Patent Publication Number: US-9842732-B2

Title: Substrate cleaning apparatus and substrate cleaning method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 14/061,686 filed Oct. 23, 2013; the entire contents are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a substrate cleaning apparatus and a substrate cleaning method for performing scrub cleaning of a surface of a substrate, such as a semiconductor wafer, with an elongated cylindrical roll cleaning member extending horizontally, by rotating the substrate and the roll cleaning member each in one direction while keeping the roll cleaning member in contact with the surface of the substrate. 
     Description of the Related Art 
     In a cleaning apparatus for performing scrub cleaning of a surface of a substrate, such as a semiconductor wafer, with a roll cleaning member, the roll cleaning member is rotated and pressed against the substrate under a predetermined pressing load (roll load) during cleaning of the substrate. Controlling and adjusting the roll load to an appropriate value is important to increase the cleanliness of the substrate or to prevent the substrate from being damaged. 
     Therefore, the applicant of the present invention has proposed a substrate cleaning apparatus having a closed-loop feedback control (CLC) system in which the pressing load (roll load) for pressing the roll cleaning member against the substrate is measured by a load cell during cleaning of the substrate to control a regulating device of an actuator, such as a motor, based on the measured values, thereby controlling the roll load to an appropriate value by feedback control, as disclosed in Japanese laid-open patent publication No. 2002-50602. 
     In the case where the pressing load (roll load) for pressing the roll cleaning member against the substrate is measured by the load cell to control the roll load based on the measured values by feedback control during cleaning of the substrate, if the roll load is not measured accurately due to breakdown of the load cell, an appropriate roll load cannot be applied to the substrate by the roll cleaning member. 
     Specifically, if the measured value (measured roll load) of the load cell becomes larger than the actual roll load due to breakdown of the load cell, a roll load which is smaller than the predetermined roll load is applied to the substrate by the roll cleaning member, thus reducing the cleaning ability for the substrate. On the other hand, if the measured value (measured roll load) of the load cell becomes smaller than the actual roll load, a roll load which is larger than the predetermined roll load is applied to the substrate by the roll cleaning member, thus possibly causing breakage of the substrate. 
     SUMMARY OF THE INVENTION 
     Based on the above knowledge obtained from various experiments, the present invention has been made. It is therefore an object of the present invention to provide a substrate cleaning apparatus and a substrate cleaning method which can rapidly detect breakdown of a load cell for measuring a roll load during cleaning of a substrate, to prevent the cleaning of the substrate, in a state in which an abnormal roll load is applied to the substrate by a roll cleaning member, from being continued. 
     In order to achieve the above object, according to an aspect of the present invention, there is provided a substrate cleaning apparatus for cleaning a substrate, comprising: a roll holder configured to support a horizontally elongated roll cleaning member and rotate the roll cleaning member; a vertical movement mechanism configured to vertically move the roll holder so that the roll cleaning member applies a roll load to the substrate at the time of cleaning the substrate by actuation of an actuator having a regulating device; a load cell configured to measure the roll load; a controller configured to perform feedback control of the roll load through the regulating device based on the measured value of the load cell; and a monitor unit configured to monitor whether an operation amount of the regulating device falls outside an allowable range of a preset reference value of an operation amount corresponding to a preset roll load. 
     If the load cell for measuring the roll load breaks down to cause an error in the measured value of the load cell, the operation amount of the regulating device is deviated from the preset reference value of the operation amount corresponding to the preset roll load. Therefore, the monitor unit monitors whether the operation amount of the regulating device falls outside the allowable range of the preset reference value of the operation amount corresponding to the preset roll load, thereby detecting breakdown of the load cell promptly during cleaning of the substrate. 
     In a preferred aspect of the present invention, the substrate cleaning apparatus further comprises an alarm configured to issue an alarm when the operation amount of the regulating device falls outside the allowable range of the preset reference value of the operation amount corresponding to the preset roll load. 
     In a preferred aspect of the present invention, the actuator comprises an air cylinder, and the regulating device comprises an electropneumatic regulator configured to control a valve opening degree of a regulating valve for regulating a pressure of air to be supplied to the air cylinder. 
     According to another aspect of the present invention, there is provided a substrate cleaning method for cleaning a substrate, comprising: applying a roll load to the substrate by a horizontally elongated roll cleaning member configured to be moved vertically by actuation of an actuator having a regulating device; measuring the roll load applied to the substrate by a load cell; and monitoring whether an operation amount of the regulating device falls outside an allowable range of a preset reference value of an operation amount corresponding to a preset roll load while performing feedback control of the roll load through the regulating device based on the measured value of the load cell. 
     In a preferred aspect of the present invention, the substrate cleaning method further comprises issuing an alarm when the operation amount of the regulating device falls outside the allowable range of the preset reference value of the operation amount corresponding to the preset roll load. 
     According to the present invention, the monitor unit monitors whether the operation amount of the regulating device falls outside the allowable range of the preset reference value of the operation amount corresponding to the preset roll load, thereby detecting breakdown of the load cell promptly during cleaning of the substrate. In this manner, the cleaning of the substrate in a state in which an abnormal roll load is applied to the substrate by the roll cleaning member can be prevented from being continued. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view showing an entire structure of a substrate processing apparatus incorporating a substrate cleaning apparatus according to an embodiment of the present invention; 
         FIG. 2  is a schematic perspective view showing the substrate cleaning apparatus, according to an embodiment of the present invention, which is incorporated in the substrate processing apparatus shown in  FIG. 1 ; 
         FIG. 3  is a schematic elevational view showing an entire structure of the substrate cleaning apparatus according to the embodiment of the present invention; 
         FIG. 4  is a graph showing an example of the relationship between a preset roll load (N), a reference value of a valve-operation-amount (kPa) of an electropneumatic regulator, and an allowable range of the reference value; 
         FIG. 5  is a graph showing another example of the relationship between the preset roll load (N), the reference value of the valve-operation-amount (kPa) of the electropneumatic regulator, and the allowable range of the reference value; and 
         FIG. 6  is a flow chart showing an embodiment of control for cleaning the front surface (upper surface) of the substrate while performing feedback control of a roll load applied to the substrate by an upper roll cleaning member of the substrate cleaning apparatus shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     A substrate cleaning apparatus and a substrate cleaning method according to embodiments of the present invention will be described below with reference to  FIGS. 1 through 6 . 
       FIG. 1  is a plan view showing an entire structure of a substrate processing apparatus incorporating a substrate cleaning apparatus according to an embodiment of the present invention. As shown in  FIG. 1 , the substrate processing apparatus includes a generally-rectangular housing  10 , and a loading port  12  for placing thereon a substrate cassette storing a large number of substrates, such as semiconductor wafers. The loading port  12  is disposed adjacent to the housing  10  and is capable of placing thereon an open cassette, a SMIF (standard manufacturing interface) pod or a FOUP (front opening unified pod). Each of the SMIF and the FOUP is a hermetically sealed container which houses therein a substrate cassette and is covered with a partition wall, and thus can keep independent internal environment isolated from an external space. 
     In the housing  10 , there are provided a plurality of (four in this embodiment) polishing units  14   a ,  14   b ,  14   c ,  14   d , a first substrate cleaning unit  16  and a second substrate cleaning unit  18  each for cleaning a substrate after polishing, and a substrate drying unit  20  for drying a substrate after cleaning. The polishing units  14   a ,  14   b ,  14   c ,  14   d  are arranged in the longitudinal direction of the substrate processing apparatus, and the substrate cleaning units  16 ,  18  and the substrate drying unit  20  are also arranged in the longitudinal direction of the substrate processing apparatus. The substrate cleaning apparatus according to the embodiment of the present invention is applied to the first substrate cleaning unit  16 . 
     A first substrate transfer robot  22  is disposed in an area surrounded by the loading port  12 , and the polishing unit  14   a  and the substrate drying unit  20  which are located near the loading port  12 . Further, a substrate transport unit  24  is disposed in parallel to the polishing units  14   a ,  14   b ,  14   c ,  14   d . The first substrate transfer robot  22  receives a substrate before polishing from the loading port  12  and transfers the substrate to the substrate transport unit  24 , and receives a substrate after drying from the substrate drying unit  20  and returns the substrate to the loading port  12 . The substrate transport unit  24  transports a substrate transferred from the first substrate transfer robot  22 , and transfers the substrate between the substrate transport unit  24  and the polishing units  14   a ,  14   b ,  14   c ,  14   d.    
     Between the first substrate cleaning unit  16  and the second substrate cleaning unit  18 , there is provided a second substrate transfer robot  26  for transferring a substrate between the first substrate cleaning unit  16  and the second substrate cleaning unit  18 . Between the second substrate cleaning unit  18  and the substrate drying unit  20 , there is provided a third substrate transfer robot  28  for transferring a substrate between the second substrate cleaning unit  18  and the substrate drying unit  20 . In the housing  10 , there is provided a control panel (operation panel)  30  for setting a controller  66 , inputting a preset value of the roll load (instructing a preset roll load), and the like, described below. 
       FIG. 2  is a schematic perspective view showing the substrate cleaning apparatus (first substrate cleaning unit  16 ), according to an embodiment of the present invention, incorporated in the substrate processing apparatus shown in  FIG. 1 .  FIG. 3  is a schematic elevational view showing an entire structure of the substrate cleaning apparatus  16  according to the embodiment of the present invention. 
     As shown in  FIGS. 2 and 3 , the substrate cleaning unit  16  includes a plurality of (four as illustrated) horizontally movable spindles  40  for supporting a periphery of a substrate W, such as a semiconductor wafer, with its front surface facing upwardly, and horizontally rotating the substrate W, a vertically movable upper roll holder  42  disposed above the substrate W rotatably supported by the spindles  40 , and a vertically movable lower roll holder  44  disposed below the substrate W rotatably supported by the spindles  40 . 
     An elongated cylindrical upper roll cleaning member (roll sponge)  46  made of PVA or the like, is rotatably supported by the upper roll holder  42 . The upper roll cleaning member  46  is rotated by a driving mechanism (not shown) in the direction shown by the arrow F 1  in  FIG. 2 . An elongated cylindrical lower roll cleaning member (roll sponge)  48 , made of PVA or the like, is rotatably supported by the lower roll holder  44 . The lower roll cleaning member  48  is rotated by a driving mechanism (not shown) in the direction shown by the arrow F 2  in  FIG. 2 . 
     An upper cleaning liquid supply nozzle  50  for supplying a cleaning liquid onto the front surface (upper surface) of the substrate W is disposed above the substrate W rotatably supported by the spindles  40 . A lower cleaning liquid supply nozzle  52  for supplying a cleaning liquid onto the back surface (lower surface) of the substrate W is disposed below the substrate W rotatably supported by the spindles  40 . 
     A concave portion  42   a  is provided at a substantially central area along the longitudinal direction of the upper roll holder  42 , and a load cell  54  is located inside of the concave portion  42   a  and is fixed to the upper roll holder  42 . In this example, there is provided a vertical movement mechanism  60  comprising an air cylinder  56  disposed in the vertical direction to serve as an actuator, a vertically movable shaft  57  vertically movable by actuation of the air cylinder (actuator)  56 , and a vertically movable arm  58 , as a vertically movable unit, extending in the horizontal direction and having a base end connected to the upper end of the vertically movable shaft  57 . The upper roll holder  42  is coupled via the load cell  54  to a lower end of a free end side of the vertically movable arm (vertically movable unit)  58 . A tilting mechanism  70  for tilting the upper roll holder  42  is disposed between the load cell  54  and the lower surface of the free end side of the vertically movable arm  58 . 
     With this configuration, the upper roll holder  42  is vertically moved together with the vertically movable shaft  57  and the vertically movable arm  58 , by actuation of the air cylinder  56 . The air cylinder  56  is provided with an electropneumatic regulator  62 , as a regulating device, for regulating a pressure of air to be supplied to an interior of the air cylinder  56 . By adjusting a valve opening degree of the electropneumatic regulator (regulating device)  62 , the pressure of air to be supplied into the air cylinder  56  is regulated. 
     In this manner, the upper roll holder  42  is coupled to the lower surface of the free end side of the vertically movable arm  58  at the substantially central area along the longitudinal direction of the upper roll holder  42  so that a vertical line, passing through the center of gravity of the upper roll holder  42  which supports and rotates the upper roll cleaning member  46 , passes through the center of the load cell  54  or a location close to the center of the load cell  54 , thereby coupling the upper roll holder  42  to the lower surface of the free end side of the vertically movable arm  58  in a horizontal state and a well-balanced manner. 
     Further, the own weight of the upper roll holder  42  can be transmitted to the load cell  54  without any loss by coupling the upper roll holder  42  to the lower surface of the free end side of the vertically movable arm  58  via the load cell  54 . When the upper roll holder  42  is lowered to bring the upper roll cleaning member  46  into contact with the substrate W at the time of cleaning the substrate W, a tensile load applied to the load cell  54  is reduced by a certain amount which substantially coincides with the roll load (pressing load) applied to the substrate W by the upper roll cleaning member  46 . 
     Accordingly, the roll load applied to the substrate W by the upper roll cleaning member  46  during cleaning of the substrate W is measured by the load cell  54  based on the reduced tensile load and the valve opening degree is adjusted by an operation amount of the electropneumatic regulator  62 , thereby regulating the roll load. 
     The measured value measured by the load cell  54  is outputted to an indicator  64 , and an analog signal is sent from the indicator  64  to the controller  66  as a control unit. Then, the analog signal sent from the controller (control unit)  66  is inputted to the electropneumatic regulator  62 . Thus, a closed-loop control system for performing a closed-loop control is constructed. Further, a preset value of the roll load (preset roll load) and the like are inputted from the control panel (operation panel)  30  to the controller  66 . 
     With this configuration, the controller  66  compares the measured value (measured roll load) measured by the load cell  54  and the preset roll load inputted from the control panel (operation panel)  30  and gives the electropneumatic regulator  62  an instruction of an operation amount of a regulating valve (valve-operation-amount) for the air cylinder, depending on a difference between the measured roll load and the preset roll load. The electropneumatic regulator  62  automatically adjusts the valve opening degree of the regulating valve in response to the instruction from the controller  66  to vary a thrust force of the air cylinder  56  with the adjusted valve opening degree, thereby performing feedback control of the roll load applied to the substrate W during cleaning of the substrate W. 
     According to this example, the load cell  54  is provided between the vertically movable arm  58  of the vertical movement mechanism  60  and the upper roll holder  42  coupled to the vertically movable arm  58 , and the load cell  54  is configured to receive the own weight of the upper roll holder  42 . Further, a bearing or a link rod which increases friction during vertical movement of the upper roll holder  42 , or a beam structure or projection which causes a loss in load transmission, is not provided between the upper roll holder  42  and the load cell  54 . Thus, the roll load applied to the substrate W during cleaning of the substrate is transmitted to the load cell  54  accurately, thereby measuring the roll load with high accuracy and controlling the roll load. 
     If the load cell  54  breaks down during cleaning of the substrate W while measuring the roll load by the load cell  54 , the accurate roll load cannot be measured, and thus an appropriate roll load cannot be applied to the substrate W by the upper roll cleaning member  46 . Accordingly, the breakdown of the load cell  54  causes an error in the measured value (measured roll load) of the load cell  54 , and thus a current valve-operation-amount of the electropneumatic regulator  62 , i.e. a valve-operation-amount outputted from the controller  66  to the electropneumatic regulator  62 , is deviated from a preset reference value of the valve-operation-amount depending on the preset roll load. 
     Thus, in this example, a monitor unit  80  connected to the controller  66  monitors whether the valve-operation-amount outputted to the electropneumatic regulator  62  falls outside an allowable range of the preset reference value of the valve-operation-amount depending on the preset roll load, thereby detecting breakdown of the load cell  54  promptly during cleaning of the substrate W. 
     Specifically, the reference value of the valve-operation-amount for each of the preset roll loads inputted from the control panel  30  to the controller  66 , is inputted in advance and stored in the monitor unit  80 , and the valve-operation-amount outputted from the controller  66  to the electropneumatic regulator  62  is inputted to the monitor unit  80 . Then, the monitor unit  80  monitors whether the valve-operation-amount outputted to the electropneumatic regulator  62  falls outside the allowable range of the reference value of the valve-operation-amount corresponding to the preset roll load inputted from the control panel  30  to the controller  66 . When the monitor unit  80  detects breakdown of the load cell  54 , the monitor unit  80  outputs a signal to an alarm  82  connected to the monitor unit  80 , and then the alarm  82  issues an alarm in response to the signal. 
       FIG. 4  shows an example of the relationship between the preset roll load (N), the reference value of the valve-operation-amount (kPa) of the electropneumatic regulator, and the allowable range of the reference value. In this example, the reference values of the valve-operation-amount (kPa), of the electropneumatic regulator, corresponding to the respective preset roll loads (N) are shown by a linearly-extending line A. A linearly extending line B obtained by subtracting a constant value from the line A shows lower thresholds, and a linearly extending line C obtained by adding a constant value to the line A shows upper thresholds. A range between the line B (lower thresholds) and the line C (upper thresholds) is determined as an allowable range. 
     Specifically, in the case where the preset roll load instructed from the control panel  30  to the controller  66  is R (N), the reference value of the valve-operation-amount of the electropneumatic regulator  62  corresponding to the preset roll load is S (kPa) which is an intersection of the preset roll load R with the line A, and the allowable range of the reference value S of the valve-operation-amount is the range (T 1  to T 2 ) between the lower threshold T 1  (kPa) which is an intersection of the preset roll load R with the line B and the upper threshold T 2  (kPa) which is an intersection of the preset roll load R with the line C. 
     Each of the preset roll loads (N) and each of the reference values of the valve-operation-amount (kPa) of the electropneumatic regulator may be one-to-one correspondence. 
     When the substrate W is actually being cleaned while controlling the roll load applied to the substrate W by the upper roll cleaning member  46  to the preset roll load R, the monitor unit  80  judges that the load cell  54  operates normally if the valve-operation-amount (kPa) outputted from the controller  66  to the electropneumatic regulator  62  falls within the allowable range (T 1  to T 2 ), and judges that the load cell  54  breaks down if the valve-operation-amount falls outside the allowable range (T 1  to T 2 ), i.e. the valve-operation-amount becomes smaller than the lower threshold T 1  or larger than the upper threshold T 2 . 
       FIG. 5  shows another example of the relationship between the preset roll load (N), the reference value of the valve-operation-amount (kPa) of the electropneumatic regulator, and the allowable range of the reference value. In this example, the reference values of the valve-operation-amount (kPa), of the electropneumatic regulator, corresponding to the respective preset roll loads (N) are shown by a linearly-extending line A. A linearly extending line D obtained by subtracting a constant percentage (%) from the line A shows lower thresholds, and a linearly extending line E obtained by adding a constant percentage (%) to the line A shows upper thresholds. A range between the line D (lower threshold) and the line E (upper threshold) is determined as an allowable range. 
     Specifically, in the case where the preset roll load instructed from the control panel  30  to the controller  66  is R (N), the reference value of the valve-operation-amount of the electropneumatic regulator  62  corresponding to the preset roll load is S (kPa) which is an intersection of the preset roll load R with the line A, and the allowable range of the reference value S of the valve-operation-amount is the range (T 3  to T 4 ) between the lower threshold T 3  (kPa) which is an intersection of the preset roll load R with the line D and the upper threshold T 4  (kPa) which is an intersection of the preset roll load R with the line E. 
     As shown in  FIG. 3 , a concave portion  44   a  is provided at a substantially central area along the longitudinal direction of the lower roll holder  44 . The lower roll holder  44  has a vertical movement mechanism  60   a  comprising an air cylinder  56   a  disposed in the vertical direction to serve as an actuator, and a vertically movable shaft  59  as a vertically movable unit vertically movable by actuation of the air cylinder (actuator)  56   a . The lower roll holder  44  is coupled via a load cell  54   a  to an upper end surface of the vertically movable shaft (vertically movable unit)  59 . With this configuration, the lower roll holder  44  is vertically moved together with the vertically movable shaft  59  by actuation of the air cylinder  56   a . The air cylinder  56   a  is provided with an electropneumatic regulator  62   a  serving as a regulating device, in the same manner as the above. A tilting mechanism  70   a  for tilting the lower roll holder  44  is disposed between the load cell  54   a  fixed to the upper end surface of the vertically movable shaft  59  and the lower roll holder  44 . 
     In this manner, the lower roll holder  44  is coupled to the upper end surface of the vertically movable shaft  59  at the substantially central area along the longitudinal direction of the lower roll holder  44  so that a vertical line, passing through the center of gravity of the lower roll holder  44  which supports and rotates the lower roll cleaning member  48 , passes through the center of the load cell  54   a  or a location closed to the center of the load cell  54   a , thereby coupling the lower roll holder  44  to the vertically movable shaft  59  in a horizontal state and a well-balanced manner. 
     Further, the own weight of the lower roll holder  44  can be transmitted to the load cell  54   a  without any loss by coupling the lower roll holder  44  to the upper end surface of the vertically movable shaft  59  via the load cell  54   a . When the lower roll holder  44  is lifted to bring the lower roll cleaning member  48  into contact with the substrate W, a compressive load applied to the load cell  54   a  is increased by a certain amount which substantially coincides with the roll load (pressing load) applied to the substrate W by the lower roll cleaning member  48 . 
     Accordingly, the roll load applied to the substrate W by the lower roll cleaning member  48  during cleaning of the substrate W is measured by the load cell  54   a  based on the increased compressive load and the valve opening degree is adjusted by an operation amount of the electropneumatic regulator (regulating device)  62   a , thereby regulating the roll load. 
     The measured value measured by the load cell  54   a  is outputted to an indicator  64   a , and an analog signal is sent from the indicator  64   a  to the controller  66 . Then, the analog signal sent from the controller  66  is inputted to the electropneumatic regulator  62   a . Thus, a closed-loop control system for performing a closed-loop control is constructed. Further, a preset value of the roll load (preset roll load) is inputted from the control panel (operation panel)  30  to the controller  66 . 
     With this configuration, the controller  66  compares the measured value (measured roll load) measured by the load cell  54   a  and the preset roll load inputted from the control panel (operation panel)  30  and gives the electropneumatic regulator  62   a  an instruction of an operation amount of a regulating valve (valve-operation-amount) depending on a difference between the measured roll load and the preset roll load. The electropneumatic regulator  62   a  automatically adjusts the valve opening degree in response to the instruction from the controller  66  to vary a thrust force of the air cylinder  56   a  with the adjusted valve opening degree, thereby performing feedback control of the roll load applied to the substrate W during cleaning of the substrate W. 
     According to this example, the load cell  54   a  is provided between the vertically movable shaft  59  of the vertical movement mechanism  60   a  and the lower roll holder  44  coupled to the vertically movable shaft  59  and the load cell  54   a  is configured to receive the own weight of the lower roll holder  44 . Further, a bearing or a link rod which increases friction during vertical movement of the lower roll holder  44 , or a beam structure or a projection which causes a loss in load transmission, is not provided between the lower roll holder  44  and the load cell  54   a . Thus, the roll load applied to the substrate W during cleaning of the substrate is transmitted to the load cell  54   a  accurately, thereby measuring the roll load with high accuracy and controlling the roll load. 
     If the load cell  54   a  breaks down during cleaning of the substrate W while measuring the roll load by the load cell  54   a , the accurate roll load cannot be measured, and thus an appropriate roll load cannot be applied to the substrate W by the lower roll cleaning member  44 . 
     Therefore, as described above, the monitor unit  80  connected to the controller  66  monitors whether the valve-operation-amount of the electropneumatic regulator  62   a  falls outside an allowable range of the reference value of the valve-operation-amount depending on the preset roll load, thereby detecting breakdown of the load cell  54   a  promptly during cleaning of the substrate W. When the monitor unit  80  detects the breakdown of the load cell  54   a , an alarm is issued from the alarm  82 . 
     In the substrate cleaning apparatus (scrub cleaning apparatus) having the above structure, as shown in  FIG. 2 , a peripheral portion of the substrate W is located in an engagement groove  90   a  formed in a circumferential surface of a spinning top  90  provided at an upper portion of each of the spindles  40 . By spinning (rotating) the spinning tops  90  while pressing them inwardly against the peripheral portion of the substrate W, the substrate W is rotated horizontally in the direction shown by the arrow E in  FIG. 2 . In this embodiment, two of the four spinning tops  90  apply a rotational force to the substrate W, while the other two spinning tops  90  function as a bearing for supporting the rotation of the substrate W. It is also possible to couple all the spinning tops  90  to a drive mechanism so that they all apply a rotational force to the substrate W. 
     While horizontally rotating the substrate W and supplying a cleaning liquid (chemical liquid) from the upper cleaning liquid supply nozzle  50  to the front surface (upper surface) of the substrate W, the upper roll cleaning member  46  is rotated and lowered to be brought into contact with the front surface of the rotating substrate W under a predetermined roll load, thereby performing scrub cleaning of the front surface of the substrate W with the upper roll cleaning member  46  in the presence of the cleaning liquid. The length of the upper roll cleaning member  46  is set to be slightly larger than the diameter of the substrate W, and thus the entire length of the front surface of the substrate W in a diametrical direction, from one end to the other end of the substrate W can be cleaned at the same time. 
     When the front surface of the substrate W is scrub-cleaned with the upper roll cleaning member  46 , the roll load applied to the substrate W by the upper roll cleaning member  46  is measured by the load cell  54 . The controller  66  compares the measured value (measured roll load) and the preset roll load which has been inputted from the control panel  30 , and gives an instruction of the operation amount of the regulating valve (valve-operation-amount) to the electropneumatic regulator  62  depending on a difference between the measured roll load and the preset roll load. The electropneumatic regulator  62  automatically adjusts the valve opening degree in response to the instruction from the controller  66  to vary a thrust force of the air cylinder  56  with the adjusted valve opening degree. Accordingly, the roll load applied to the substrate W during cleaning of the substrate W is feedback-controlled so that the roll load becomes equal to the preset roll load. 
       FIG. 6  is a flow chart showing an embodiment of the control for cleaning the front surface (upper surface) of the substrate W while performing feedback control of the roll load applied to the substrate W by the upper roll cleaning member  46  of the substrate cleaning apparatus shown in  FIG. 3 . An embodiment of the control for cleaning the back surface (lower surface) of the substrate W while performing feedback control of the roll load applied to the substrate W by the lower roll cleaning member  48  of the substrate cleaning apparatus shown in  FIG. 3  is substantially the same as the above, and therefore the explanation will be omitted hereinafter. 
     As shown in  FIG. 6 , the reference values of the valve-operation-amount of the electropneumatic regulator  62  corresponding to the respective preset roll loads, e.g. values on the line A shown in  FIG. 4  or  FIG. 5 , are inputted from the control panel  30 , and these reference values of the valve-operation-amount (line A) are stored in the monitor unit  80  (step  1 ). Then, the roll load (preset roll load), which should be applied to the substrate W from the upper roll cleaning member  46  during cleaning, e.g. the preset roll load R (N) shown in  FIG. 4  or  FIG. 5  is inputted from the control panel  30  to the controller  66  (step  2 ). 
     Next, while horizontally rotating the substrate W and supplying a cleaning liquid (chemical liquid) from the upper cleaning liquid supply nozzle  50  to the front surface (upper surface) of the substrate W, the upper roll cleaning member  46  is rotated and lowered to be brought into contact with the front surface of the rotating substrate W, thereby performing scrub cleaning of the front surface of the substrate W with the upper roll cleaning member  46  in the presence of the cleaning liquid. The roll load applied to the substrate W during cleaning of the substrate W is feedback-controlled so that the roll load becomes equal to the preset roll load (step  3 ). 
     The monitor unit  80  compares the reference value of the valve-operation-amount, of the stored reference values of the valve-operation-amount, corresponding to the preset roll load, e.g. the reference value S of the valve-operation-amount at which the preset roll load R (N) intersects with the line A (the reference value of the valve-operation-amount) shown in  FIG. 4  or  FIG. 5 , and the valve-operation-amount (current valve-operation-amount) which is outputted from the controller  66  to the electropneumatic regulator  62  (step  4 ). Then, the monitor unit  80  judges whether the current valve-operation-amount falls outside the allowable range of the reference value of the valve-operation-amount corresponding to the preset roll load. For example, the monitor unit  80  judges whether the valve-operation-amount falls outside the range (T 1  to T 2 ) shown in  FIG. 4  between the lower threshold T 1  (kPa) as an intersection at which the preset roll load R intersects with the line B, and the upper threshold T 2  (kPa) as an intersection at which the preset roll load R intersects with the line C, or the range (T 3  to T 4 ) shown in  FIG. 5  between the lower threshold T 3  (kPa) as an intersection at which the preset roll load R intersects with the line D, and the upper threshold T 4  (kPa) as an intersection at which the preset roll load R intersects with the line E (steps). 
     When the current valve-operation-amount falls outside the allowable range of the reference value of the valve-operation-amount corresponding to the preset roll load, e.g. the current valve-operation-amount becomes smaller than the lower threshold T 1  (kPa) shown in  FIG. 4  or the lower threshold T 3  (kPa) shown in  FIG. 5 , or the current valve-operation-amount becomes larger than the upper threshold T 2  (kPa) shown in  FIG. 4  or the upper threshold T 4  (kPa) shown in  FIG. 5 , the monitor unit  80  judges breakdown of the load cell  54  and issues an alarm from the alarm  82  (step  6 ). Then, the cleaning process is forcibly terminated and the apparatus running is stopped after retrieving the substrates inside the apparatus (step  7 ). 
     On the other hand, when the current valve-operation-amount does not fall outside the allowable range of the reference value of the valve-operation-amount corresponding to the preset roll load, the monitor unit  80  judges whether a predetermined cleaning time has elapsed (step  8 ). If the predetermined cleaning time has not elapsed, the processing is returned to step  4 . If the predetermined cleaning time has elapsed, the cleaning is terminated (step  9 ). Then, a process of a subsequent substrate is started (step  10 ), and the processing is returned to step  2 . 
     In this manner, the breakdown of the load cell  54  can be detected promptly during cleaning of the substrate W, and the cleaning of the substrate in a state in which an abnormal roll load is applied to the substrate W by the roll cleaning member  46  can be prevented from being continued. 
     Simultaneously, while supplying a cleaning liquid (chemical liquid) from the lower cleaning liquid supply nozzle  52  to the back surface (lower surface) of the substrate W, the lower roll cleaning member  48  is rotated and raised to be brought into contact with the back surface of the rotating substrate W under a predetermined roll load, thereby performing scrub cleaning of the back surface of the substrate W with the lower roll cleaning member  48  in the presence of the cleaning liquid. The length of the lower roll cleaning member  48  is set to be slightly larger than the diameter of the substrate W, and thus the entire back surface of the substrate W is cleaned, as with the above-described cleaning of the front surface of the substrate W. 
     During scrub cleaning of the back surface of the substrate W with the lower roll cleaning member  48 , the roll load applied to the substrate W during cleaning of the substrate W is feedback-controlled so that the roll load becomes equal to the preset roll load, as with the case of the above-described upper roll cleaning member  46 . 
     In the substrate processing apparatus shown in  FIG. 1 , the substrate taken out from a substrate cassette inside the loading port  12  is transferred to one of the polishing units  14   a ,  14   b ,  14   c ,  14   d , and the surface of the substrate is polished by the specified polishing unit. The surface of the substrate which has been polished is cleaned (primarily cleaned) in the first substrate cleaning unit  16 , and is then cleaned (finally cleaned) in the second substrate cleaning unit  18 . Then, the cleaned substrate is removed from the second substrate cleaning unit  18  and transferred to the substrate drying unit  20  where the substrate is spin-dried. Thereafter, the dried substrate is returned into the substrate cassette inside the loading port  12 . 
     In the above examples, although both of the roll load applied to the front surface (upper surface) of the substrate by the upper roll cleaning member  46  and the roll load applied to the back surface (lower surface) of the substrate by the lower roll cleaning member  48  are feedback-controlled using a closed-loop control system, either one of them may be feedback-controlled depending on use conditions (process, property of the substrate, pressing load, and the like). 
     Although preferred embodiments have been described in detail above, it should be understood that the present invention is not limited to the illustrated embodiments, but many changes and modifications can be made therein without departing from the appended claims.