Abstract:
A concentration control apparatus of a liquid chemical is provided, which is capable of real-time concentration/composition control of a liquid chemical without sampling the same. This apparatus is comprised of (a) a liquid chemical supplier section for supplying a liquid chemical; the liquid chemical being a mixture of source liquids; (b) a wafer treater section for applying a specific treatment to a semiconductor wafer using the liquid chemical supplied from the liquid chemical supplying section; the wafer treating section being connected to the liquid chemical supplying section through a communication line; (c) a concentration detector located in the communication line for detecting the concentration and/or composition of the liquid chemical flowing through the communication line; the concentration detector outputting a data signal about the detected concentration and/or composition of the liquid chemical; and (d) a data processor section for processing the data signal outputted from the concentration detector to output a control signal to the liquid chemical supplier section. The concentration and/or composition of the liquid chemical supplied from the liquid chemical supplier section is/are controlled by the control signal outputted from the data processor section.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a concentration control apparatus and more particularly, to a concentration control apparatus of a liquid chemical that is capable of real-time measurement and management of the concentration and/or composition of a liquid chemical used for various treatments, such as wafer etching, wafer cleaning, and so on, in semiconductor device fabrication. 
     2. Description of the Prior Art 
     Conventionally, various cleaning apparatuses for cleaning a semiconductor wafer have been known, an example of which is disclosed in the Japanese Non-Examined Patent Publication No. 5-121388 published in May 1993. This prior-art cleaning apparatus is comprised of a wafer heating means for heating a semiconductor wafer, a wafer holding means for holding the wafer, and a cleaning liquid diffusing means for diffusing a cleaning liquid over the wafer held by the wafer holding means. 
     FIG. 1 schematically shows the configuration of this prior-art cleaning apparatus. 
     As seen from FIG. 1, this apparatus has a halogen lamp  161  for heating an underlying semiconductor wafer  160 , a wafer holder  162  for holding the wafer  160 , an arm  163  for operating the holder  162 , a motor  164  for rotating the wafer  160  in the horizontal plane, a rotational shaft  165  for transmitting the rotation motion of the motor  164  to the wafer  160  through the arm  163 , a nozzle  166  for spraying a liquid cleaning chemical toward the wafer  60  held by the holder  162 , the two nozzles for spraying pure water toward the wafer  60  to rinse the same. The lamp  161 , the wafer holder  162 , the arm  163 , and the nozzles  166  and  167  are provided in a chamber  170 . The motor  164  is located outside the chamber  170 . The rotational shaft  165  penetrates the bottom wall of the chamber  170 . 
     On operation, the wafer  160  held by the holder  162  is heated by the halogen lamp  161  in the chamber  170  while supplying a nitrogen (N 2 ) gas through a gas port  171  of the chamber  170  and rotating the wafer  160  by the motor  164  in the horizontal plane. Then, a specific cleaning liquid is sprayed from the nozzle  166  toward the wafer  160 , thereby cleaning the wafer  160 . The drain generated in the chamber  170  is flown out of the chamber  170  through a drain port  172 . After the cleaning process is completed, pure water is sprayed from the nozzles  167  toward the wafer  160 , thereby rinsing the wafer  160 . 
     With the prior-art cleaning apparatus shown in FIG. 1, since the halogen lamp  161  is provided in the chamber  170 , the wafer  160  can be heated to a specific temperature in a short time during the cleaning process. Also, the N 2  gas is introduced into the chamber  170  during the cleaning process and therefore, the cleaning atmosphere is stabilized, resulting in highly-reproducible cleaning operation. 
     On the other hand, various concentration control apparatuses for controlling the concentration of liquid chemicals been known. An example of the apparatuses of this sort is disclosed in the Japanese Non-Examined Patent Publication No. 60-223131 published in Nov. 1985. In this apparatus, the concentration of a cleaning liquid stored in a container is detected or monitored. Based on the detection result of the concentration thus obtained, the supply of source liquids to the container is controlled to stabilize the concentration of the liquid in the container. Taking the fact that the concentration detection operation necessitates a little time (i.e., a little time lag) into consideration and it is performed intermittently, the concentration control includes prediction of the concentration change to occur in the container after the detection of the liquid concentration (i.e., at the concentration detection operation next time). 
     FIG. 2 schematically shows the configuration of this prior-art concentration control apparatus. 
     As seen from FIG. 2, this prior-art apparatus has a storage container  270  in which a specific cleaning liquid CO having a specific concentration is stored. A source liquid supplying section  271  is provided over the container  270  for supplying two source liquids C 1  and C 2  to the container  270 . The section  271  includes two storage tanks  272   a  and  272   b  in which the source liquids C 1  and C 2  are respectively stored, two valves  273   a  and  273   b  located respectively in the flow paths communicating with the tanks  272   a  and  272   b , and two valves  274   a  and  274   b  located respectively in these flow paths. The section  271  supplies the source liquids C 1  and C 2  to the container  270  while controlling the flow rates of the liquids C 1  and C 2  by using the pumps  273   a  and  273   b  and the valves  274   a  and  274   b.    
     The storage container  270  is equipped with a concentration monitoring section  275  for monitoring or detecting the concentration of the cleaning liquid CO in the container  270 . The section  275  includes a pump  276  for pumping up the solution CO from the container  270  for sampling the same, and a monitoring device  277  for monitoring the concentration of the liquid CO thus pumped up. 
     A control section  278  is provided between the supplying section  271  and the monitoring section  275 . The control section  278  controls the operation of the supplying section  271  according to the output of the monitoring section  275 . Specifically, the control section  278  has a feedback control system  279   a  and a predictive control system  279   b . The section  278  controls the concentration of the cleaning liquid CO in the container  270  under the cooperation of these systems  279   a  and  279   b.    
     In the monitoring operation of the monitoring section  275 , the pump  276  is activated to deliver the liquid CO in the container  270  to the monitoring device  277  for the purpose of sampling the same. Then, after a little time period passes required for detecting the concentration, the value of the concentration is read out. 
     The concentration data thus obtained in the monitoring section  275  is sent to the control section  278 . Then, the control section  278  predicts the concentration change that will occur in the container  270  at the next-time monitoring operation, and controls the valves  274   a  and  274   b  and the pumps  273   a  and  273   b  so as to increase or decrease the supply rates of the source liquids C 1  and C 2  according to the result of the prediction operation. 
     With the above-described prior-art apparatuses shown in FIGS. 1 and 2, however, the following problems occur. 
     In single-wafer treating apparatuses such as single-wafer etchers designed for treating a single semiconductor wafer in each step, an advantage that the treatment condition can be easily controlled as desired arises. However, if an easily-decomposable liquid chemical or a mixture of several liquid chemicals is used as a treating agent, the prior-art apparatus shown in FIG. 1 has a problem that it is unable to find the deterioration and/or abnormal initial concentration of the agent in early stages. This is because the apparatus has no means for detecting or monitoring the concentration change of the agent during the ongoing treatment step for each wafer. 
     On the other hand, the prior-art apparatus shown in FIG. 2 has the monitoring section  275  for monitoring the concentration of the cleaning liquid CO stored in the container  270 . Therefore, the above-described problem of the prior-art apparatus of FIG. 1 does not occur. However, the sampling of the liquid CO from the container  270  by the monitoring section  275  causes some pressure loss in the supply lines (not shown) of the liquid CO to a specific cleaning chamber. As a result, not only the liquid CO is required in surplus but also the required amount of the liquid CO for each cleaning step tends to fluctuate. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a concentration control apparatus of a liquid chemical that is capable of real-time concentration/composition control of a liquid chemical without sampling the same. 
     Another object of the present invention is to provide a concentration control apparatus of a liquid chemical that makes it possible to find the deterioration of a liquid chemical and the abnormal concentration or composition thereof immediately. 
     Still another object of the present invention is to provide a concentration control apparatus of a liquid chemical that is capable of precise control of the concentration and/or composition of a liquid chemical. 
     A further object of the present invention is to provide a concentration control apparatus of a liquid chemical that minimizes the damage to be caused by the unallowable concentration and/or composition of a liquid chemical. 
     The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description. 
     A concentration control apparatus of a liquid chemical according to the present invention is comprised of 
     (a) a liquid chemical supplier section for supplying a liquid chemical; the liquid chemical being a mixture of source liquids; 
     (b) a wafer treater section for applying a specific treatment to a semiconductor wafer using the liquid chemical supplied from the liquid chemical supplying section; the wafer treating section being connected to the liquid chemical supplying section through a communication line; 
     (c) a concentration detector located in the communication line for detecting the concentration and/or composition of the liquid chemical flowing through the communication line; the concentration detector outputting a data signal about the detected concentration and/or composition of the liquid chemical; and 
     (d) a data processor section for processing the data signal outputted from the concentration detector to output a control signal to the liquid chemical supplier section. 
     The concentration and/or composition of the liquid chemical supplied from the liquid chemical supplier section is/are controlled by the control signal outputted from the data processor section. 
     With the concentration control apparatus of a liquid chemical according to the present invention, the liquid chemical supplier section for supplying a liquid chemical is connected to the wafer treater section for applying a specific treatment to a semiconductor wafer suing the liquid chemical by the communication line. Also, the concentration detector is located in the communication line to detect the concentration and/or composition of the liquid chemical flowing through the communication line. 
     Accordingly, real-time concentration/composition control of a liquid chemical can be realized without sampling the same. This solves the sampling-induced problem of the above-described prior-art apparatus shown in FIG.  2 . 
     Also, since the concentration/composition of the liquid chemical can be real-time monitored by the concentration detector located in the communication line, the deterioration of the liquid chemical and the abnormal concentration or composition thereof can be found immediately. This minimizes the damage to be caused by the unallowable concentration and/or composition of the liquid chemical. 
     Furthermore, based on the data signal about the concentration and/or composition of the liquid chemical, the data processor section outputs in real time the control signal to the liquid chemical supplier section, thereby controlling the concentration and/or composition of the liquid chemical supplied from the liquid chemical supplier section. As a result, the concentration/composition of the liquid chemical can be controlled precisely. 
     In the present invention, any liquid or any liquid chemical substance may be used as each of the source liquids. Typically, the liquid chemical supplied from the liquid chemical supplier section is a mixture of two or more liquid chemical substances. However, this liquid chemical may be a diluted chemical substance, in which, for example, one of the two source liquids is a liquid chemical substance such as hydrogen fluoride (HF) and the other is a diluting liquid such as pure water. 
     Typically, each of the source liquids is a liquid chemical substance necessary for producing the desired liquid chemical in the liquid chemical supplier section. Usually, the liquid chemical substance contains a single component. For example, undiluted sulfuric acid (H 2 SO 4 ), 30% -hydrogen peroxide solution, or the like may be used. However, the liquid chemical substance may contain two or more components. 
     The liquid chemical supplier section may have any configuration if it can supplies a liquid chemical and the concentration and/or composition of the liquid chemical can be controlled by the control signal outputted from the data processor section. 
     The wafer treater section may have any configuration if it is used for applying a specific treatment to a semiconductor wafer using the liquid chemical supplied from the liquid chemical supplying section. 
     The concentration detector may have any configuration if it is located in the communication line and it detects the concentration and/or composition of the liquid chemical flowing through the communication line to thereby output a data signal about the detected concentration and/or composition of the liquid chemical. 
     The data processor section may have any configuration if it processes the data signal outputted from the concentration detector to thereby output a control signal to the liquid chemical supplier section. 
     In a preferred embodiment of the apparatus according to the invention, the liquid chemical is a cleaning liquid for cleaning the semiconductor wafer. In this embodiment, for example, 89% -sulfuric acid and 31% hydrogen peroxide solution are used as the source liquids, thereby producing a well-known sulfuric-peroxide mixture (SPM) as the liquid chemical in the liquid chemical supplier section. Alternately, 29% - ammonia solution and 31% hydrogen peroxide solution are used as the source liquids, thereby producing a well-known ammonia-peroxide mixture (APM) as the liquid chemical in the liquid chemical supplier section. 
     In another preferred embodiment of the apparatus according to the invention, the liquid chemical is an etching liquid for etching the semiconductor wafer. In this embodiment, for example, 50%-hydrogen fluoride (HF) and pure water are used as the source liquids, thereby producing a well-known 0.3% -diluted hydrogen fluoride (DHF) as the liquid chemical in the liquid chemical supplier section. 
     In still another preferred embodiment of the apparatus according to the invention, the liquid chemical supplier section is comprised of storage tanks for storing the source liquids, and pumps respectively connected to the storage tanks. The pumps are controlled by the control signal outputted from the data processor section in such a way that the source liquids stored in the tanks are mixed together to have a desired ratio. The mixture of the source liquids is supplied to the wafer treater section as the liquid chemical. 
     It is preferred that a reference tank for storing a reference liquid is additionally provided for supplying the reference liquid to the concentration detector. The reference liquid is used for calibration. Alternately, a pure water tank for storing pure water may be additionally provided, in which the pure water is supplied to the concentration detector for calibration. 
     In a further preferred embodiment of the apparatus according to the invention, the concentration detector measures molarities of the source liquids contained in the liquid chemical by detecting an absorption wavelength or absorption characteristic of the liquid chemical using a ultraviolet, visible, or near-infrared spectrophotometer. 
     In a still further preferred embodiment of the apparatus according to the invention, the data processor section is comprised of a data converter and a pump controller. The data converter converts the control signal from the data signal about the concentration/composition of the liquid chemical to the control signal. The pump controller controls pumping rates of the source liquids according to the control signal. 
     In this embodiment, preferably, the data processor section compares concentrations or composition of the source liquids contained in the liquid chemical with predetermined values, and produces correction values for the source liquids based on result of comparison. The pump controller changes the pumping rates of the source liquids according to the correction values. 
     It is preferred that the data processor section has a specific stabilization period of time, and the correction values are produced after the stabilization period passes. In this case, it is preferred that the data processor section performs only a displaying operation of concentration values during the stabilization period. 
     Preferably, the data processor section performs the production operation of the correction values after each stabilization period. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawings. 
     FIG. 1 is a schematic cross-sectional view of a prior-art cleaning apparatus of a semiconductor wafer. 
     FIG. 2 is a schematic cross-sectional view of a prior-art concentration control apparatus of a liquid chemical. 
     FIG. 3 is a schematic cross-sectional view of a concentration control apparatus of a liquid chemical according to a first embodiment of the present invention. 
     FIG. 4A is a schematic side view of a measurement cell of the spectrophotometer used in the apparatus according to the first embodiment of FIG.  3 . 
     FIG. 4B is a schematic front view of the measurement cell shown in FIG.  4 A. 
     FIG. 5A is a schematic side view of another measurement cell of the spectrophotometer used in the apparatus according to the first embodiment of FIG.  3 . 
     FIG. 5B is a schematic front view of the measurement cell shown in FIG.  5 A. 
     FIG. 6 is a flow chart showing the operation of the concentration control apparatus according to the first embodiment of FIG.  3 . 
     FIG. 7 is a timing diagram showing the operation of the concentration control apparatus according to the first embodiment of FIG. 3, in which the time-dependent change of the concentration of the liquid chemical is shown. 
     FIG. 8 is a diagram showing the operation of the concentration detector of the concentration control apparatus according to the first embodiment of FIG. 3, in which a calibration curve or line is used. 
     FIG. 9 is a schematic cross-sectional view of a concentration control apparatus of a liquid chemical according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail below while referring to the drawings attached. 
     FIRST EMBODIMENT 
     A concentration control apparatus of liquid chemicals according to a first embodiment of the present invention is shown in FIG. 3, which is comprised of a liquid chemical supplier section  1 , a wafer treater section  2 , and a data processor section  3 . 
     In the liquid chemical supplier section  1 , storage tanks  4   a  and  4   b  are provided for storing source liquids S 1  and S 2 , respectively. Pumps  5   a  and  5   b  are respectively connected to the tanks  4   a  and  4   b  through communication lines or pipes  14   a  and  14   b . The pipes  14   a  and  14   b  are coupled together and connected in common to a valve  9 . In other words, the pumps  5   a  and  5   b  are connected in common to the valve  9  through the pipes  14   c  and  14   d . The valve  9  is further connected to a concentration detector  15  through a communication line or pipe  14   e . The detector  15  is further connected to a nozzle  13  of the wafer treater section  2  through a communication line or pipe  14   f . 
     The source liquids S 1  and S 2  stored in the tanks  4   A  AND  4   B  are respectively sucked up by the pumps  5   a  and  5   b  at specific suction or pumping rates while the valve  9  is opened. Then, they are automatically mixed with each other at the coupling part of the pipes  14   c  and  14   d  before the valve  9 , thereby producing the mixture of these liquids S 1  and S 2  as a liquid chemical SO. The mixture of the liquids S 1  and S 2  (i.e., the liquid chemical SO) is sent to the nozzle  13  through the concentration detector  15  and then, it is sprayed from the nozzle  13  toward a semiconductor wafer  12  located in the wafer treater section  2 . 
     The liquid chemical SO is, for example, a cleaning liquid for cleaning the wafer  12  or an etching liquid for etching the same. 
     Moreover, in the liquid chemical supplier section  1 , a storage tank  6  is provided for storing a reference liquid S 3 . A pump  7  is connected to the tank  6  through a communication line or pipe  14   g . The pump  7  is further connected to a valve  8  through a communication line or pipe  14   h . The valve  8  is further connected to the pipe  14   e  communicating with the concentration detector  15  and the nozzle  13  through a communication line or pipe  14   i . 
     The reference liquid S 3  stored in the tank  6  is sucked up by the pump  7  at a specific suction rate while the valves  8  and  9  are opened and then, it is sent to the nozzle  13  through the concentration detector  15 . The detector  15  detects the concentration of the reference liquid S 3  and sends the concentration data to the data processor  3 . The concentration data thus obtained is used for calibration. 
     The wafer treater section  2  includes a wafer stage  16  on which the semiconductor wafer  12  to be treated is placed and the nozzle  13  which communicates with the liquid chemical supplier section  1 . The nozzle  13  and the stage  16  are located in a treatment chamber  17 . 
     The data processor section  3  is comprised of a data converter  10  and a pump controller  11 . The data converter  10  is electrically connected to the concentration detector  15  located in the flowing path connecting the wafer treater section  2  with the liquid chemical supplier section  1 . The concentration detector  15  outputs a concentration or composition data signal SAD about the liquid chemical SO flowing through the detector  15  to the data converter  10  at each detection timing. The data converter  10  receives the concentration/composition data signal SAD and converts it to a control data signal CLD. The pump controller  11  receives the control data signal CLD and then, sends two control signals CL 1  and CL 2  to the pumps  5   a  and  5   b  in the chemical liquid supplier section  1 , respectively, thereby controlling the discharging or pumping rates of the pumps  5   a  and  5   b  according to the concentration data signal SAD sent from the detector  15 . Thus, the concentration and/or composition of the liquid chemical SO (i.e., the mixture of the two source liquids S 1  and S 2 ) is always kept at a desired value or within a desired range. 
     The concentration detector  15  measures or detects the concentration or composition of the liquid chemical SO or reference liquid S 3  by measuring the absorption wavelengths (i.e., the absorption characteristic) of the detection light in the liquid S) or S 3  using a known near-infrared, visible, or ultraviolet spectrophotometer. 
     FIGS. 4A and 4B show an example of the measurement cell  41  used in the near-infrared, visible, or ultraviolet spectrophotometer serving as the detector  15 . As shown in FIG. 4B, this cell  41  has a square cross-section having four equal side lengths that is equal to the optical path length L of the incident or detection light. The length L is set as a specific value. The two opening ends of the cell  41  are connected to the pipes  14   e  and  14   f , respectively. 
     FIGS. 5A and 5B show another example of the measurement cell  41 A used in the detector  15 . Unlike the cell  41 , this cell  41 A has a circular cross-section having a diameter that is equal to the optical path length L. The tow opening ends of the cell  41 A are connected to the pipes  14   e  and  14   f , respectively. 
     Each of the cells  41  and  41 A is made of a material having no absorption within the near-infrared, visible, or ultraviolet region, a typical one of which is quartz. 
     If the optical path length L is known and the optical path of the detection light is fixed to penetrate the same position of the cell  41  or  41 A, the cross-sectional shape may be circular or polygonal due to the Lambert-Beer Law. This Law states that when monochromatic light propagates through a substance or medium with light-absorption property, the intensity of the light decreases exponentially with respect to the propagation distance through the substance, in other words, the intensity change of the light is expressed by an exponential function with respect to the thickness of the substance and a constant relating to the sort of the substance and the wavelength of the light. 
     To perform the calibration operation of the concentration or composition of the liquid chemical SO, the reference liquid S 3  is used. Specifically, the value  8  is opened and then, the reference liquid S 3  is pumped up by the pump  7  while the valve  9  is closed, thereby supplying the liquid S 3  to the nozzle  13 . At this time, the concentration detector  15  detects the concentration of the reference liquid S 3  and sends the concentration data signal SAD to the data processor  3 . By comparing the concentration of the liquid chemical SO with that of the reference liquid S 3 , the concentration calibration can be accomplished. 
     Next, the operation of the concentration control apparatus according to the first embodiment is explained below with reference to FIG.  6 . 
     While the liquid chemical SO is supplied to the wafer treater section  2  through the pipes  14   e  and  14   f , the concentration detector  15  always detects the concentration or composition of the chemical SO flowing through the measurement cell  41  or  41 A of the detector  15  and then, it outputs the concentration data signal SAD to the data processor  3  (Step  31 ). The concentration or composition value obtained by this measurement is displayed on the screen (not shown) of the processor  3  (Step  32 ). 
     As seen from FIG. 3, the detector  15  is located apart from the pumps  5   a  and  5   b  provided upstream for pumping the source liquids S 1  and S 2  in the tanks  4   a  and  4   b  by a comparatively long distance. Therefore, even if the discharge or pumping rate of the pumps  5   a  and/or  5   b  is changed, the concentration change of the liquid chemical SO (i.e., the mixture of the source liquids S 1  and S 2 ) occurs after a time lag. Accordingly, it is judged whether or not a specific concentration stabilization period Δt has not passed yet, the flow is returned from the step  33  to the step  31 . If the time Δt has already passed, the next step  34  is carried out, in which the measured concentration value of the liquid chemical SO is compared with its preset value. Then, in the next Step  35 , it is judged whether or not the discharging or pumping rate of the pump  5   a  and/or  5   b  needs to be changed. If the measured concentration value is included within a specific permissible range, the flow is returned from the step  35  to the step  31 . On the other hand, if the measured concentration value is outside the specific permissible range, the next step  36  is carried out in order to increase or decrease the discharging rate of at least one of the pumps  5   a  and  5   b . 
     In the step  36 , the discharging rate change of the pumps  5   a  and/or  5   b  is accomplished using the calibration line X as shown in FIG.  8 . The data of the line X is inputted into the data converter  10  in advance. For example, it is supposed in FIG. 8 that when the discharging rates of the pumps  5   a  and  5   b  and V 1  and V 2 , the concentrations of the source liquids S 1  and S 2  are C 1  and C 2 , respectively. In this case, the concentrations and discharging rates are expressed by the point C, where the ordinates (i.e., the concentrations of the liquids S 1  and S 2  or their composition ratio) are C 1  and C 2 . Then, if, due to some cause, the concentration of the source liquid S 1  is decreased to C 1 ′ and at the same time, that of the source liquid S 2  is increased to C 2 ′, the point C is shifted along the calibration line X to the point C′, where the concentrations (i.e., the composition ratio) of the liquids S 1  and S 2  are given as C 1 ′ and C 2 ′. At this point C′, the discharging rates of the pumps  5   a  and  5   b  are given as V 1 ′ and V 2 ′, respectively. As seen from FIG. 8, the necessary change of the discharging rate of the pump  5   a  is −ΔV and that of the pump  5   b  is +ΔV. 
     As a result, in the step  37 , the data converter  10  of the data processor  3  outputs the control signal CLD reflecting this calibration result to the pump controller  11  and then, the controller  11  outputs the control signals CL 1  and CL 2  to the pumps  5   a  and  5   b  in such a way that the discharging rates of the pumps  5   a  and  5   b  are changed to be V 1 ′ and V 2 ′, respectively. Thus, the concentration or composition of the liquid chemical SO can be kept at the preset value or within the preset range. 
     FIG. 7 shows a timing diagram showing the operation of the apparatus according to the first embodiment of FIG.  3 . 
     As seen from FIG. 7, it is supposed that the concentration or composition calibration of the liquid chemical SO is carried out at the time t 0 . In this case, the same calibration operation occurs at the time t 1  after the specific stabilization period Δt is passed. During the period Δt between the times t 0  and t 1 , the measured concentration is increased from C B  less than the preset concentration value C O  to C A  greater than C O . In this period Δt, the data converter  10  simply displays the value of the measured concentration or composition of the chemical SO. 
     Similarly, the same calibration operation occurs at the time t 2  after the specific stabilization period Δt is passed from t 1 . During the period Δt between the times t 1  and t 2 , the measured concentration is decreased from C A  to C B . In this period Δt, the data converter  10  simply displays the value of the measured concentration or composition of the chemical SO. 
     The same calibration operation occurs at the time t 3  after the specific stabilization period Δt is passed from t 2 . During the period Δt between the times t 2  and t 3 , the measured concentration is kept at the preset value C O . In this period Δt, the data converter  10  simply displays the value of the measured concentration or composition of the chemical SO. 
     Between the times t 4  and t 5 , the wafer  12  in the chamber  17  of the wafer treater section  2  is replaced with a new one. Thereafter, the same measurement operation is repeated intermittently. 
     With the concentration control apparatus of a liquid chemical according to the first embodiment of FIG. 3, the liquid chemical supplier section  1  is connected to the wafer treater section  2 , and the concentration detector  15  is located in the communication line between the sections  1  and  2  to detect the concentration and/or composition of the liquid chemical SO flowing through the detector  15 . Accordingly, real-time concentration/composition control of the liquid chemical SO can be realized without sampling the same. This solves the sampling-induced problem of the above-described prior-art apparatus shown in FIG.  2 . 
     Also, since the concentration/composition of the liquid chemical SO can be real-time monitored by the concentration detector  15 , the deterioration of the liquid chemical SO and the abnormal concentration or composition thereof can be found immediately. This minimizes the damage to be caused by the unallowable concentration and/or composition of the liquid chemical SO. 
     Furthermore, based on the data signal SAD about the concentration and/or composition of the liquid chemical SO, the data processor section  3  outputs in real time the control signals CL 1  and CL 2  to the liquid chemical supplier section  1 , thereby controlling the concentration and/or composition of the liquid chemical SO supplied from the liquid chemical supplier section  1 . As a result, the concentration/composition of the liquid chemical SO can be controlled precisely. 
     In the apparatus according to the first embodiment, as described above, the mixture of the source liquids S 1  and S 2  is kept at the desired concentration, resulting in the desired composition of the liquid chemical SO. However, the measured concentration may be simply displayed for monitoring without the concentration adjustment. 
     SECOND EMBODIMENT 
     In the concentration control apparatus according to the first embodiment of FIG. 3, the concentration correction of the liquid chemical SO is performed by comparing the concentration of the chemical SO with that of the reference liquid S 3 . However, it can be realized without using the reference liquid S 3  as explained in the following way. 
     FIG. 9 shows a concentration control apparatus according to a second embodiment. This apparatus has the same configuration as that of the apparatus according to the first embodiment, except that a pure water tank  66 , a pump  67  for sucking the pure water S 4  from the tank  66 , and a valve  68  for controlling the flow of the pure water S 4  are provided, instead of the reference liquid tank  6 , the pump  7 , and the valve  8 . Therefore, the explanation about the same configuration is omitted here by attaching the same reference symbols as those used in the first embodiment to the same elements in FIG.  9 . 
     As seen from FIG. 9, the pure water tank  66 , the pump  67 , and the valve  68  are located outside the liquid chemical supplier section  1 . However, they may be located in the section  1 . The tank  66  is connected to the pump  67  through a pipe  64   a . The pump  67  is connected to the valve  68  through a pipe  64   b . The valve  68  is connected to the pipe  14   e  communicating with the concentration detector  15 . 
     After the valve  9  is closed, the valve  68  is opened and the pump  67  sucks the pure water S 4  from the tank  66 , thereby replacing the liquid chemical SO existing in the detector  15  with the pure water S 4 . Thereafter, the pure water S 4  in the detector  15  is evacuated therefrom and the inside of the detector  15  is dried. Subsequently, the detector  15  performs its detection and measurement operations with respect to no liquid, in other words, it carries out so-called “blank measurement”. Thus, the concentration/composition calibration of the liquid chemical SO can be attained. 
     Needless t say, the apparatus according to the second embodiment of FIG. 9 has the same advantages as those in the first embodiment. 
     While the preferred forms of the present invention have been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of the present invention, therefore, is to be determined solely by the following claims.