Patent Publication Number: US-2020289994-A1

Title: Mixing apparatus, mixing method and substrate processing system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Japanese Patent Application No. 2019-046145 filed on Mar. 13, 2019, the entire disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The exemplary embodiments described herein pertain generally to a mixing apparatus, a mixing method and a substrate processing system. 
     BACKGROUND 
     Conventionally, there has been known a substrate processing system that performs an etching processing on a substrate by immersing the substrate in an etching solution containing a phosphoric acid aqueous solution and an additive for suppressing the precipitation of a silicon dioxide (SiO 2 ) (see Patent Document 1). 
     Patent Document 1: Japanese Patent Laid-open Publication No. 2017-118092 
     SUMMARY 
     In an exemplary embodiment, a mixing apparatus includes a phosphoric acid aqueous solution supply, an additive supply, a tank, a phosphoric acid aqueous solution supply path and an additive supply path. The phosphoric acid aqueous solution supply is configured to supply a phosphoric acid aqueous solution. The additive supply is configured to supply an additive configured to suppress precipitation of a silicon oxide. The phosphoric acid aqueous solution supply path is configured to connect the phosphoric acid aqueous solution supply with the tank. The additive supply path is configured to connect the additive supply with the tank. The additive is supplied while fluidity is imparted to the phosphoric acid aqueous solution supplied from the phosphoric acid aqueous solution supply into the tank. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, exemplary embodiments, and features described above, further aspects, exemplary embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description that follows, exemplary embodiments are described as illustrations only since various changes and modifications will become apparent to those skilled in the art from the following detailed description. The use of the same reference numbers in different figures indicates similar or identical items. 
         FIG. 1  is a schematic block diagram illustrating a substrate processing system according to an exemplary embodiment; 
         FIG. 2  is a timing chart illustrating an example of operation patterns of respective components of a mixing apparatus in an etching solution production processing according to the exemplary embodiment; 
         FIG. 3  is a schematic block diagram illustrating a configuration of the mixing apparatus according to a first modification example of the exemplary embodiment; 
         FIG. 4  is a timing chart illustrating an example of operation patterns of respective components of the mixing apparatus in an etching solution production processing according to the first modification example of the exemplary embodiment; 
         FIG. 5  is a schematic block diagram illustrating a configuration of the mixing apparatus according to a second modification example of the exemplary embodiment; 
         FIG. 6  is a schematic block diagram illustrating a configuration of the mixing apparatus according to a third modification example of the exemplary embodiment; 
         FIG. 7  is a schematic block diagram illustrating a configuration of the mixing apparatus according to a fourth modification example of the exemplary embodiment; 
         FIG. 8  is a schematic block diagram illustrating a configuration of the mixing apparatus according to a fifth modification example of the exemplary embodiment; 
         FIG. 9  is a schematic block diagram illustrating a configuration of the mixing apparatus according to a sixth modification example of the exemplary embodiment; 
         FIG. 10  is a schematic block diagram illustrating a configuration of the mixing apparatus according to a seventh modification example of the exemplary embodiment; 
         FIG. 11  is a schematic block diagram illustrating a configuration of the mixing apparatus according to an eighth modification example of the exemplary embodiment; 
         FIG. 12  is a schematic block diagram illustrating a configuration of the mixing apparatus according to a ninth modification example of the exemplary embodiment; 
         FIG. 13  is a schematic block diagram illustrating a configuration of the mixing apparatus according to a tenth modification example of the exemplary embodiment; 
         FIG. 14  is a timing chart illustrating an example of operation patterns of respective components of the mixing apparatus in an etching solution production processing according to the tenth modification example of the exemplary embodiment; 
         FIG. 15  is a schematic block diagram illustrating a configuration of the mixing apparatus according to an eleventh modification example of the exemplary embodiment; 
         FIG. 16  is a schematic block diagram illustrating a configuration of the mixing apparatus according to a twelfth modification example of the exemplary embodiment; 
         FIG. 17  is a schematic block diagram illustrating a configuration of the mixing apparatus according to a thirteenth modification example of the exemplary embodiment; 
         FIG. 18  is a timing chart illustrating an example of operation patterns of respective components of the mixing apparatus in an etching solution production processing according to the thirteenth modification example of the exemplary embodiment; 
         FIG. 19  is a schematic block diagram illustrating a configuration of a substrate processing system according to a fourteenth modification example of the exemplary embodiment; and 
         FIG. 20  is a flowchart showing a processing sequence of an etching solution production processing and a substrate processing according to the exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part of the description. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Furthermore, unless otherwise noted, the description of each successive drawing may reference features from one or more of the previous drawings to provide clearer context and a more substantive explanation of the current exemplary embodiment. Still, the exemplary embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings, may be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     Hereinafter, exemplary embodiments of a mixing apparatus, a mixing method and a substrate processing system according to the present disclosure will be described in detail with reference to the accompanying drawings. Further, the present disclosure is not limited to the following exemplary embodiments. Furthermore, it is to be noted that the drawings are illustrative of the invention, and a relationship between the sizes of components and the proportions of the respective components can be different from the real ones. Also, the drawings may be different from each other in a relationship between the sizes of components and the proportions of the respective components. 
     Conventionally, there has been known a substrate processing system that performs an etching processing on a substrate by immersing the substrate in an etching solution containing a phosphoric acid aqueous solution and an additive for suppressing precipitation of a silicon oxide. 
     For example, it is possible to selectively etch, between a silicon nitride film (SiN) and a silicon dioxide film (SiO 2 ) stacked on a substrate, the silicon nitride film by immersing the substrate in a phosphoric acid (H 3 PO 4 ) aqueous solution. 
     Also, it is possible to suppress the precipitation of the silicon oxide on the silicon oxide film during an etching processing by adding the additive (hereinafter, also referred to as “precipitation inhibitor”) for suppressing the precipitation of silicon oxide to the phosphoric acid aqueous solution. 
     However, when the etching solution is produced, if the phosphoric acid aqueous solution and the precipitation inhibitor are not well mixed, etching non-uniformity may occur during the etching processing. Meanwhile, if a lot of time is spent on the mixing processing to mix well the phosphoric acid aqueous solution and the precipitation inhibitor, a sufficient liquid amount required for the etching processing may not be supplied. 
     Accordingly, a technique capable of efficiently mixing the phosphoric acid aqueous solution and the precipitation inhibitor has been expected. 
     &lt;Configuration of Substrate Processing System&gt; 
     First, a configuration of a substrate processing system  1  according to an exemplary embodiment will be described with reference to  FIG. 1 .  FIG. 1  is a schematic block diagram illustrating the configuration of the substrate processing system  1  according to the exemplary embodiment. 
     The substrate processing system  1  includes a mixing apparatus  10  and a substrate processing apparatus  30 . The mixing apparatus  10  is configured to produce an etching solution E by mixing a phosphoric acid aqueous solution L, a precipitation inhibitor for suppressing the precipitation of silicon oxide and a silicon-containing compound aqueous solution (hereinafter, also referred to as “silicon solution”). The precipitation inhibitor is an example of an additive, and the etching solution E is an example of a mixed solution. 
     That is, the etching solution E according to the exemplary embodiment contains the phosphoric acid aqueous solution L, the precipitation inhibitor and the silicon solution. Also, the etching solution E according to the exemplary embodiment does not necessarily contain the silicon solution. 
     The substrate processing apparatus  30  is configured to perform an etching processing on a wafer W by immersing the wafer W in the etching solution E produced by the mixing apparatus  10 . The wafer W is an example of a substrate. In the exemplary embodiment, it is possible to selectively etch, between a silicon nitride film (SiN) and a silicon dioxide film (SiO 2 ) formed on the wafer W, for example, the silicon nitride film. 
     The mixing apparatus  10  includes a phosphoric acid aqueous solution supply  11 , a precipitation inhibitor supply  12 , a silicon solution supply  13 , a tank  14  and a circulation path  15 . The precipitation inhibitor supply  12  is an example of an additive supply. 
     The phosphoric acid aqueous solution supply  11  supplies the phosphoric acid aqueous solution L into the tank  14 . The phosphoric acid aqueous solution supply  11  is equipped with a phosphoric acid aqueous solution source  11   a , a phosphoric acid aqueous solution supply path  11   b  and a flow rate controller  11   c.    
     The phosphoric acid aqueous solution source  11   a  is, for example, a tank that stores the phosphoric acid aqueous solution L. The phosphoric acid aqueous solution supply path  11   b  connects the phosphoric acid aqueous solution source  11   a  and the tank  14 , and supplies the phosphoric acid aqueous solution L from the phosphoric acid aqueous solution source  11   a  into the tank  14 . 
     The flow rate controller  11   c  is provided at the phosphoric acid aqueous solution supply path  11   b  and controls a flow rate of the phosphoric acid aqueous solution L to be supplied into the tank  14 . The flow rate controller  11   c  is composed of an opening/closing valve, a flow rate control valve, a flowmeter and the like. 
     The precipitation inhibitor supply  12  supplies the precipitation inhibitor into the tank  14 . The precipitation inhibitor supply  12  is equipped with a precipitation inhibitor source  12   a , a precipitation inhibitor supply path  12   b  and a flow rate controller  12   c . The precipitation inhibitor supply path  12   b  is an example of an additive supply path. 
     The precipitation inhibitor source  12   a  is, for example, a tank that stores the precipitation inhibitor. The precipitation inhibitor supply path  12   b  connects the precipitation inhibitor source  12   a  and the tank  14  and supplies the precipitation inhibitor from the precipitation inhibitor source  12   a  into the tank  14 . 
     Further, the precipitation inhibitor supply path  12   b  is equipped with a precipitation inhibitor supply opening  12   d  at an outlet thereof. The precipitation inhibitor supply opening  12   d  is an example of an additive supply opening. Furthermore, the precipitation inhibitor is discharged from the precipitation inhibitor supply opening  12   d  onto a liquid surface La of the phosphoric acid aqueous solution L stored in the tank  14 . 
     The flow rate controller  12   c  is provided at the precipitation inhibitor supply path  12   b  and controls a flow rate of the precipitation inhibitor to be supplied into the tank  14 . The flow rate controller  12   c  is composed of an opening/closing valve, a flow rate control valve, a flowmeter and the like. 
     The precipitation inhibitor according to the exemplary embodiment just needs to contain a component for suppressing the precipitation of silicon oxide. For example, the precipitation inhibitor may contain a component configured to suppress the precipitation of silicon oxide by stabilizing silicon ions dissolved in the phosphoric acid aqueous solution L. Also, the precipitation inhibitor may contain a component configured to suppress the precipitation of silicon oxide by other known methods. 
     Examples of the precipitation inhibitor according to the exemplary embodiment may include hexafluorosilicic acid (H 2 SiF 6 ) aqueous solution containing fluorine. Further, the precipitation inhibitor may contain an additive such as ammonia in order to stabilize hexafluorosilicic acid in the aqueous solution. 
     Examples of the precipitation inhibitor according to the exemplary embodiment may include ammonium hexafluorosilicate ((NH 4 ) 2 SiF 6 ) or sodium hexafluorosilicate (Na 2 SiF 6 ). 
     The precipitation inhibitor according to the exemplary embodiment may be a compound containing cations having an ionic radius of from 0.2 Å to 0.9 Å. Herein, the term “ionic radius” refers to the radius of an ion calculated by experience from the sum of the radiuses of anions and cations obtained from a lattice constant of a crystal lattice. 
     The precipitation inhibitor according to the exemplary embodiment may contain an oxide of at least one element of, for example, aluminum, potassium, lithium, sodium, magnesium, calcium, zirconium, tungsten, titanium, molybdenum, hafnium, nickel and chromium. 
     Further, the precipitation inhibitor according to the exemplary embodiment may contain at least one of a nitride, a chloride, a bromide, a hydroxide and a nitrate of any one of the above-described elements instead of or in addition to an oxide of any one of the above-described elements. 
     The precipitation inhibitor according to the exemplary embodiment may contain at least one of, for example, Al(OH) 3 , AlCl 3 , AlBr 3 , Al(NO 3 ) 3 , Al 2 (SO 4 ) 3 , AlPO 4  and Al 2 O 3 . 
     Further, the precipitation inhibitor according to the exemplary embodiment may contain at least one of KCl, KBr, KOH and KNO 3 . Furthermore, the precipitation inhibitor according to the exemplary embodiment may contain at least one of LiCl, NaCl, MgCl 2 , CaCl 2  and ZrCl 4 . 
     The silicon solution supply  13  supplies the silicon solution into the tank  14 . The silicon solution according to the exemplary embodiment is, for example, a solution in which colloidal silicon is dispersed. The silicon solution supply  13  is equipped with a silicon solution source  13   a , a silicon solution supply path  13   b  and a flow rate controller  13   c.    
     The silicon solution source  13   a  is, for example, a tank that stores the silicon solution. The silicon solution supply path  13   b  connects the silicon solution source  13   a  and the tank  14  and supplies the silicon solution from the silicon solution source  13   a  into the tank  14 . 
     The flow rate controller  13   c  is provided at the silicon solution supply path  13   b  and controls a flow rate of the silicon solution to be supplied into the tank  14 . The flow rate controller  13   c  is composed of an opening/closing valve, a flow rate control valve, a flowmeter and the like. 
     The tank  14  stores the phosphoric acid aqueous solution L supplied from the phosphoric acid aqueous solution supply  11 , the precipitation inhibitor supplied from the precipitation inhibitor supply  12  and the silicon solution supplied from the silicon solution supply  13 . Also, the tank  14  stores the etching solution E produced by mixing the phosphoric acid aqueous solution L, the precipitation inhibitor and the silicon solution. 
     The circulation path  15  is a circulation line that comes out of the tank  14  and returns to the tank  14 . The circulation path  15  has an inlet  15   a  provided at a lower portion of the tank  14  and an outlet  15   b  provided at an upper portion of the tank  14  and forms a circulation flow flowing from the inlet  15   a  toward the outlet  15   b . Further, in the exemplary embodiment, the outlet  15   b  is placed above the liquid surface La of the phosphoric acid aqueous solution L stored in the tank  14 . 
     The circulation path  15  is equipped with a pump  16 , a heater  17 , an opening/closing valve  18 , a filter  19  and a branch portion  15   c  that are provided in sequence from an upstream side of the tank  14 . Further, a solution sending path  22  through which the etching solution E is sent to a processing tank  31  of the substrate processing apparatus  30  is branched from the branch portion  15   c.    
     The pump  16  forms a circulation flow of the phosphoric acid aqueous solution L that starts from the tank  14  and returns to the tank  14  through the circulation path  15 . 
     The heater  17  heats the phosphoric acid aqueous solution L circulating in the circulation path  15 . In the exemplary embodiment, by heating the phosphoric acid aqueous solution L, the heater  17  heats the phosphoric acid aqueous solution L stored in the tank  14 . 
     The filter  19  removes contaminants such as particles contained in the etching solution E circulating in the circulation path  15 . Further, the circulation path  15  is equipped with a bypass flow path  20  that bypasses the filter  19 , and the bypass flow path  20  is equipped with an opening/closing valve  21 . 
     By alternately opening and closing the opening/closing valve  18  provided at the circulation path  15  and the opening/closing valve  21  provided at the bypass flow path  20 , it is possible to form any one of a circulation flow flowing through the filter  19  and a circulation flow bypassing the filter  19 . 
     In the exemplary embodiment, to efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor, the precipitation inhibitor is supplied while fluidity is imparted to the phosphoric acid aqueous solution L. For example, in the exemplary embodiment, the pump  16  is operated to form the circulation flow in the circulation path  15 , and, thus, fluidity is imparted to the phosphoric acid aqueous solution L. 
     As such, since the precipitation inhibitor is supplied while fluidity is imparted to the phosphoric acid aqueous solution L, a contact area between the phosphoric acid aqueous solution L and the precipitation inhibitor can be increased. Therefore, according to the exemplary embodiment, it is possible to efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Further, in the exemplary embodiment, the precipitation inhibitor supply opening  12   d  through which the precipitation inhibitor is supplied from the precipitation inhibitor supply path  12   b  into the tank  14  just needs to be provided adjacent to the outlet  15   b  of the circulation path  15 . Thus, the precipitation inhibitor can be directly supplied to the phosphoric acid aqueous solution L that has been discharged from the outlet  15   b  to have high fluidity. 
     Therefore, according to the exemplary embodiment, the contact area between the phosphoric acid aqueous solution L and the precipitation inhibitor can be further increased, and, thus, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Details of an etching solution production processing performed by the mixing apparatus  10  will be described with reference to  FIG. 2 .  FIG. 2  is a timing chart illustrating an example of operation patterns of respective components of the mixing apparatus  10  in the etching solution production processing according to the exemplary embodiment. Also, the components of the mixing apparatus  10  are controlled by a controller (not illustrated) provided in the substrate processing system  1 . 
     The controller controls the operations of the respective components (the mixing apparatus  10 , the substrate processing apparatus  30  and the like) of the substrate processing system  1  illustrated in  FIG. 1 . The controller controls the operations of the respective components of the substrate processing system  1  based on signals from a switch and various sensors. 
     The controller is, for example, a computer and includes a computer-readable recording medium (not illustrated). The recording medium stores therein a program for controlling various processings performed by the substrate processing system  1 . 
     The controller controls the operations of the substrate processing system  1  by reading the program stored in the recording medium and executing the program. The program may be recorded in a computer-readable recording medium and may be installed into the recording medium of the controller from other recording medium. 
     The computer-readable recording medium includes, for example, a hard disk (HD), a flexible disk (FD), a compact disk (CD), a magneto-optical disk (MO), and a memory card. 
     As illustrated in  FIG. 2 , in the etching solution production processing according to the exemplary embodiment, a mixing processing, a heating processing and a filtration processing are sequentially performed. First, the controller starts the mixing processing by operating the phosphoric acid aqueous solution supply  11  (ON state) from a time point T 0  to supply the phosphoric acid aqueous solution L into the tank  14 . 
     At the time point T 0 , the precipitation inhibitor supply  12 , the silicon solution supply  13 , the pump  16  and the heater  17  do not operate (OFF state). Also, at the time point T 0 , the opening/closing valve  18  is closed and the opening/closing valve  21  is opened, and, thus, the filter  19  is in a bypass state (a filter bypass is in an ON state) on the bypass flow path  20 . 
     Then, at a time point T 1  when a predetermined amount of the phosphoric acid aqueous solution L is stored in the tank  14 , the controller operates the pump  16  (ON state) to form the circulation flow in the circulation path  15 . Thus, it is possible to impart fluidity to the phosphoric acid aqueous solution L stored in the tank  14 . 
     Also, by operating the pump  16  after the predetermined amount of the phosphoric acid aqueous solution L is stored in the tank  14 , it is possible to suppress air from being mixed into the circulation path  15  and the occurrence of trouble in the pump  16 . 
     Then, at a time point T 2  when a predetermined time has elapsed from the time point T 1  and fluidity is imparted sufficiently to the phosphoric acid aqueous solution L, the controller operates the precipitation inhibitor supply  12  (ON state) to supply the precipitation inhibitor into the tank  14 . 
     Accordingly, the precipitation inhibitor can be mixed with the phosphoric acid aqueous solution L to which fluidity is imparted sufficiently, and, thus, it is possible to efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Further, in the exemplary embodiment, the precipitation inhibitor just needs to be supplied to be diffused on the liquid surface La of the phosphoric acid aqueous solution L flowing in the tank  14 . That is, in the exemplary embodiment, the precipitation inhibitor just needs to be supplied a little at a time according to the fluidity of the phosphoric acid aqueous solution L. In other words, in the exemplary embodiment, the amount of the precipitation inhibitor to be supplied just needs to be set based on the fluidity of the phosphoric acid aqueous solution L. 
     Thus, it is possible to suppress a concentration of the precipitation inhibitor in the phosphoric acid aqueous solution L from being locally increased. Therefore, according to the exemplary embodiment, it is possible to suppress the gelation of the precipitation inhibitor caused as the concentration of the precipitation inhibitor is locally increased. 
     Then, at a time point T 3  when a predetermined time has elapsed from the time point T 2 , the controller operates the silicon solution supply  13  (ON state) to supply the silicon solution into the tank  14 . Then, at a time point T 4  when predetermined amounts of the precipitation inhibitor and the silicon solution have been supplied into the tank  14 , the controller stops the precipitation inhibitor supply  12  and the silicon solution supply  13  (OFF state). 
     Thereafter, at a time point T 5  when a predetermined amount of the phosphoric acid aqueous solution L has been supplied into the tank  14 , the controller stops the phosphoric acid aqueous solution supply  11  (OFF state). Then, the circulation flow is formed in the circulation path  15  to mix the chemical liquid in the tank  14  until a time point T 6 , and, thus, the mixing processing is completed. 
     Although  FIG. 2  illustrates an example where the silicon solution starts to be supplied later than the precipitation inhibitor, the supply of the precipitation inhibitor and the supply of the silicon solution may start at the same timing (time point T 2 ). 
     Then, the controller starts the heating processing by operating the heater  17  (ON state) from the time point T 6  to heat the etching solution E circulating in the circulation path  15 . The controller heats the etching solution E stored in the tank  14  by heating the etching solution E with the heater  17 . 
     Further, when a liquid amount of the phosphoric acid aqueous solution L or the precipitation inhibitor is weighed with a liquid surface sensor (not illustrated) provided in the tank  14 , a temperature change of the stored phosphoric acid aqueous solution L may have a bad influence on the accuracy in the weighing. 
     Therefore, in the exemplary embodiment, the heating processing starts from a time point (time point T 6 ) when the weighing of each chemical liquid has been completed and the mixing processing has been completed. Thus, the accuracy in the weighing of each chemical liquid can be well maintained. 
     Then, at a time point T 7  when the etching solution E in the tank  14  has been heated to a predetermined temperature (e.g., 165° C.), the heating processing is completed. As such, in the exemplary embodiment, the heater  17  that performs the heating processing is provided in the mixing apparatus  10 , and, thus, the heated etching solution E can be supplied into the substrate processing apparatus  30 . 
     Further, in the exemplary embodiment, the heater  17  is provided at the circulation path  15  of the mixing apparatus  10 , and, thus, the etching solution E can be heated efficiently. 
     Furthermore, in the etching solution production processing according to the exemplary embodiment, the heating processing starts after the mixing processing is completed. This is because, if the precipitation inhibitor containing an organic solvent is supplied to the phosphoric acid aqueous solution L whose temperature is increased by being heated, the precipitation inhibitor may bump. 
     That is, according to the exemplary embodiment, the heating processing starts after the mixing processing is completed, and, thus, it is possible to suppress the bumping of the precipitation inhibitor during the supply of the precipitation inhibitor. 
     Likewise, if the silicon solution containing water is supplied to the phosphoric acid aqueous solution L whose temperature is increased by being heated, the silicon solution may bump. That is, according to the exemplary embodiment, the heating processing starts after the mixing processing is completed, and, thus, it is possible to suppress the bumping of the silicon solution during the supply of the silicon solution. 
     Then, the controller starts the filtration processing by turning the filter bypass in an OFF state from the time point T 7 . That is, the controller changes the opening/closing valve  18  to an open state and the opening/closing valve  21  to a closed state from the time point T 7  to form the circulation flow flowing in the filter  19  in the circulation path  15 . Thus, the contaminants such as particles contained in the etching solution E are removed. 
     Then, at a time point T 8  when the contaminants such as particles contained in the etching solution E have been removed sufficiently, the filtration processing is completed. In this way, the etching solution production processing according to the exemplary embodiment is completed. 
     Further, in the etching solution production processing according to the exemplary embodiment, the filter bypass is in the ON state during the mixing processing and the heating processing. Accordingly, a pressure loss that occurs in the filter  19  can be reduced in the circulation path  15 , and, thus, it is possible to efficiently circulate the phosphoric acid aqueous solution L stored in the tank  14 . 
     Therefore, according to the exemplary embodiment, since the filter bypass is turned in the ON state, it is possible to efficiently impart fluidity to the phosphoric acid aqueous solution L. Also, the filter  19  does not need to filter the phosphoric acid aqueous solution L or the like until the heating processing is completed, and, thus, there is nothing wrong even if the phosphoric acid aqueous solution L is circulated through the bypass flow path  20 . 
     Returning to  FIG. 1 , other components of the substrate processing system  1  will be described. The substrate processing apparatus  30  performs the etching processing on the wafer W by immersing the wafer W in the etching solution E produced by the mixing apparatus  10 . 
     The substrate processing apparatus  30  includes the processing tank  31 , a circulation path  32 , a DIW supply  33  and an etching solution drain unit  34 . The processing tank  31  is equipped with an inner tank  31   a  and an outer tank  31   b.    
     The inner tank  31   a  has an open top, and, thus, the etching solution E is supplied near an upper portion of the inner tank  31   a . In the inner tank  31   a , a plurality of wafers W is immersed in the etching solution E by using a substrate elevating mechanism  35  so that the etching processing is performed on the wafers W. The substrate elevating mechanism  35  is configured to be movable up and down and holds the plurality of wafers W arranged back and forth in a standing posture. 
     The outer tank  31   b  is provided around the upper portion of the inner tank  31   a  and has an open top. The etching solution E overflowing from the inner tank  31   a  is introduced into the outer tank  31   b . Further, the etching solution E from the mixing apparatus  10  through the solution sending path  22  is supplied into the outer tank  31   b  and deionized water (DIW) from the DIW supply  33  is supplied thereinto. 
     Furthermore, a flow rate controller  23  is provided at the solution sending path  22 . The flow rate controller  23  controls a flow rate of the etching solution E to be supplied into the processing tank  31 . The flow rate controller  23  is composed of an opening/closing valve, a flow rate control valve, a flowmeter and the like. 
     The DIW supply  33  is equipped with a DIW source  33   a , a DIW supply path  33   b  and a flow rate controller  33   c . The DIW supply  33  supplies DIW into the outer tank  31   b  to supplement water that has evaporated from the heated etching solution E. 
     The DIW supply path  33   b  connects the DIW source  33   a  and the outer tank  31   b  and supplies DIW having a predetermined temperature from the DIW source  33   a  into the outer tank  31   b.    
     The flow rate controller  33   c  is provided at the DIW supply path  33   b  and controls the amount of DIW to be supplied into the outer tank  31   b . The flow rate controller  33   c  is composed of an opening/closing valve, a flow rate control valve, a flowmeter and the like. Since the amount of DIW to be supplied is controlled by the flow rate controller  33   c , the temperature of the etching solution E, the concentration of phosphoric acid, the concentration of silicon and the concentration of the precipitation inhibitor can be controlled. 
     Further, the outer tank  31   b  is equipped with a temperature sensor  36  and a phosphoric acid concentration sensor  37 . The temperature sensor  36  detects the temperature of the etching solution E, and the phosphoric acid concentration sensor  37  detects the concentration of phosphoric acid in the etching solution E. Signals output by the temperature sensor  36  and the phosphoric acid concentration sensor  37  are input to the above-described controller. 
     The outer tank  31   b  and the inner tank  31   a  are connected by the circulation path  32 . One end of the circulation path  32  is connected to a lower portion of the outer tank  31   b  and the other end of the circulation path  32  is connected to a processing liquid supply nozzle  38  provided inside the inner tank  31   a.    
     The circulation path  32  is equipped with a pump  39 , a heater  40 , a filter  41  and a silicon concentration sensor  42  that are provided in sequence from the outer tank  31   b  side. 
     The pump  39  forms a circulation flow of the etching solution E that is sent from the outer tank  31   b  into the inner tank  31   a  through the circulation path  32 . Further, the etching solution E overflows from the inner tank  31   a  into the outer tank  31   b . As such, the circulation flow of the etching solution E is formed inside the substrate processing apparatus  30 . That is, the circulation flow is formed in the outer tank  31   b , the circulation path  32  and the inner tank  31   a.    
     The heater  40  controls the temperature of the etching solution E circulating in the circulation path  32 . The filter  41  filters the etching solution E circulating in the circulation path  32 . The silicon concentration sensor  42  detects the concentration of silicon in the etching solution E circulating in the circulation path  32 . A signal output by the silicon concentration sensor  42  is input to the controller. 
     When all or some of the etching solution E used in the etching processing is replaced, the etching solution drain unit  34  drains the etching solution E to a drain DR. The etching solution drain unit  34  is equipped with a drain path  34   a , a flow rate controller  34   b  and a cooling tank  34   c.    
     The drain path  34   a  is connected to the circulation path  32 . The flow rate controller  34   b  is provided at the drain path  34   a  and controls the amount of the etching solution E to be drained. The flow rate controller  34   b  is composed of an opening/closing valve, a flow rate control valve, a flowmeter and the like. 
     The cooling tank  34   c  temporarily stores the etching solution E flown through the drain path  34   a  and cools the etching solution E. In the cooling tank  34   c , the amount of the etching solution E to be drained is controlled by the flow rate controller  34   b.    
     Modification Example 
     Hereinafter, various modification examples of the mixing apparatus  10  according to the exemplary embodiment will be described with reference to  FIG. 3  to  FIG. 18 .  FIG. 3  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to a first modification example of the exemplary embodiment. 
     In the following modification examples, the same parts will be assigned same reference numerals, and redundant description thereof will be omitted. Further, in the drawings referred to below, a state in which the phosphoric acid aqueous solution L is stored in the tank  14  will be illustrated for easy understanding of the mixing processing. 
     As illustrated in  FIG. 3 , the mixing apparatus  10  according to the first modification example is different in the configuration of the precipitation inhibitor supply path  12   b  of the precipitation inhibitor supply  12  from the exemplary embodiment. Specifically, the precipitation inhibitor supply path  12   b  is branched into a plurality of flow paths. 
     Further, in the first modification example, the precipitation inhibitor supply opening  12   d  is divided into a plurality of parts in a horizontal direction at the upper portion of the tank  14 . That is, in the first modification example, a plurality of precipitation inhibitor supply openings  12   d  is provided at different locations, respectively, in the horizontal direction. 
     Further, in the present disclosure, the term “upper portion of the tank  14 ” refers to the upper side from the center in a height direction of the tank  14  and the term “lower portion of the tank  14 ” refers to the lower side from the center in the height direction of the tank  14 . 
     In the first modification example, the precipitation inhibitor supply  12  divides the precipitation inhibitor to a plurality of points by using the plurality of precipitation inhibitor supply openings  12   d  and supplies the precipitation inhibitor onto the liquid surface La of the stored phosphoric acid aqueous solution L. Thus, the contact area between the phosphoric acid aqueous solution L and the precipitation inhibitor can be increased. Therefore, according to the first modification example, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Further, in the first modification example, the precipitation inhibitor is divided to the plurality of points and supplied to the phosphoric acid aqueous solution L, and, thus, it is possible to suppress the concentration of the precipitation inhibitor in the phosphoric acid aqueous solution L from being locally increased. Therefore, it is possible to suppress the gelation of the precipitation inhibitor caused as the concentration of the precipitation inhibitor is locally increased. 
     Therefore, according to the first modification example, the precipitation inhibitor that is in good condition without being gelated can be mixed with the phosphoric acid aqueous solution L. Although  FIG. 3  illustrates an example where the precipitation inhibitor supply path  12   b  is branched into four flow paths, the number of flow paths to be branched is not limited to four. 
     Further, in the first modification example, the precipitation inhibitor just needs to be supplied to be diffused on the liquid surface La of the phosphoric acid aqueous solution L flowing in the tank  14 . That is, in the first modification example, the precipitation inhibitor just needs to be supplied a little at a time according to the fluidity of the phosphoric acid aqueous solution L. In other words, in the first modification example, the amount of the precipitation inhibitor to be supplied just needs to be set based on the fluidity of the phosphoric acid aqueous solution L. 
     Accordingly, since it is possible to further suppress the concentration of the precipitation inhibitor in the phosphoric acid aqueous solution L from being locally increased, it is possible to further suppress the gelation of the precipitation inhibitor. 
       FIG. 4  is a timing chart illustrating an example of operation patterns of respective components of the mixing apparatus  10  in the etching solution production processing according to the first modification example of the exemplary embodiment. As illustrated in  FIG. 4 , the etching solution production processing according to the first modification example is different in the supply timing of the precipitation inhibitor from the exemplary embodiment. 
     Specifically, in the first modification example, after the mixing processing starts at the time point T 0 , the supply of the precipitation inhibitor starts at the same timing as the pump  16  is operated (time point T 1 ). The following processings are the same as those of the exemplary embodiment, and, thus, a detailed description thereof will be omitted. 
       FIG. 5  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to a second modification example of the exemplary embodiment. As illustrated in  FIG. 5 , the mixing apparatus  10  according to the second modification example is different in the arrangement of the plurality of precipitation inhibitor supply openings  12   d  from the first modification example. 
     Specifically, the plurality of precipitation inhibitor supply openings  12   d  is arranged to be divided in the height direction as well as in the horizontal direction. In other words, in the second modification example, the plurality of precipitation inhibitor supply openings  12   d  is provided at different locations, respectively, in the horizontal direction and the height direction. 
     Accordingly, since the precipitation inhibitor can be supplied to a plurality of points in a wider range, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Also, in the second modification example, the precipitation inhibitor can be supplied onto the liquid surface La of the stored phosphoric acid aqueous solution L as well as into the phosphoric acid aqueous solution L. Further, since the precipitation inhibitor according to the exemplary embodiment contains the organic solvent, it has a smaller specific gravity than the phosphoric acid aqueous solution L. 
     Therefore, as in the second modification example, the precipitation inhibitor is supplied into the phosphoric acid aqueous solution L, and, thus, it is possible to suppress the precipitation inhibitor from staying only on the liquid surface La of the phosphoric acid aqueous solution L. 
     That is, in the second modification example, the precipitation inhibitor is supplied into the phosphoric acid aqueous solution L, and, thus, it is possible to suppress the concentration of the precipitation inhibitor on the liquid surface La from being locally increased to suppress the gelation of the precipitation inhibitor. Therefore, according to the second modification example, the precipitation inhibitor that is in good condition without being gelated can be mixed with the phosphoric acid aqueous solution L. 
     Further, in the second modification example, the precipitation inhibitor just needs to be supplied into the phosphoric acid aqueous solution L flowing in the tank  14  so as not to degrade the fluidity of the phosphoric acid aqueous solution L. That is, the precipitation inhibitor just needs to be supplied at a lower flow velocity than the phosphoric acid aqueous solution L. 
     Accordingly, it is possible to suppress the degradation of the fluidity of the phosphoric acid aqueous solution L and suppress poor mixing of the precipitation inhibitor. 
     Also, the etching solution production processing according to the second modification example just needs to be performed according to the timing chart as illustrated in  FIG. 4 . 
       FIG. 6  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to a third modification example of the exemplary embodiment. As illustrated in  FIG. 6 , the mixing apparatus  10  according to the third modification example is equipped with a shower nozzle  12   e  at the precipitation inhibitor supply opening  12   d . The shower nozzle  12   e  is provided at the upper portion of the tank  14  and supplies the precipitation inhibitor onto the liquid surface La of the phosphoric acid aqueous solution L. 
     In the third modification example, the shower nozzle  12   e  supplies the precipitation inhibitor so as to be thinly diffused on the liquid surface La of the stored phosphoric acid aqueous solution L. Thus, the contact area between the phosphoric acid aqueous solution L and the precipitation inhibitor can be further increased. Therefore, according to the third modification example, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Further, in the third modification example, the precipitation inhibitor is supplied by the shower nozzle  12   e  to the phosphoric acid aqueous solution L so as to be thinly diffused, and, thus, it is possible to suppress the concentration of the precipitation inhibitor in the phosphoric acid aqueous solution L from being locally increased. 
     Therefore, according to the third modification example, it is possible to suppress the gelation of the precipitation inhibitor when the precipitation inhibitor is supplied to the phosphoric acid aqueous solution L. 
     Furthermore, in the third modification example, the precipitation inhibitor just needs to be supplied to be diffused on the liquid surface La of the phosphoric acid aqueous solution L flowing in the tank  14 . That is, in the third modification example, the precipitation inhibitor just needs to be supplied a little at a time according to the fluidity of the phosphoric acid aqueous solution L. In other words, in the third modification example, the amount of the precipitation inhibitor to be supplied just needs to be set based on the fluidity of the phosphoric acid aqueous solution L. 
     Accordingly, since it is possible to further suppress the concentration of the precipitation inhibitor in the phosphoric acid aqueous solution L from being locally increased, it is possible to further suppress the gelation of the precipitation inhibitor. 
     Also, the etching solution production processing according to the third modification example just needs to be performed according to the timing chart as illustrated in  FIG. 4 . Further, the shower nozzle  12   e  is provided at the upper portion of the tank  14 , but may be provided at the lower portion of the tank  14 . Furthermore, the precipitation inhibitor may be supplied from the shower nozzle  12   e  provided at the lower portion of the tank  14  into the stored phosphoric acid aqueous solution L. 
       FIG. 7  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to a fourth modification example of the exemplary embodiment. As illustrated in  FIG. 7 , the mixing apparatus  10  according to the fourth modification example is equipped with a mixer  11   d  on the phosphoric acid aqueous solution supply path  11   b . The mixer  11   d  is, for example, an inline mixer or a static mixer. 
     The precipitation inhibitor supply  12  supplies the precipitation inhibitor into the mixer  11   d . Thus, it is possible to supply the precipitation inhibitor to the phosphoric acid aqueous solution L to which high fluidity is imparted by the mixer  11   d.    
     Therefore, according to the fourth modification example, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Further, in the fourth modification example, the precipitation inhibitor just needs to be supplied into the phosphoric acid aqueous solution L flowing in the phosphoric acid aqueous solution supply path  11   b  so as not to degrade the fluidity of the phosphoric acid aqueous solution L. That is, the precipitation inhibitor just needs to be supplied at a lower flow velocity than the phosphoric acid aqueous solution L. 
     Accordingly, it is possible to suppress the degradation of the fluidity of the phosphoric acid aqueous solution L and suppress poor mixing of the precipitation inhibitor. Also, the etching solution production processing according to the fourth modification example just needs to be performed according to the timing chart as illustrated in  FIG. 4 . 
       FIG. 8  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to a fifth modification example of the exemplary embodiment. As illustrated in  FIG. 8 , the mixing apparatus  10  according to the fifth modification example is equipped with the precipitation inhibitor supply opening  12   d  of the precipitation inhibitor supply  12  at the lower portion of the tank  14 . Further, the precipitation inhibitor is supplied into the phosphoric acid aqueous solution L from the precipitation inhibitor supply opening  12   d.    
     Accordingly, since it is possible to suppress the precipitation inhibitor from staying only on the liquid surface La of the phosphoric acid aqueous solution L, it is possible to suppress the concentration of the precipitation inhibitor on the liquid surface La from being locally increased and suppress the gelation of the precipitation inhibitor. 
     Therefore, according to the fifth modification example, the precipitation inhibitor that is in good condition without being gelated can be mixed with the phosphoric acid aqueous solution L. 
     Further, in the fifth modification example, the precipitation inhibitor just needs to be supplied into the phosphoric acid aqueous solution L flowing in the tank  14  so as not to degrade the fluidity of the phosphoric acid aqueous solution L. That is, the precipitation inhibitor just needs to be supplied at a lower flow velocity than the phosphoric acid aqueous solution L. 
     Accordingly, it is possible to suppress the degradation of the fluidity of the phosphoric acid aqueous solution L and suppress poor mixing of the precipitation inhibitor. Also, the etching solution production processing according to the fifth modification example just needs to be performed according to the timing chart as illustrated in  FIG. 2 . 
       FIG. 9  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to a sixth modification example of the exemplary embodiment. As illustrated in  FIG. 9 , the mixing apparatus  10  according to the sixth modification example is different in the configuration of the precipitation inhibitor supply path  12   b  of the precipitation inhibitor supply  12  from the fifth modification example. 
     Specifically, the precipitation inhibitor supply path  12   b  is branched into a plurality of flow paths and the precipitation inhibitor supply opening  12   d  is arranged to be divided into a plurality of parts in the horizontal direction at the lower portion of the tank  14 . Further, in the sixth modification example, the precipitation inhibitor is divided to a plurality of points by a plurality of precipitation inhibitor supply openings  12   d  to be supplied into the stored phosphoric acid aqueous solution L. 
     Thus, the contact area between the phosphoric acid aqueous solution L and the precipitation inhibitor can be increased. Therefore, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Further, in the sixth modification example, since it is possible to suppress the precipitation inhibitor from staying only on the liquid surface La of the phosphoric acid aqueous solution L, it is possible to suppress the concentration of the precipitation inhibitor on the liquid surface La from being locally increased and suppress the gelation of the precipitation inhibitor. 
     Therefore, according to the sixth modification example, the precipitation inhibitor that is in good condition without being gelated can be mixed with the phosphoric acid aqueous solution L. 
     Furthermore, in the sixth modification example, the precipitation inhibitor just needs to be supplied into the phosphoric acid aqueous solution L flowing in the tank  14  so as not to degrade the fluidity of the phosphoric acid aqueous solution L. That is, the precipitation inhibitor just needs to be supplied at a lower flow velocity than the phosphoric acid aqueous solution L. 
     Accordingly, it is possible to suppress the degradation of the fluidity of the phosphoric acid aqueous solution L and suppress poor mixing of the precipitation inhibitor. 
     Also, the etching solution production processing according to the sixth modification example just needs to be performed according to the timing chart as illustrated in  FIG. 2 . Although  FIG. 9  illustrates an example where the precipitation inhibitor supply path  12   b  is branched into five flow paths, the number of flow paths to be branched is not limited to five. 
       FIG. 10  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to a seventh modification example of the exemplary embodiment. As illustrated in  FIG. 10 , the mixing apparatus  10  according to the seventh modification example is equipped with the precipitation inhibitor supply opening  12   d  of the precipitation inhibitor supply  12  that is provided adjacent to the inlet  15   a  of the circulation path  15  at the lower portion of the tank  14 . Further, the precipitation inhibitor is supplied toward the inlet  15   a  of the circulation path  15  from the precipitation inhibitor supply opening  12   d.    
     Accordingly, since it is possible to suppress the precipitation inhibitor from staying only on the liquid surface La of the phosphoric acid aqueous solution L, it is possible to suppress the concentration of the precipitation inhibitor on the liquid surface La from being locally increased and suppress the gelation of the precipitation inhibitor. Therefore, according to the seventh modification example, the precipitation inhibitor that is in good condition without being gelated can be mixed with the phosphoric acid aqueous solution L. 
     Further, in the seventh modification example, the precipitation inhibitor is rapidly supplied to the circulation path  15 . Thus, it is possible to supply the precipitation inhibitor to the phosphoric acid aqueous solution L, to which high fluidity is imparted, inside the circulation path  15 . Therefore, according to the seventh modification example, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Furthermore, in the seventh modification example, an influence of pulsation of the pump  16  on the precipitation inhibitor supply  12  can be reduced. Therefore, according to the seventh modification example, it is possible to improve the supplying accuracy of the precipitation inhibitor from the precipitation inhibitor supply  12 . 
     Moreover, in the seventh modification example, the precipitation inhibitor just needs to be supplied into the phosphoric acid aqueous solution L flowing in the tank  14  so as not to degrade the fluidity of the phosphoric acid aqueous solution L. That is, the precipitation inhibitor just needs to be supplied at a lower flow velocity than the phosphoric acid aqueous solution L. 
     Accordingly, it is possible to suppress the degradation of the fluidity of the phosphoric acid aqueous solution L and suppress poor mixing of the precipitation inhibitor. Also, the etching solution production processing according to the seventh modification example just needs to be performed according to the timing chart as illustrated in  FIG. 2 . 
       FIG. 11  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to an eighth modification example of the exemplary embodiment. As illustrated in  FIG. 11 , the mixing apparatus  10  according to the eighth modification example is equipped with a mixer  15   d  on a more downstream side of the circulation path  15  than the branch portion  15   c . The mixer  15   d  is, for example, an inline mixer or a static mixer. 
     The precipitation inhibitor supply  12  supplies the precipitation inhibitor into the mixer  15   d . Thus, it is possible to supply the precipitation inhibitor to the phosphoric acid aqueous solution L to which fluidity is imparted by the pump  16  and further imparted by the mixer  15   d . Therefore, according to the eighth modification example, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Further, in the eighth modification example, the mixer  15   d  is provided at a more downstream side of the circulation path  15  than the pump  16  and the filter  19 . Thus, even if the precipitation inhibitor is gelated, it is possible to suppress the gelated precipitation inhibitor from being deposited in the pump  16  and the filter  19 . 
     Furthermore, in the eighth modification example, the precipitation inhibitor just needs to be supplied into the phosphoric acid aqueous solution L flowing in the circulation path  15  so as not to degrade the fluidity of the phosphoric acid aqueous solution L. That is, the precipitation inhibitor just needs to be supplied at a lower flow velocity than the phosphoric acid aqueous solution L. 
     Accordingly, it is possible to suppress the degradation of the fluidity of the phosphoric acid aqueous solution L and suppress poor mixing of the precipitation inhibitor. Also, the etching solution production processing according to the eighth modification example just needs to be performed according to the timing chart as illustrated in  FIG. 2 . 
       FIG. 12  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to a ninth modification example of the exemplary embodiment. As illustrated in  FIG. 12 , the mixing apparatus  10  according to the ninth modification example is equipped with a joint portion  15   e  on a more upstream side of the circulation path  15  than the pump  16 . 
     The precipitation inhibitor supply  12  supplies the precipitation inhibitor to the joint portion  15   e . Thus, it is possible to supply the precipitation inhibitor to the phosphoric acid aqueous solution L, to which fluidity is imparted, inside the circulation path  15 . Therefore, according to the ninth modification example, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Further, in the ninth modification example, the joint portion  15   e  is provided at the more upstream side than the pump  16 . Thus, it is possible to mix the phosphoric acid aqueous solution L and the precipitation inhibitor inside the pump  16 . That is, in the ninth modification example, the pump  16  also functions as a mixer. 
     Thus, there is no need to add a separate mixer. Therefore, it is possible to efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor at low cost. 
     Furthermore, in the ninth modification example, the precipitation inhibitor just needs to be supplied into the phosphoric acid aqueous solution L flowing in the circulation path  15  so as not to degrade the fluidity of the phosphoric acid aqueous solution L. That is, the precipitation inhibitor just needs to be supplied at a lower flow velocity than the phosphoric acid aqueous solution L. 
     Accordingly, it is possible to suppress the degradation of the fluidity of the phosphoric acid aqueous solution L and suppress poor mixing of the precipitation inhibitor. Also, the etching solution production processing according to the ninth modification example just needs to be performed according to the timing chart as illustrated in  FIG. 2 . 
       FIG. 13  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to a tenth modification example of the exemplary embodiment. As illustrated in  FIG. 13 , the mixing apparatus  10  according to the tenth modification example is equipped with the precipitation inhibitor supply opening  12   d  of the precipitation inhibitor supply  12  at the upper portion of the tank  14 . 
     Further, the mixing apparatus  10  according to the tenth modification example is equipped with a stirrer at the tank  14 . In an example illustrated in  FIG. 13 , a bubbling device  24  as an example of the stirrer is provided at the tank  14 . 
     The bubbling device  24  makes bubbles of the phosphoric acid aqueous solution L stored in the tank  14  with a bubbling gas. The bubbling device  24  is equipped with a bubbling gas source  24   a , a bubbling gas supply path  24   b , a flow rate controller  24   c  and a bubbling nozzle  24   d.    
     In the bubbling device  24 , the bubbling gas is supplied from the bubbling gas source  24   a  to the bubbling nozzle  24   d  through the bubbling gas supply path  24   b . The bubbling nozzle  24   d  is provided, for example, at the lower portion of the tank  14  and extends in the horizontal direction. 
     Further, on the bubbling nozzle  24   d , a plurality of discharge holes (not illustrated) for discharging the bubbling gas is provided side by side in the horizontal direction. Furthermore, since the bubbling gas is discharged from the plurality of discharge holes, the phosphoric acid aqueous solution L stored in the tank  14  can be bubbled. The bubbling gas is, for example, an inert gas such as a nitrogen gas. 
     Furthermore, in the tenth modification example, when the bubbling device  24  is operated, fluidity caused by an upward flow can be imparted to the phosphoric acid aqueous solution L stored in the tank  14 . 
     Accordingly, the precipitation inhibitor is supplied while new fluidity is imparted to the phosphoric acid aqueous solution L. Thus, the contact area between the phosphoric acid aqueous solution L and the precipitation inhibitor can be further increased. Therefore, according to the tenth modification example, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Moreover, in the tenth modification example, the bubbling device  24  without an actuator is used as the stirrer, and, thus, it is possible to suppress impurities from being mixed into the phosphoric acid aqueous solution L stored in the tank  14 . 
       FIG. 14  is a timing chart illustrating an example of operation patterns of respective components of the mixing apparatus  10  in the etching solution production processing according to the tenth modification example of the exemplary embodiment. First, the controller starts a mixing processing by operating the phosphoric acid aqueous solution supply  11  (ON state) from the time point T 0  to supply the phosphoric acid aqueous solution L into the tank  14 . 
     At the time point T 0 , the precipitation inhibitor supply  12 , the silicon solution supply  13 , the pump  16  and the heater  17  do not operate (OFF state). Also, at the time point T 0 , the filter bypass is in the ON state and the stirrer (the bubbling device  24 ) does not operate (OFF state). 
     Then, at a time point T 1   a  when a predetermined amount of the phosphoric acid aqueous solution L is stored in the tank  14 , the controller operates the precipitation inhibitor supply  12  (ON state) to supply the precipitation inhibitor into the tank  14 . 
     Also, at the same timing as the supply of the precipitation inhibitor starts (time point T 1   a ), the controller operates the stirrer (the bubbling device  24 ) (ON state). Thus, it is possible to impart fluidity to the phosphoric acid aqueous solution L. 
     Accordingly, the precipitation inhibitor can be mixed with the phosphoric acid aqueous solution L to which fluidity is imparted, and, thus, it is possible to efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Further, in the tenth modification example, the precipitation inhibitor just needs to be supplied to be diffused on the liquid surface La of the phosphoric acid aqueous solution L flowing in the tank  14 . That is, in the tenth modification example, the precipitation inhibitor just needs to be supplied a little at a time according to the fluidity of the phosphoric acid aqueous solution L. In other words, in the tenth modification example, the amount of the precipitation inhibitor to be supplied just needs to be set based on the fluidity of the phosphoric acid aqueous solution L. 
     Accordingly, since it is possible to suppress the concentration of the precipitation inhibitor in the phosphoric acid aqueous solution L from being locally increased, it is possible to suppress the gelation of the precipitation inhibitor. 
     Then, at a time point T 2   a  when a predetermined time has elapsed from the time point T 1   a , the controller operates the pump  16  (ON state) to form the circulation flow in the circulation path  15 . Thus, it is possible to impart new fluidity to the phosphoric acid aqueous solution L. 
     Then, at a time point T 3   a  when a predetermined amount of the phosphoric acid aqueous solution L has been supplied into the tank  14 , the controller stops the phosphoric acid aqueous solution supply  11  (OFF state). Then, at a time point T 4   a  when a predetermined amount of the precipitation inhibitor has been supplied into the tank  14 , the controller stops the precipitation inhibitor supply  12  (OFF state). 
     At the same timing as the supply of the precipitation inhibitor is stopped (time point T 4   a ), the controller operates the silicon solution supply  13  (ON state) to supply the silicon solution into the tank  14 . 
     Thereafter, at a time point T 5   a  when a predetermined amount of the silicon solution has been supplied into the tank  14 , the controller stops the silicon solution supply  13  (OFF state). Thus, the mixing processing is completed. 
     Although  FIG. 14  illustrates an example where the silicon solution starts to be supplied later than the precipitation inhibitor, the supply of the precipitation inhibitor and the supply of the silicon solution may start at the same timing (time point T 1   a ). 
     Then, the controller starts a heating processing by operating the heater  17  (ON state) from the time point T 5   a  to heat the etching solution E circulating in the circulation path  15 . The controller heats the etching solution E stored in the tank  14  by heating the etching solution E with the heater  17 . 
     Then, at a time point T 6   a  when the etching solution E in the tank  14  has been heated to a predetermined temperature (e.g., 165° C.), the heating processing is completed. Then, the controller starts a filtration processing by turning the filter bypass in the OFF state from the time point T 6   a.    
     Thereafter, at a time point T 7   a  when the contaminants such as particles contained in the etching solution E are removed sufficiently, the filtration processing is completed. In this way, the etching solution production processing according to the tenth modification example is completed. 
       FIG. 15  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to an eleventh modification example of the exemplary embodiment. As illustrated in  FIG. 15 , the mixing apparatus  10  according to the eleventh modification example is equipped with a stirring blade  25  as another example of the stirrer at the lower portion of the tank  14 . 
     Further, in the eleventh modification example, by operating an actuator (not illustrated) configured to rotate the stirring blade  25 , fluidity caused by a vortex flow can be imparted to the phosphoric acid aqueous solution L stored in the tank  14 . 
     Accordingly, the precipitation inhibitor is supplied while new fluidity is imparted to the phosphoric acid aqueous solution L. Thus, the contact area between the phosphoric acid aqueous solution L and the precipitation inhibitor can be further increased. Therefore, according to the eleventh modification example, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Furthermore, in the eleventh modification example, by minutely controlling the actuator of the stirring blade  25 , it is possible to minutely control a stirring speed. Accordingly, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Moreover, in the eleventh modification example, the tank  14  just needs to be formed into a cylindrical shape. Accordingly, it is possible to readily form the vortex flow in the phosphoric acid aqueous solution L inside the tank  14 . Therefore, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Also, in the eleventh modification example, the precipitation inhibitor just needs to be supplied to be diffused on the liquid surface La of the phosphoric acid aqueous solution L flowing in the tank  14 . That is, in the eleventh modification example, the precipitation inhibitor just needs to be supplied a little at a time according to the fluidity of the phosphoric acid aqueous solution L. In other words, in the eleventh modification example, the amount of the precipitation inhibitor to be supplied just needs to be set based on the fluidity of the phosphoric acid aqueous solution L. 
     Accordingly, since it is possible to suppress the concentration of the precipitation inhibitor in the phosphoric acid aqueous solution L from being locally increased, it is possible to suppress the gelation of the precipitation inhibitor. 
     Further, in the eleventh modification example, as illustrated in  FIG. 8  and other drawings, the precipitation inhibitor may be supplied into the phosphoric acid aqueous solution L from the precipitation inhibitor supply opening  12   d  provided at the lower portion of the tank  14 . 
     Accordingly, it is possible to actively attract the precipitation inhibitor to a vortex flow formed by the stirring blade  25 . Therefore, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     In this case, the precipitation inhibitor just needs to be supplied into the phosphoric acid aqueous solution L flowing in the circulation path  15  so as not to degrade the fluidity of the phosphoric acid aqueous solution L. That is, the precipitation inhibitor just needs to be supplied at a lower flow velocity than the vortex flow formed in the phosphoric acid aqueous solution L. 
     Accordingly, it is possible to suppress the degradation of the fluidity of the phosphoric acid aqueous solution L and suppress poor mixing of the precipitation inhibitor. Also, the etching solution production processing according to the eleventh modification example just needs to be performed according to the timing chart as illustrated in  FIG. 14 . 
       FIG. 16  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to a twelfth modification example of the exemplary embodiment. As illustrated in  FIG. 16 , the mixing apparatus  10  according to the twelfth modification example is equipped with an ultrasonic generator  26  as another example of the stirrer at the lower portion of the tank  14 . 
     The ultrasonic generator  26  can generate ultrasonic waves toward the phosphoric acid aqueous solution L stored in the tank  14 . Further, in the twelfth modification example, by operating the ultrasonic generator  26 , fluidity caused by the ultrasonic waves can be imparted to the phosphoric acid aqueous solution L stored in the tank  14 . 
     Accordingly, the precipitation inhibitor is supplied while new fluidity is imparted to the phosphoric acid aqueous solution L. Thus, the contact area between the phosphoric acid aqueous solution L and the precipitation inhibitor can be further increased. Furthermore, in the twelfth modification example, the ultrasonic waves from the ultrasonic generator  26  are transmitted to the entire phosphoric acid aqueous solution L, and, thus, the stirring is performed throughout the phosphoric acid aqueous solution L. 
     Therefore, according to the twelfth modification example, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Also, in the twelfth modification example, the ultrasonic waves from the ultrasonic generator  26  cause cavitation in the phosphoric acid aqueous solution L. For this reason, even if the precipitation inhibitor is gelated in the phosphoric acid aqueous solution L, it is possible to break the gel into smaller pieces. 
     That is, in the twelfth modification example, the dissolution of the gelated precipitation inhibitor can be accelerated, and, thus, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Further, in the twelfth modification example, the precipitation inhibitor just needs to be supplied to be diffused on the liquid surface La of the phosphoric acid aqueous solution L flowing in the tank  14 . That is, in the twelfth modification example, the precipitation inhibitor just needs to be supplied a little at a time according to the fluidity of the phosphoric acid aqueous solution L. In other words, in the twelfth modification example, the amount of the precipitation inhibitor to be supplied just needs to be set based on the fluidity of the phosphoric acid aqueous solution L. 
     Accordingly, since it is possible to suppress the concentration of the precipitation inhibitor in the phosphoric acid aqueous solution L from being locally increased, it is possible to suppress the gelation itself of the precipitation inhibitor. Also, the etching solution production processing according to the twelfth modification example just needs to be performed according to the timing chart as illustrated in  FIG. 14 . 
       FIG. 17  is a schematic block diagram illustrating a configuration of the mixing apparatus  10  according to a thirteenth modification example of the exemplary embodiment. As illustrated in  FIG. 17 , the mixing apparatus  10  according to the thirteenth modification example is different in the configuration of the tank  14  from the exemplary embodiment. Specifically, the tank  14  according to the thirteenth modification example is equipped with an inner tank  14   a  and an outer tank  14   b.    
     The inner tank  14   a  has an open top, and, thus, the phosphoric acid aqueous solution L, the precipitation inhibitor and the silicon solution are supplied near an upper portion of the inner tank  14   a . That is, the phosphoric acid aqueous solution supply  11  supplies the phosphoric acid aqueous solution L into the inner tank  14   a , the precipitation inhibitor supply  12  supplies precipitation inhibitor into the inner tank  14   a  and the silicon solution supply  13  supplies the silicon solution into the inner tank  14   a.    
     The outer tank  14   b  is provided around the inner tank  14   a  and has an open top. The phosphoric acid aqueous solution L overflowing from the inner tank  14   a  is supplied into the outer tank  14   b.    
     Further, the inlet  15   a  of the circulation path  15  is provided at a lower portion of the outer tank  14   b . Furthermore, the outlet  15   b  of the circulation path  15  is provided at a lower portion of the inner tank  14   a . That is, in the thirteenth modification example, the circulation flow of the phosphoric acid aqueous solution L is formed by the outer tank  14   b , the circulation path  15  and the inner tank  14   a.    
     Moreover, in the mixing apparatus  10  according to the thirteenth modification example, the phosphoric acid aqueous solution L is allowed to overflow from the inner tank  14   a  to the outer tank  14   b , and, thus, fluidity caused by an upward flow can be imparted to the phosphoric acid aqueous solution L. 
     Accordingly, the precipitation inhibitor is supplied while new fluidity is imparted to the phosphoric acid aqueous solution L. Thus, the contact area between the phosphoric acid aqueous solution L and the precipitation inhibitor can be further increased. Therefore, according to the thirteenth modification example, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Further, in the thirteenth modification example, the upward flow is formed in the inner tank  14   a  and the outer tank  14   b  that do not have an actuator, and, thus, it is possible to suppress the impurities from being mixed into the phosphoric acid aqueous solution L stored in the tank  14 . 
     Furthermore, in the thirteenth modification example, the precipitation inhibitor just needs to be supplied into the inner tank  14   a  of the tank  14 . Thus, it is possible to spread the precipitation inhibitor and make it thin on the liquid surface La of the phosphoric acid aqueous solution L overflowing from the inner tank  14   a . That is, the contact area between the phosphoric acid aqueous solution L and the precipitation inhibitor can be further increased. 
     Therefore, according to the thirteenth modification example, it is possible to more efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     Moreover, in the thirteenth modification example, the precipitation inhibitor just needs to be supplied to be diffused on the liquid surface La of the phosphoric acid aqueous solution L flowing in the inner tank  14   a . That is, in the thirteenth modification example, the precipitation inhibitor just needs to be supplied a little at a time according to the fluidity of the phosphoric acid aqueous solution L. In other words, in the thirteenth modification example, the amount of the precipitation inhibitor to be supplied just needs to be set based on the fluidity of the phosphoric acid aqueous solution L. 
     Accordingly, since it is possible to suppress the concentration of the precipitation inhibitor in the phosphoric acid aqueous solution L from being locally increased, it is possible to suppress the gelation of the precipitation inhibitor. 
       FIG. 18  is a timing chart illustrating an example of operation patterns of respective components of the mixing apparatus  10  in the etching solution production processing according to the thirteenth modification example of the exemplary embodiment. First, the controller starts a mixing processing by operating the phosphoric acid aqueous solution supply  11  (ON state) from the time point T 0  to supply the phosphoric acid aqueous solution L into the tank  14 . 
     At the time point T 0 , the precipitation inhibitor supply  12 , the silicon solution supply  13 , the pump  16  and the heater  17  do not operate (OFF state). Also, at the time point T 0 , the filter bypass is in the ON state. 
     Then, at a time point T 1   b  when a predetermined amount of the phosphoric acid aqueous solution L is stored in the inner tank  14   a  and the outer tank  14   b  of the tank  14 , the controller stops the phosphoric acid aqueous solution supply  11  (OFF state). Herein, the term “predetermined amount” refers to the amount in which at least the phosphoric acid aqueous solution L can overflow from the inner tank  14   a  and circulate in the circulation path  15 . 
     At the same timing as the supply of the phosphoric acid aqueous solution L is stopped (time point T 1   b ), the controller operates the precipitation inhibitor supply  12  and the pump  16  (ON state) to supply the precipitation inhibitor into the tank  14  and form the circulation flow in the circulation path  15 . Thus, it is possible to supply the precipitation inhibitor to the phosphoric acid aqueous solution L overflowing from the inner tank  14   a.    
     Then, at a time point T 2   b  when a predetermined amount of the precipitation inhibitor has been supplied into the inner tank  14   a , the controller stops the precipitation inhibitor supply  12  (OFF state). Then, the circulation flow is formed in the circulation path  15  to mix a chemical liquid in the tank  14  until a time point T 3   b , and, thus, the mixing processing is completed. 
     Then, the controller starts a heating processing by operating the heater  17  (ON state) from the time point T 3   b  to heat the phosphoric acid aqueous solution L circulating in the circulation path  15 . The controller heats the phosphoric acid aqueous solution L stored in the tank  14  by heating the phosphoric acid aqueous solution L with the heater  17 . 
     At the same timing as the operation of the heater  17  is started (time point T 3   b ), the controller operates the silicon solution supply  13  (ON state) to supply the silicon solution into the tank  14 . 
     Thereafter, at a time point T 4   b  when a predetermined amount of the silicon solution has been supplied into the tank  14 , the controller stops the silicon solution supply  13  (OFF state). Also, at a time point T 5   b  when the phosphoric acid aqueous solution L in the tank  14  has been heated to a predetermined temperature (e.g., 165° C.), the heating processing is completed. 
     Then, the controller starts a filtration processing by turning the filter bypass in the OFF state from the time point T 5   b.    
     Thereafter, at a time point T 6   b  when the contaminants such as particles contained in the phosphoric acid aqueous solution L are removed sufficiently, the filtration processing is completed. In this way, the etching solution production processing according to the thirteenth modification example is completed. 
       FIG. 19  is a schematic block diagram illustrating a configuration of a substrate processing system  1 A according to a fourteenth modification example of the exemplary embodiment. The substrate processing system  1 A illustrated in  FIG. 19  is different from the exemplary embodiment in that the substrate processing system  1 A includes a substrate processing apparatus  50  configured to perform a single-wafer processing on each wafer W instead of the substrate processing apparatus  30  configured to perform a batch-type processing to a plurality of wafers W. Further, in  FIG. 19 , the same components as those in the exemplary embodiment illustrated in  FIG. 1  will be assigned same reference numerals, and redundant description thereof will be omitted. 
     In the substrate processing system  1 A illustrated in  FIG. 19 , the etching solution E circulating in the circulation path  15  is supplied into the substrate processing apparatus  50  via the solution sending path  22 . The substrate processing apparatus  50  is equipped with a substrate holder  51  and a rotation mechanism  52 . 
     The substrate holder  51  horizontally holds a wafer W. The rotation mechanism  52  rotates the substrate holder  51  and the wafer W held by the substrate holder  51 . Further, the substrate processing system  1 A may perform a single-wafer etching processing on the wafer W by discharging the etching solution E through the circulation path  15  and the solution sending path  22  to a top surface of the wafer W held by the substrate holder  51 . 
     Although  FIG. 19  illustrates an example where the mixing apparatus  10  according to the exemplary embodiment is combined with the substrate processing apparatus  50  that can perform a single-wafer processing, the mixing apparatus  10  according to the first to thirteenth modification examples may be combined with the substrate processing apparatus  50  configured to perform the single-wafer processing. 
     The mixing apparatus  10  according to the exemplary embodiment is equipped with the phosphoric acid aqueous solution supply  11 , an additive supply (the precipitation inhibitor supply  12 ), the tank  14 , the phosphoric acid aqueous solution supply path  11   b  and an additive supply path (the precipitation inhibitor supply path  12   b ). The phosphoric acid aqueous solution supply  11  is configured to supply the phosphoric acid aqueous solution L. The additive supply (the precipitation inhibitor supply  12 )  11  is configured to supply an additive (the precipitation inhibitor) configured to suppress the precipitation of the silicon oxide. The phosphoric acid aqueous solution supply path  11   b  is configured to connect the phosphoric acid aqueous solution supply  11  with the tank  14 . The additive supply path (the precipitation inhibitor supply path  12   b ) is configured to connect the additive supply (the precipitation inhibitor supply  12 ) with the tank  14 . Further, the additive (the precipitation inhibitor) is supplied while fluidity is imparted to the phosphoric acid aqueous solution L supplied from the phosphoric acid aqueous solution supply  11  into the tank  14 . Accordingly, it is possible to efficiently mix the phosphoric acid aqueous solution L and the precipitation inhibitor. 
     The mixing apparatus  10  according to the exemplary embodiment is further equipped with the circulation path  15  that comes out of the tank  14  and returns to the tank  14  and the pump  16  provided on the circulation path  15 . Also, the fluidity is imparted to the phosphoric acid aqueous solution L by operating the pump  16  to form a circulation flow in the circulation path  15 . Accordingly, it is possible to efficiently impart the fluidity to the phosphoric acid aqueous solution L. 
     Further, in the mixing apparatus  10  according to the exemplary embodiment, an additive supply opening (the precipitation inhibitor supply opening  12   d ) through which the additive (the precipitation inhibitor) is supplied from the additive supply path (the precipitation inhibitor supply path  12   b ) into the tank  14  is provided adjacent to the outlet  15   b  of the circulation path  15 . Thus, the precipitation inhibitor can be directly supplied into the phosphoric acid aqueous solution L discharged from the outlet  15   b  and has high fluidity. 
     Furthermore, in the mixing apparatus  10  according to the exemplary embodiment, an additive supply opening (the precipitation inhibitor supply opening  12   d ) through which the additive (the precipitation inhibitor) is supplied from the additive supply path (the precipitation inhibitor supply path  12   b ) into the tank  14  is provided adjacent to the inlet  15   a  of the circulation path  15 . Thus, the precipitation inhibitor can be rapidly supplied to the circulation path  15 . Therefore, it is possible to supply the precipitation inhibitor into the phosphoric acid aqueous solution L, to which the high fluidity is imparted, within the circulation path  15 . 
     Moreover, the mixing apparatus  10  according to the exemplary embodiment is further equipped with a stirrer provided in the tank  14 . Also, the fluidity is imparted to the phosphoric acid aqueous solution L by operating the stirrer. Thus, it is possible to efficiently impart the fluidity to the phosphoric acid aqueous solution L. 
     In the mixing apparatus  10  according to the exemplary embodiment, the stirrer is the bubbling device  24  configured to supply a bubbling gas into the phosphoric acid aqueous solution L stored in the tank  14 . Thus, it is possible to impart the fluidity caused by the upward flow to the phosphoric acid aqueous solution L stored in the tank  14 . 
     Further, in the mixing apparatus  10  according to the exemplary embodiment, the stirrer is the stirring blade  25  configured to stir the phosphoric acid aqueous solution L stored in the tank  14 . Thus, it is possible to impart the fluidity caused by the vortex flow to the phosphoric acid aqueous solution L stored in the tank  14 . 
     Furthermore, in the mixing apparatus  10  according to the exemplary embodiment, the stirrer is the ultrasonic generator  26  configured to generate ultrasonic waves toward the phosphoric acid aqueous solution L stored in the tank  14 . Thus, it is possible to impart the fluidity caused by the ultrasonic waves to the phosphoric acid aqueous solution L stored in the tank  14 . 
     Moreover, in the mixing apparatus  10  according to the exemplary embodiment, multiple additive supply openings (the precipitation inhibitor supply openings  12   d ) through which the additive (the precipitation inhibitor) is supplied from the additive supply path (the precipitation inhibitor supply path  12   b ) into the tank  14  are provided at the upper portion of the tank  14 . Thus, the contact area between the phosphoric acid aqueous solution L and the precipitation inhibitor can be increased. Therefore, it is possible to more efficiently mix the phosphoric acid aqueous solution L with the precipitation inhibitor. 
     Further, in the mixing apparatus  10  according to the exemplary embodiment, the additive supply opening (the precipitation inhibitor supply opening  12   d ) through which the additive (the precipitation inhibitor) is supplied from the additive supply path (the precipitation inhibitor supply path  12   b ) into the tank  14  is provided at the lower portion of the tank  14 . Thus, it is possible to suppress the precipitation inhibitor from staying only on the liquid surface La of the phosphoric acid aqueous solution L. Therefore, it is possible to suppress the concentration of the precipitation inhibitor on the liquid surface La from being locally increased and suppress the gelation of the precipitation inhibitor. 
     Furthermore, in the mixing apparatus  10  according to the exemplary embodiment, the tank  14  is equipped with the inner tank  14   a  and the outer tank  14   b . Moreover, the fluidity is imparted to the phosphoric acid aqueous solution L by overflowing the phosphoric acid aqueous solution L from the inner tank  14   a  to the outer tank  14   b . Accordingly, it is possible to impart the fluidity caused by the upward flow to the phosphoric acid aqueous solution L stored in the tank  14 . 
     Also, in the mixing apparatus  10  according to the exemplary embodiment, the additive (the precipitation inhibitor) is supplied into the inner tank  14   a . Thus, it is possible to thinly diffuse the precipitation inhibitor on the liquid surface La of the phosphoric acid aqueous solution L overflowing from the inner tank  14   a . Therefore, the contact area between the phosphoric acid aqueous solution L and the precipitation inhibitor can be further increased. 
     The mixing apparatus  10  according to the exemplary embodiment is further equipped with the heater  17  configured to heat the phosphoric acid aqueous solution L stored in the tank  14 . Thus, the heated etching solution E can be supplied into the substrate processing apparatus  30 . 
     &lt;Details of Etching Solution Production Processing and Substrate Processing&gt; 
     Hereinafter, an etching solution production processing and a substrate processing performed by the substrate processing system  1  according to the exemplary embodiment will be described in detail with reference to  FIG. 20 .  FIG. 20  is a flowchart showing a processing sequence of the etching solution production processing and the substrate processing according to the exemplary embodiment. 
     First, the controller operates the mixing apparatus  10  to perform a mixing processing of mixing the phosphoric acid aqueous solution L, the precipitation inhibitor and the silicon solution (process S 101 ). For example, the controller mixes the phosphoric acid aqueous solution L, the precipitation inhibitor and the silicon solution by supplying the precipitation inhibitor and the silicon solution to the phosphoric acid aqueous solution L while fluidity is imparted to the phosphoric acid aqueous solution L stored in the tank  14 . 
     Then, the controller operates the heater  17  of the mixing apparatus  10  to perform a heating processing of heating a mixed solution of the phosphoric acid aqueous solution L, the precipitation inhibitor and the silicon solution (process S 102 ). 
     Then, the controller performs a filtration processing of filtering the mixed solution of the phosphoric acid aqueous solution L, the precipitation inhibitor and the silicon solution through the filter  19  (process S 103 ). When the filtration processing is completed, the etching solution production processing according to the exemplary embodiment is completed. 
     Then, the controller operates the mixing apparatus  10  and the substrate processing apparatus  30  to perform a supply processing in which the etching solution E is supplied from the mixing apparatus  10  to the substrate processing apparatus  30  (process S 104 ). Thus, the etching solution E is stored in the processing tank  31  of the substrate processing apparatus  30 . 
     Then, the controller operates the substrate processing apparatus  30  to perform an etching processing of etching a wafer W with the etching solution E stored in the processing tank  31  (process S 105 ). Then, when the etching processing is completed, the substrate processing according to the exemplary embodiment is completed. 
     The mixing method according to the exemplary embodiment includes a mixing process (process S 101 ) and a heating process (process S 102 ). The mixing process (process S 101 ) supplies an additive (the precipitation inhibitor) for suppressing the precipitation of silicon oxide to the flowing phosphoric acid aqueous solution L and mixes them. The heating process (process S 102 ) heats the mixed solution of the phosphoric acid aqueous solution L and the additive (the precipitation inhibitor). Thus, it is possible to heat the etching solution E that is efficiently mixed and supply the heated etching solution E into the substrate processing apparatus  30 . 
     Further, in the mixing method according to the exemplary embodiment, the mixing process (process S 101 ) includes supplying the additive (the precipitation inhibitor) to be diffused on the liquid surface La of the flowing phosphoric acid aqueous solution L. Thus, it is possible to further suppress the concentration of the precipitation inhibitor in the phosphoric acid aqueous solution L from being locally increased. Therefore, it is possible to further suppress the gelation of the precipitation inhibitor. 
     Furthermore, in the mixing method according to the exemplary embodiment, the mixing process includes supplying the additive (the precipitation inhibitor) into the flowing phosphoric acid aqueous solution L so as not to degrade the fluidity of the phosphoric acid aqueous solution L. Thus, it is possible to suppress the degradation of the fluidity of the phosphoric acid aqueous solution L and suppress poor mixing of the precipitation inhibitor. 
     According to the exemplary embodiments, it is possible to efficiently mix the additive configured to suppress the precipitation of the silicon oxide and the phosphoric acid aqueous solution. 
     While the present disclosure has been described with reference to the exemplary embodiments, the present disclosure is not limited to the exemplary embodiments but may be variously modified without departing from the spirit thereof. 
     The exemplary embodiments disclosed herein are illustrative in all aspects and not limited thereto. In fact, the above exemplary embodiments can be embodied in various forms. Further, the above-described exemplary embodiments may be omitted, substituted, or changed in various forms without departing from the scope of the appended claims. 
     From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting. The scope of the inventive concept is defined by the following claims and their equivalents rather than by the detailed description of the exemplary embodiments. It shall be understood that all modifications and embodiments conceived from the meaning and scope of the claims and their equivalents are included in the scope of the inventive concept.