  10  through the bottom thereof. As shown in  FIG. 2 , the inner tank  10  is provided with a pair of wafer holders  10   a  for holding a plurality of wafers W that are arranged in order. 
     As shown in  FIGS. 1 and 2 , an intake  14   a  of the return pipe  14  is connected to a bottom of the outer tank  12 . A sulfuric-acid supply port  28   a  of the sulfuric-acid supply pipe  28  is positioned above the inner tank  10  near a side periphery thereof. A hydrogen-peroxide-solution supply port  34   a  of the hydrogen-peroxide-solution supply pipe  34  is positioned above the inner tank  10  near a corner thereof. 
     As shown in  FIG. 1 , the sulfuric-acid storage tank  24 , the sulfuric-acid supply valve  26 , and the sulfuric-acid supply pipe  28  constitute a sulfuric-acid supply unit for supplying a sulfuric acid into the inner tank  10 . On the other hand, the hydrogen-peroxide-solution storage tank  30 , the hydrogen-peroxide-solution supply valve  32 , and the hydrogen-peroxide-solution supply pipe  34  constitute a hydrogen-peroxide-solution supply unit for supplying a hydrogen peroxide solution into the inner tank  10 . 
     As shown in  FIG. 1 , the controller  40  is connected to and communicated with the return pump  16 , the heater  20 , the sulfuric-acid supply valve  26 , and the hydrogen-peroxide-solution supply valve  32 . The controller  40  controls each of these members by sending control signals thereto. Specifically, the controller  40  opens at first the sulfuric-acid supply valve  26  to fill the inner tank  10  with a sulfuric acid, so that the overflowing sulfuric acid from the inner tank  10  flows into the outer tank  12 . Then, the controller  40  opens the hydrogen-peroxide-solution supply valve  32  to supply a hydrogen peroxide solution into the inner tank  10 , so that the hydrogen peroxide solution flows into the outer tank  12 , whereby the two kinds of liquids, i.e., the hydrogen peroxide solution and the sulfuric acid, are stored in the outer tank  12 . Then, the controller  40  opens the hydrogen-peroxide-solution supply valve  32 , and simultaneously activates the return pump  16 . In addition, the controller  40  activates the heater  20  after an elapse of a preset period (e.g., 15 minutes) from the activation of the return pump  16 . Details of the control operations by the controller  40  are described hereafter. 
     Next, operations of the chemical-liquid mixing apparatus in this embodiment are described. Specifically, with reference to  FIG. 3 , there is described a method of producing a mixture liquid by using the above chemical-liquid mixing apparatus shown in  FIG. 1  and so on. 
     As shown in  FIG. 3( a ) , the inner tank  10  and the outer tank  12  are empty in their initial states. At this time, the return pump  16  and the heater are in OFF state. 
     At first, as shown in  FIG. 3( b ) , while the return pump  16  and the heater  20  are kept OFF, the sulfuric-acid supply valve  26  is opened by a control command from the controller  40  so as to supply a sulfuric acid into the inner tank  10 . The supply of the sulfuric acid is continued until the inner tank  10  is filled up with the sulfuric acid and the sulfuric acid overflows into the outer tank  12 . 
     Then, as shown in  FIG. 3( c ) , in accordance with a control command from the controller  40 , the sulfuric-acid supply valve  26  is closed, and the hydrogen-peroxide-solution supply valve  32  is opened so as to supply a hydrogen peroxide solution into the inner tank  10 . Since the hydrogen-peroxide-solution supply port  34   a  of the hydrogen-peroxide-solution supply pipe  34  is positioned near the corner of the inner tank  10 , a hydrogen peroxide solution falling into the inner tank  10  by gravity from the hydrogen-peroxide-solution supply port  34   a  immediately overflows into the outer tank  12 . In this manner, the hydrogen peroxide solution flows into the outer tank  12 , and thus the two kinds of liquids of the hydrogen peroxide solution and the sulfuric acid are stored in the outer tank  12 . 
     As shown in  FIG. 3( d ) , the hydrogen-peroxide-solution supply valve  32  is closed by a control command from the controller  40 . Simultaneously, the controller  40  activates the return pump  16 . Thus, while the hydrogen peroxide solution is being continuously supplied into the outer tank  12 , the two kinds of liquids of the hydrogen peroxide solution and the sulfuric acid are withdrawn from the outer tank  12  by the return pump  16  through the return pipe  14 . In this manner, the liquid is circulated through the combination unit of the inner tank  10  and the outer tank  12 . At this time, there is performed a so-called stirring of the two kinds of liquids of the hydrogen peroxide solution and the sulfuric acid, in the return pipe  14  by the return pump  16 , whereby the two kinds of liquids are mixed with each other. 
     Subsequently, as shown in  FIG. 3( e ) , when the mixture liquid reaches a certain level in the outer tank  12 , the hydrogen-peroxide-solution supply valve  32  is closed by a control command from the controller  40  to stop the supply of the hydrogen peroxide solution. However, the operation of the return pump  16  is continued so that the liquid is continuously circulated through the combination unit of the inner tank  10  and the outer tank  12 . Due to the circulation of the liquid, the mixture operation of the hydrogen peroxide solution and the sulfuric acid is consecutively carried out. 
     Finally, as shown in  FIG. 3( f ) , after an elapse of a preset period (e.g., 15 minutes) from the activation of the return pump  16 , the controller  40  activates the heater  20 . Thus, the liquid passing through the return pipe  14  is heated, and a temperature of the liquid in the inner tank  10  is raised at a temperature suitable for cleaning wafers W, specifically, 100° C. to 150° C., for example. 
     At last, when the temperature of the liquid in the inner tank  10  reaches a certain temperature, the controller  40  stops the return pump  16  and the heater  20 . Following thereto, a plurality of wafers W are all together immersed into the inner tank  10 , to decompose a resist adhering to each wafer by the mixture liquid of the sulfuric acid and the hydrogen peroxide solution, more specifically a peroxomonosulfuric acid generated by mixing the sulfuric acid and the hydrogen peroxide solution, to thereby remove the resist from the wafer. In this manner, a series of steps for cleaning the wafers is completed. 
     According to the chemical-liquid mixing method and the chemical-liquid mixing apparatus in this embodiment, the inner tank  10  is firstly filled up with a sulfuric acid, and the sulfuric acid overflowing from the inner tank  10  flows into the outer tank  12 . Then, by supplying a hydrogen peroxide solution into the inner tank  10 , the hydrogen peroxide solution flows into the outer tank  12 , whereby the two kinds of liquids of the hydrogen peroxide solution and the sulfuric acid are stored in the outer tank  12 . The return pump  16  is activated simultaneously when the hydrogen peroxide solution flows into the outer tank  12 . Since the hydrogen peroxide solution is added to the sulfuric acid which has been already stored in the outer tank  12 , a ratio of the sulfuric acid relative to the hydrogen peroxide solution is increased in the outer tank  12 . Therefore, a generation ratio of a peroxomonosulfuric acid can be elevated. Further, since the return pump  16  is activated simultaneously when the hydrogen peroxide solution flows into the outer tank  12 , there is performed a so-called stirring of the sulfuric acid and the hydrogen peroxide solution in the return pipe  14 . During the stirring operation, since an amount of the hydrogen peroxide solution is relatively small, generation of a peroxomonosulfuric acid is further promoted in the return pipe  14 . Therefore, a ratio of the amount of peroxomonosulfuric acid contained in a finally produced mixture liquid of the sulfuric acid and the hydrogen peroxide solution is increased. As a result, a mixture liquid that is effective in removing a resist from a substrate can be provided. 
     In addition, the return pipe  14  is provided with the heater  20  which is activated after a preset period has elapsed from the activation of the return pump  16 . In generating a peroxomonosulfuric acid by mixing a sulfuric acid and a hydrogen peroxide solution with each other, if temperatures of the liquids which have not been mixed yet are excessively high, a peroxomonosulfuric acid cannot be sufficiently generated. However, since there is an enough period of time in which the return pump  16  is operated while the heater  20  is not operated, in other words, since there is a period of time in which only a circulation of the liquids is performed, an enough amount of peroxomonosulfuric acid can be generated. 
     The chemical-liquid mixing method and the chemical-liquid mixing apparatus are not limited to the above embodiment, and various changes and modifications are possible. For example, in place of using the sulfuric-acid supply valve  26  and the hydrogen-peroxide-solution supply valve  32  shown in  FIG. 1  as the sulfuric-acid supply unit and the hydrogen-peroxide-solution supply unit, supply pumps may be respectively provided on the sulfuric-acid supply pipe  28  and the hydrogen-peroxide-solution supply pipe  34  so as to supply a sulfuric acid and a hydrogen peroxide solution by these supply pumps from the sulfuric-acid storage tank  24  and the hydrogen-peroxide-solution storage tank  30 , respectively. In this case, the controller  40  controls these supply pumps, in place of controlling the sulfuric-acid supply valve  26  and the hydrogen-peroxide-solution supply valve  32 . 
     Further, in place of positioning the hydrogen-peroxide-solution supply port  34   a  of the hydrogen-peroxide-solution supply pipe  34  above the inner tank  10 , the hydrogen-peroxide-solution supply port  34   a  may be positioned above the outer tank  12 , which is shown in  FIG. 4 . In this case, a hydrogen peroxide solution can be directly supplied from the hydrogen-peroxide-solution storage tank  30  into the outer tank  12 . 
     An amount of a peroxomonosulfuric acid contained in the mixture liquid is decreased by repeating the wafer cleaning process. In order to cope with this, the present method may further include a hydrogen-peroxide-solution replenishing step for replenishing a hydrogen peroxide solution. A hydrogen peroxide solution is replenished through a hydrogen-peroxide-solution replenishing pipe  36  shown in  FIG. 5 . One end of the hydrogen-peroxide-solution replenishing pipe  36  is communicated with the hydrogen-peroxide-solution storage tank  30 , and the other end thereof passes through the outer tank  12  to extend into an inside of the return pipe  14 . The hydrogen-peroxide-solution replenishing pipe  36  is provided with a hydrogen-peroxide-solution replenishing valve  38  for switching on and off the replenishment of a hydrogen peroxide solution. 
     Since the other end of the hydrogen-peroxide-solution replenishing pipe  36  reaches the inside of the return pipe  14 , when a hydrogen peroxide solution is replenished, the hydrogen peroxide solution transferred from the hydrogen-peroxide-solution storage tank  30  is directly sent to the return pipe  14 . Thus, a so-called stirring of the replenished hydrogen peroxide solution and the sulfuric acid is performed in the return pipe  14 . During the stirring operation, since an amount of the hydrogen peroxide solution is relatively small, an enough peroxomonosulfuric acid can be generated in the return pipe  14 . Therefore, a ratio of the amount of a peroxomonosulfuric acid generated by the replenishment of the hydrogen peroxide solution is increased. As a result, a resist removing efficiency of the mixture liquid can be maintained. 
     EXAMPLE 
     Next, a Present Example of the chemical-liquid mixing method and the chemical-liquid mixing apparatus as shown in  FIGS. 1 to 3  is described below. A Comparative Example of a chemical-liquid mixing method as shown in  FIG. 8  is described for comparison. 
     Present Example 
     A chemical-liquid mixing apparatus as shown in  FIG. 1  was prepared. The chemical-liquid mixing apparatus includes an inner tank  10  in which a wafer W is accommodated, and an outer tank  12  disposed around a circumference of the inner tank  10 , into which a liquid overflowing from the inner tank  10  flows. There is disposed a return pipe  14  for returning the liquid in the outer tank  12  into the inner tank  10 . In the return pipe  14 , there are serially arranged a return pump  16  for returning a liquid in the outer tank  12  to the inner tank  10 , a dumper  18  for reducing vibrations or the like of the return pipe  14 , a heater  20  for heating a liquid passing through the return pipe  14 , and a filter  22  for filtering a liquid passing through the return pipe  14 . 
     The chemical-liquid mixing apparatus in the Present Example also includes a sulfuric-acid storage tank  24  for storing a sulfuric acid (H 2 SO 4 ), and a hydrogen-peroxide-solution storage tank  30  for storing a hydrogen peroxide (H 2 O 2 ) solution. The sulfuric acid and the hydrogen peroxide solution stored in the respective storage tanks  24  and  30  are supplied into the inner tank  10  by a sulfuric-acid supply pipe  28  and a hydrogen-peroxide-solution supply pipe  34 , respectively. A concentration of the sulfuric acid was 98 wt %, and a concentration of the hydrogen peroxide solution was 30 wt %. Supply operations of the respective chemical liquids are adjusted by a sulfuric-acid supply valve  26  and a hydrogen-peroxide-solution supply valve  32 , respectively. A supply rate of the sulfuric acid supplied by the sulfuric-acid supply unit was 4 liters/min, and a supply rate of the hydrogen peroxide solution by the hydrogen-peroxide-solution supply unit was 1 liter/min. 
     The chemical-liquid mixing apparatus in the Present Example further includes a controller  40  for controlling the return pump  16 , the heater  20 , the sulfuric-acid supply valve  26 , and the hydrogen-peroxide-solution supply valve  32 .  FIG. 3  shows control operations of the controller  40 , and details thereof are described below. 
     As shown in  FIG. 3( a ) , the inner tank and the outer tank were empty in their initial states. At first, as shown in  FIG. 3( b ) , while the return pump and the heater were kept OFF, the sulfuric-acid supply valve was opened to supply a sulfuric acid into the inner tank. The supply of the sulfuric acid was continued until the inner tank was filled up with sulfuric acid and the sulfuric acid overflowed into the outer tank. 
     Then, as shown in  FIG. 3( c ) , the sulfuric-acid supply valve was closed, and the hydrogen-peroxide-solution supply valve was opened to supply a hydrogen peroxide solution into the inner tank. At this time, the hydrogen peroxide solution falling from a hydrogen-peroxide-solution supply port by gravity immediately overflowed into the outer tank. In this manner, the hydrogen peroxide solution flowed into the outer tank, and thus the two kinds of liquids of the hydrogen peroxide solution and the sulfuric acid were stored in the outer tank. 
     As shown in  FIG. 3( d ) , the hydrogen-peroxide-solution supply valve was opened, and the return pump was activated at the same time. Thus, while the hydrogen peroxide solution was being continuously supplied into the outer tank, the two kinds of liquids of the hydrogen peroxide solution and the sulfuric acid were withdrawn from the outer tank by the return pump through the return pipe. In this manner, the liquids are circulated through the combination of the inner tank and the outer tank, whereby the hydrogen peroxide solution and the sulfuric acid were mixed with each other. 
     Subsequently, as shown in  FIG. 3( e ) , when an amount of the liquid stored in the outer tank reached about 90 percent of a total storage amount of the outer tank, the hydrogen-peroxide-solution supply valve was closed to stop the supply of the hydrogen peroxide solution. However, the return pump continued its operation so that the liquids were continuously circulated through the combination unit of the inner tank and the outer tank. Due to the circulation of the liquids, the hydrogen peroxide solution and the sulfuric acid were consecutively mixed with each other. 
     Finally, as shown in  FIG. 3( f ) , after a passage of 10 minutes from the activation of the return pump, the heater was activated. Thus, the liquid passing through the return pipe was heated, and a temperature of the liquid in the inner tank was raised at 100° C. in the last place. 
     At last, the return pump and the heater were stopped. Following thereto, a wafer was immersed into the inner tank so as to remove a resist adhering to the wafer therefrom by the mixture liquid of the sulfuric acid and the hydrogen peroxide solution.  FIG. 9( a )  shows a condition of residues of a resist on the wafer which had been immersed in the mixture liquid in the inner tank and was taken up therefrom. 
     Comparative Example 
     Also in the Comparative Example, a chemical-liquid mixing apparatus as shown in  FIG. 1  was prepared. Volumes of an inner tank  10  and an outer tank  10  of the chemical-liquid mixing apparatus were identical to those in the chemical-liquid mixing apparatus of the Present Example. Similarly, structures of a return pipe and a return pump  16 , and structures of sulfuric-acid supply unit and hydrogen-peroxide-solution supply unit were identical to those in the chemical-liquid mixing apparatus of the Present Example. However, control operations conducted by a controller  40  was different from the method of the Present Example as shown in  FIG. 3 . The control operations in the Comparative Example are shown in  FIG. 8 , which are described in detail below. 
     As shown in  FIG. 8( a ) , the inner tank and the outer tank were empty in their initial states, Under this condition, as shown in  FIG. 8( b ) , a sulfuric-acid supply valve was opened to supply a sulfuric acid into the inner tank until the inner tank was filled up with the sulfuric acid. Then, as shown in  FIG. 8( c ) , the sulfuric-acid supply valve was closed, and a hydrogen-peroxide-solution supply valve was opened to supply a hydrogen peroxide solution into the outer tank. After that, as shown in  FIG. 8( d ) , the hydrogen-peroxide-solution supply valve was closed, and the sulfuric-acid supply valve was again opened to supply a sulfuric acid into the inner tank. Since the inner tank had been already filled up with the sulfuric acid, the sulfuric acid overflowed into the outer tank where the sulfuric acid and the hydrogen peroxide solution were mixed with each other. 
     Subsequently, as shown in  FIG. 8( e ) , the sulfuric-acid supply valve was again closed, and the return pump was switched on. Thus, the liquid in the outer tank was returned to the inner tank and the liquid in the inner tank overflowed into the outer tank, whereby the liquid was circulated in the combination unit of the inner tank and the outer tank. After about 10 minutes had passed, as shown in  FIG. 8( f ) , the return pump was temporarily made OFF, and the sulfuric-acid supply valve was opened to supply a sulfuric acid into the inner tank. Thereafter, as shown in  FIG. 8( g ) , the sulfuric-acid supply valve was again closed, and the return pump was switched on so as to again circulate the liquid. Finally, after a passage of about 10 minutes, as shown in  FIG. 8( h ) , the heater was switched on to heat the liquid passing through the return pipe. Thus, a temperature of the liquid in the inner tank was raised at 100° C. 
     At last, the return pump and the heater were stopped. Following thereto, a wafer was immersed into the inner tank so as to remove a resist adhering to the wafer therefrom by the mixture liquid of the sulfuric acid and the hydrogen peroxide solution.  FIG. 9( b )  shows a condition of residues of a resist on the wafer which had been immersed in the mixture liquid in the inner tank and was taken up therefrom. 
     As described above, a peroxomonosulfuric acid is generated by mixing a sulfuric acid and a hydrogen peroxide solution. However, a method of directly measuring an amount of the peroxomonosulfuric acid has not been established yet. Thus, the following method is generally carried out. Namely, a wafer to which a resist adheres is immersed in a mixture liquid of a sulfuric acid and a hydrogen peroxide solution in which a peroxomonosulfuric acid is contained, and a relative amount of the peroxomonosulfuric acid is estimated based on a degree of removal of the resist. 
     As described above,  FIG. 9( a )  and  FIG. 9( b )  show the conditions of the residual resist on the cleaned wafers of the Present Example and the Comparative Example. In the drawings, the black particles depict residues of the resist. When  FIG. 9( a )  and  FIG. 9( b )  are compared to each other, it can be understood that an amount of the resist (an amount of the residual resist) remaining on the wafer of the Present Example is significantly smaller than that of the wafer of the Comparative Example. That is to say, as compared with the chemical-liquid mixing method of the Comparative Example, it can be seen that the chemical-liquid mixing method of the Present Example can remove a larger amount of the resist when the wafer is immersed in the mixture liquid. Therefore, it was found that a peroxomonosulfuric acid that is effective in removing a resist from a substrate can be sufficiently generated with the use of the chemical-liquid mixing method and the chemical-liquid mixing apparatus of the Present Example.