Patent Publication Number: US-2023163018-A1

Title: Substrate holding apparatus and substrate processing apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a substrate holding apparatus and a substrate processing apparatus. 
     BACKGROUND ART 
     In a semiconductor manufacturing process, the presence of a foreign matter on a substrate such as a semiconductor wafer causes a defect such as an insulation defect or a short circuit of wiring. These foreign matters are mixed in various states such as those generated from a movable portion such as a conveyance device, those generated from a human body, those generated by a reaction in a processing device by a process gas, and those mixed in chemicals and materials. The same applies to a process of manufacturing a magnetic disk or a liquid crystal display element, and adhesion of generated foreign matter to a substrate (magnetic disk or liquid crystal display element) causes a defect. 
     Therefore, by detecting and managing the foreign matters on the substrate surface using the surface inspection device in the manufacturing process, the dust generation status of each manufacturing device, the cleanliness of each process, and the like are monitored and controlled to improve the quality of the product, the yield, and the like. In the foreign matter inspection method, the substrate surface is irradiated with light such as laser light, and scattered light from the foreign matter is detected to inspect the size, the attachment position, and the like of the foreign matter, and acquire the foreign matter information as unique information. Therefore, when there is undulation or the like of the substrate surface, the angle of the scattered light varies, and the accuracy of the size and the attachment position of the foreign matter is affected, and the reliability of the foreign matter information of the substrate is lowered. 
     As a method of holding the substrate, which is one factor of the undulation of the substrate surface, there are roughly classified into a back surface adsorption type and a back surface non-contact type. In the back surface adsorption type, an air adsorption port is provided in a flat table to adsorb the back surface of the substrate, and the undulation of the substrate depends on the flatness of the table. On the other hand, the back surface non-contact type is a holding type in which the substrate is held in the vicinity of the outer periphery of the substrate from the outside and the substrate surface floats in the air, and an advanced mechanism is required to suppress the undulation of the substrate and secure the flatness. 
     As a technique related to a back surface non-contact type, PTL 1 discloses a rotating wafer chuck mechanism in which a plurality of pressurized gas elements and a plurality of vacuum elements for adsorbing out the gas are arranged on a chuck surface, a wafer is floated in the air to be held in a vertical direction, and is held in a horizontal direction at a wafer edge. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: JP 2017-504199 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     PTL 1 has a structure that can maintain the flatness of the wafer while avoiding contact of the back surface of the wafer with the chuck surface. A gas supply unit as one pressurized gas element and gas exhaust units as a plurality of vacuum elements are arranged adjacent to each other, so that the gas from the pressurized gas element flows to the plurality of vacuum elements and is exhausted. That is, the gas supplied from the pressurized gas element is exhausted to the adjacent vacuum element. Since the wafer chuck mechanism is rotationally driven, the pressurized gas element and the vacuum element rotate in the same manner. Therefore, the gas from the pressurized gas element tends to flow outward due to the action of the centrifugal force depending on the rotational radius position. Since the wafer is held by the balance between the positive pressure and the negative pressure of the gas, the air flow distribution, that is, the pressure distribution changes between the stationary state and the rotating state. As a result, the gas holding force distribution for the wafer fluctuates, which may adversely affect the flatness of the wafer. 
     The present disclosure provides a technology for holding a substrate with a high degree of flatness and high precision. 
     Solution to Problem 
     A substrate holding apparatus of the present disclosure includes: a rotary stage; and a clamp part that supports an edge of a substrate which is an object to be rotated by the rotary stage in a planar direction of the substrate, in which the rotary stage is provided with: a plurality of gas supply openings that supply a gas toward the substrate; and one or more gas exhaust openings that are provided to each of the plurality of gas supply openings so as to surround peripheries of the gas supply openings. 
     Further features related to the present disclosure will become apparent from the description of the present specification and the accompanying drawings. In addition, the aspects of the present disclosure are achieved and realized by elements, combinations of various elements, the following detailed description, and aspects of the appended claims. 
     The description of the present specification is merely exemplary, and does not limit the scope of claims or application examples of the present disclosure in any sense. 
     Advantageous Effects of Invention 
     With the substrate holding apparatus of the present disclosure, the substrate can be held with high flatness and high accuracy. Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic diagram illustrating a wafer processing system according to a first embodiment. 
         FIG.  2    is a perspective view illustrating a wafer chuck of the first embodiment. 
         FIG.  3 A  is a plan view of an air bearing pad, and  FIG.  3 B  is a cross-sectional view taken along line C-C of  FIG.  3 A . 
         FIG.  4 A  is a plan view showing an aspect of an air flow in the air bearing pad, and  FIG.  4 B  is a cross-sectional view taken along line D-D of  FIG.  4 A  (lower part) and a view showing a pressure distribution (upper part). 
         FIG.  5    is a cross-sectional view taken along line B-B of  FIG.  2   , and illustrates the operation of each component of a clamp part before holding a wafer in the in-plane direction. 
         FIG.  6    is a cross-sectional view taken along line B-B of  FIG.  2   , and illustrates the operation of each component of the clamp part when the wafer is held by a holding claw. 
         FIG.  7    is a cross-sectional perspective view taken along line A-A of  FIG.  2   . 
         FIG.  8    is a flowchart illustrating an operation of loading the wafer into the wafer chuck. 
         FIG.  9    is a cross-sectional view illustrating a state of the wafer chuck in step S 4  of  FIG.  8   . 
         FIG.  10    is a cross-sectional view illustrating a state of the wafer chuck in step S 7  of  FIG.  8   . 
         FIG.  11    is a cross-sectional view illustrating a configuration of a wafer chuck according to Modification 1. 
         FIG.  12 A  is a plan view illustrating an air bearing pad according to Modification 2, and  FIG.  12 B  is a cross-sectional view taken along line E-E of  FIG.  12 A . 
         FIG.  13 A  is a plan view illustrating an air bearing pad according to Modification 3, and  FIG.  13 B  is a cross-sectional view taken along line F-F of  FIG.  13 A . 
         FIG.  14    is a cross-sectional view illustrating a configuration of a wafer chuck according to a second embodiment. 
         FIG.  15    is a cross-sectional view illustrating a structure of a wafer chuck according to a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In each embodiment, a substrate held by a substrate holding apparatus will be described as a wafer, but the substrate of the present disclosure is not limited to a wafer, and may be any type as long as the substrate has a flat plate shape, such as a glass substrate, a liquid crystal panel, an electronic circuit board, an optical disk, or a magnetic disk. 
     In the present disclosure, a vertical direction of a wafer surface is referred to as an “out-of-plane direction” or a “vertical direction”, and a direction along the wafer surface is referred to as an “in-plane direction” or a “horizontal direction”. In addition, a member on which a rotary stage and the substrate (wafer) holding mechanism are mounted is referred to as a wafer chuck (substrate holding apparatus). 
     First Embodiment 
     &lt;Configuration Example of Wafer Processing System&gt; 
       FIG.  1    is a schematic diagram illustrating a wafer processing system  10  according to a first embodiment. As illustrated in  FIG.  1   , the wafer processing system  10  (substrate processing apparatus) includes an introduction unit  11 , a conveyance device  12 , an inspection chamber  13 , and a control device  14 . 
     The wafer  205  housed in a wafer cassette (not illustrated) is loaded into the introduction unit  11 . The conveyance device  12  takes out the wafers  205  from the wafer cassette loaded in the introduction unit  11 , and conveys the wafers one by one to the inspection chamber  13  (for example, a foreign matter inspection device). 
     The inspection chamber  13  includes a wafer chuck  200 , an optical measurement unit  131 , a motor  132 , and a linear motion moving unit  133 . The wafer  205  conveyed to the inspection chamber  13  is arranged on the wafer chuck  200  and held with high flatness by the wafer chuck  200 . A method of holding the wafer  205  by the wafer chuck  200  will be described later. The optical measurement unit  131  is fixed at a position above the wafer chuck  200 , and optically measures the position and size of the foreign matter on the wafer  205 . As described above, the optical measurement unit  131 , the wafer  205 , and the wafer chuck  200  are arranged in this order from the top in the direction of gravity. The measurement result by the optical measurement unit  131  is transmitted to the control device  14 . 
     The wafer chuck  200  is rotationally supported by the motor  132  so that foreign matter can be measured while the wafer  205  is rotationally moved. The linear motion moving unit  133  moves the motor  132  in a vertical direction to the rotation axis of the motor  132 . With such a configuration, the control device  14  can map the size, position, and the like of the foreign matter on the entire surface of the wafer  205  and record the size, position, and the like as foreign matter data of the wafer  205 . The wafer  205  for which the measurement is completed is transferred again from the wafer chuck  200  by the conveyance device  12  and returned to the wafer cassette of the introduction unit  11 . The above process is repeated to inspect all the wafers  205  in the cassette for foreign matters. 
     The control device  14  controls operations of the conveyance device  12 , the wafer chuck  200 , the motor  132 , and the linear motion moving unit  133 . The control device  14  can be configured with, for example, a computer terminal such as a personal computer, a smartphone, or a tablet. 
     The wafer processing system  10  is installed in a space where cleanliness is maintained so as not to allow foreign matter to adhere to the wafer  205 . 
     &lt;Configuration Example of Wafer Chuck&gt; 
       FIG.  2    is a perspective view illustrating the wafer chuck  200 , and the wafer  205  is illustrated as a transparent view separated from the wafer chuck  200 . As illustrated in  FIG.  2   , the wafer chuck  200  includes a rotary stage  201 , a wafer support unit  202 , a plurality of air bearing pads  300 , and a plurality of clamp parts  206 . 
     The rotary stage  201  is connected to the motor  132  so as to rotate about the center as a rotation axis. The wafer support unit  202  is a protrusion provided along the outer periphery of the upper surface of the rotary stage  201 , and supports the edge portion of the wafer  205  from the lower surface of the wafer  205 . 
     The air bearing pad  300  is provided on the upper surface side of the rotary stage  201 . The air bearing pad  300  has a gas supply opening  203  and a gas exhaust opening  204 , and holds the out-of-plane direction of the wafer  205  using a gas force. The arrangement pattern of the air bearing pads  300  when the wafer chuck  200  is viewed in a plan view from above may be point symmetric with the rotation center of the rotary stage  201  as a symmetric point, or may be random arrangement. By making the arrangement pattern of the air bearing pads  300  point symmetric, it is easy to control the vibration of the wafer  205 . 
     The gas supply opening  203  is provided at the center of the air bearing pad  300 . The gas exhaust opening  204  has an annular shape centered on the gas supply opening  203  in a plan view, and is arranged so as to surround the gas supply opening  203 . Therefore, the distance between the gas supply opening  203  and the gas exhaust opening  204  is constant. As described above, the gas supply opening  203  forms a pair of supply and exhaust ports including one gas exhaust opening  204  for one gas supply opening  203 . 
     Here, the gas supply opening  203  of a certain (first) air bearing pad  300  is defined as a first gas supply opening, and the gas exhaust opening  204  arranged around the gas supply opening is defined as a first gas exhaust opening. Similarly, the gas supply opening  203  of another (second) air bearing pad  300  is defined as a second gas supply opening, and the gas exhaust opening  204  arranged around the gas supply opening is defined as a second gas exhaust opening. In addition, a circle centered on the first gas supply opening and having a radius equal to a distance between the first gas supply opening and the first gas exhaust opening is defined as a first virtual circumference. Similarly, a circle centered the second gas supply opening and having a radius equal to a distance between the second gas supply opening and the second gas exhaust opening is defined as a second virtual circumference. At this time, a part of the first virtual circumference and a part of the second virtual circumference exist between the first gas supply opening and the second gas supply opening. In other words, in the present embodiment, the gas exhaust opening  204  is positioned between the respective gas supply openings  203  in any two air bearing pads  300 . 
     In one air bearing pad  300 , conditions of a structure and pressure are given such that the amount of gas supplied to the gas supply opening  203  and the amount of gas exhausted from the gas exhaust opening  204  are equal. As a result, the flow path is completed for each air bearing pad  300 , and the air flow does not interfere with the other air bearing pads  300 . 
     The material of the rotary stage  201  and the material of the air bearing pad  300  may be the same or different. 
     The clamp part  206  holds the wafer  205  by pressing the edge of the wafer  205  in the in-plane direction. In the example illustrated in  FIG.  2   , six clamp parts  206  are provided at equal intervals, but the number is not limited to six. 
     &lt;Holding Structure in Out-of-Plane Direction&gt; 
     The holding structure in the out-of-plane direction of the wafer  205  will be described. 
       FIG.  3 A  is a plan view of the air bearing pad  300 .  FIG.  3 B  is a cross-sectional view taken along line C-C of  FIG.  3 A . 
     As shown in  FIGS.  3 A and  3 B , in the flow path with the gas supply opening  203  as the upstream and the gas exhaust opening  204  as the downstream, a space  203   a  having a radius larger than the radius of the gas exhaust opening  204  is provided in the gas supply opening  203  as the upstream. That is, the upper end of the gas supply opening  203  is provided at a position lower than the upper end of the gas exhaust opening  204 . Downstream of the space  203   a , a clearance closest to the wafer  205  is assumed. The gas is exhausted by the gas exhaust opening  204  at the most downstream. 
     As described above, for each air bearing pad  300 , it is possible to define a virtual circumference C centered on the gas supply opening  203  and having a radius that is the distance between the gas supply opening  203  and the gas exhaust opening  204 . The distance between the gas supply opening  203  and the gas exhaust opening  204  is the distance between the center of the gas supply opening  203  and the center in the width direction of the annular ring formed by the gas exhaust opening  204 . 
     As illustrated in  FIG.  3 B , gas is supplied to the gas supply opening  203  by application of a positive pressure, and a negative pressure is applied to the gas exhaust opening  204 . A gas supply source (not illustrated) that supplies gas by applying a positive pressure is connected to a lower end of the gas supply opening  203 , and a gas exhaust source (not illustrated) that exhausts gas by applying a negative pressure is connected to a lower end of the gas exhaust opening  204 . 
       FIG.  4 A  is a plan view showing an aspect of an air flow in the air bearing pad  300 .  FIG.  4 B  is a cross-sectional view taken along line D-D of  FIG.  4 A  (lower part) and a view illustrating a pressure distribution in the air bearing pad  300  (upper part). In  FIGS.  4 A and  4 B , gas streamlines  350  are indicated by dotted lines. 
     As illustrated in  FIGS.  4 A and  4 B , by supplying gas from the gas supply opening  203  to the space  203   a  with the wafer  205  at a pressure equal to or higher than the atmospheric pressure, a force for pushing up the wafer  205  can be applied. The space  203   a  functions as a buffer space because an area facing the wafer  205  and a space volume can be secured, so that a region facing the wafer  205  has an even pressure distribution. Therefore, even if the clearance between the wafer  205  and the gas supply opening  203  fluctuates, the fluctuation of the pressure can be reduced as much as possible. 
     The gas overflowing the space  203   a  flows in the 360° direction. Since the gas flowing out of the space  203   a  passes through a region narrower than the gap between the wafer  205  and the space  203   a , the passing speed is increased. As a result, a negative pressure region lower than the atmospheric pressure is generated in the region between the space  203   a  and the gas exhaust opening  204 , and a force for pulling the wafer  205  toward the wafer chuck  200  is generated. 
     Since the gas flowing to the gas exhaust opening  204  is exhausted here, the region of the pressure acting on the wafer  205  is completed at the gas exhaust opening  204 . 
     This pressure action is drawn as a pressure distribution from the gas supply opening  203  to the gas exhaust opening  204  as illustrated in the upper part of  FIG.  4 B . Since the surface of the air bearing pad  300  facing the wafer  205  has a pressure distribution in which a positive pressure region and a negative pressure region exist, a pressing force and a tensile force are generated with respect to the wafer  205 . As a result, a gap G that is balanced with the wafer weight is generated between the wafer  205  and the air bearing pad  300 . In the upper graph of  FIG.  4 B , the gap G indicates a force acting on the wafer  205  when the pressure curve is integrated in the plane of the air bearing pad  300 , that is, a distance at which a value on the positive pressure side (repulsive force to the wafer  205 ) and a value on the negative pressure side (attractive force to the wafer  205 ) are equal with the equilibrium pressure point as a boundary. In a case where the distance between the wafer  205  and the air bearing pad  300  deviates from the balanced gap G, that is, for example, in a case where the wafer  205  approaches the air bearing pad  300  side, a repulsive force acts because the positive pressure with respect to the wafer  205  is superior. On the other hand, when the distance is increased, the attractive force acts because the tensile force is superior. This action generates holding rigidity for the wafer  205 . Thus, if the ratio between the supply pressure of the gas supply opening  203  and the exhaust pressure of the gas exhaust opening  204  are not changed, and the values of the both are increased, the holding rigidity for the wafer  205  can be increased. 
     The wafer holding rigidity of the wafer chuck  200  may be any rigidity that can hold the weight of one wafer  205 . By arranging a plurality of air bearing pads  300  on the wafer chuck  200 , the holding rigidity of the air bearing pads  300  can be shared by the number of air bearing pads arranged on the wafer chuck  200 , so that design with a margin is possible. The gap between the wafer  205  and the wafer chuck  200  is controlled by the above action, and the wafer  205  can be held with high flatness in the out-of-plane direction. 
     &lt;Holding Structure in In-Plane Direction&gt; 
       FIG.  5    is a cross-sectional view taken along line B-B of  FIG.  2   , and illustrates the operation of each component of the clamp part  206  before the wafer  205  is held in the in-plane direction. As illustrated in  FIG.  5   , the clamp part  206  is built in the wafer chuck  200 , and includes a cam  211 , an air cylinder  212 , a bearing  213 , a bearing holding unit  214 , a compression spring  215 , and a rod  216 . As will be described later with reference to  FIG.  7   , a link that allows displacement in the circumferential direction is connected to the rod  216 , and a holding claw (pressing portion) that comes into contact with the wafer  205  is mounted on the link. 
     The air cylinder  212  is arranged on the central axis below the rotary stage  201  which is a base of the wafer chuck  200 , and the cam  211  is attached to the air cylinder  212  in a vertically movable manner. Air is supplied to the air cylinder  212  by a pump (not illustrated) or the like. The supply of air to the air cylinder  212  is controlled by the control device  14  described above. The bearing  213  is held by the bearing holding unit  214  to come into contact with the cam  211  and converts the movement of the cam  211  in the vertical direction into the radial direction of the rotary stage  201 . The rod  216  is movable relative to the bearing holding unit  214  in the radial direction. The compression spring  215  radially supports the bearing holding unit  214  and the rod  216 . 
     As indicated by a white arrow in  FIG.  5   , when the air cylinder  212  operates and the cam  211  moves upward, the rod  216  moves radially inward. 
       FIG.  6    is a cross-sectional view taken along line B-B in  FIG.  2    and illustrates the operation of each component of the clamp part  206  when the wafer  205  is held by a holding claw  218  after the wafer  205  is mounted on the rotary stage  201 . As indicated by a black arrow in  FIG.  6   , when the operation of the air cylinder  212  is stopped and the cam  211  moves downward, the rod  216  moves radially outward. 
       FIG.  7    is a cross-sectional perspective view taken along line A-A in  FIG.  2   , and illustrates the periphery of the holding claw  218 . As illustrated in  FIG.  7   , the rod  216  extends to an outer peripheral end (radial end) of the rotary stage  201 , and a link  217  is connected to the end of the rod  216 . The link  217  is rotatable with the vertical direction (the out-of-plane direction of the wafer  205 ) as the rotation axis direction. The link  217  is provided with the holding claw  218  supported in a rotatable manner in an in-plane direction of the wafer  205  with a vertical direction as a rotation axis direction. 
     The surface of the holding claw  218  in contact with the wafer  205  is a surface perpendicular to the surface of the wafer  205  and is an upright cylindrical surface in the present embodiment. Of course, even if the surface of the holding claw  218  in contact with the wafer  205  is a flat surface, the wafer  205  can be held in the horizontal direction. 
     The mechanisms excluding the air cylinder  212  and the cam  211  of the clamp part  206  are arranged symmetrically with respect to the central axis of the wafer chuck  200 . The number of mechanisms such as the holding claws  218  can be determined in consideration of the generated holding force of each mechanism and the necessary holding force of the wafer  205 . 
     The operation of the clamp part  206  will be described. The control device  14  ( FIG.  1   ) supplies the air to the air cylinder  212  after the wafer  205  is brought into the inspection chamber  13  by the conveyance device  12 . As indicated by the white arrow in  FIG.  5   , the cam  211  moves upward and the rod  216  moves radially inward. Since the holding claw  218  is attached at a position symmetrical to the link  217  with respect to the rotation axis, the link  217  is displaced radially inward following the movement of the rod  216 , and the holding claw  218  is displaced radially outward as indicated by the white arrow in  FIG.  7   . As a result, since the position of the holding claw  218  moves to the outside of the outer peripheral end of the wafer  205 , the wafer chuck  200  is ready to receive the wafer  205 . 
     Thereafter, the conveyance device  12  is arranged on the rotary stage  201 , and places the wafer  205  on the wafer support unit  202  having an annular shape that holds the outer peripheral portion of the lower surface of the wafer  205  in the vertical direction. 
     The control device  14  ( FIG.  1   ) stops air supply to the air cylinder  212  after the conveyance device  12  is retracted. As a result, as indicated by the black arrow in  FIG.  6   , the cam  211  moves downward and the rod  216  moves radially outward, and as indicated by a black arrow in  FIG.  7   , the holding claw  218  horizontally rotates and abuts on the outer peripheral end of the wafer  205  to generate holding force. During this time, the bearing holding unit  214 , the rod  216 , and the compression spring  215  connecting the bearing holding unit  214  and the rod  216  move integrally to the outer peripheral side in the direction of the black arrows in  FIGS.  6  and  7    until the holding claw  218  abuts on the outer peripheral end of the wafer  205 . 
     Thereafter, with the stop of the rod  216 , the bearing holding unit  214  continues to move radially outward until the movement of the cam  211  is completed. At this time, the compression spring  215  contracts due to the contraction of the relative distance between the bearing holding unit  214  and the rod  216 . As a result, the compression spring  215  generates an axial force due to the reaction force, and this force is transmitted to the holding claw  218 . As a result, the spring force of the compression spring  215  becomes the holding force of the holding claw  218  in the in-plane direction of the wafer  205 . The spring force of compression spring  215  is a ratio of a distance from the rotation center of the holding claw  218  (supporting point) to the joint shaft (force point) of the link  217  and a distance from the rotation center (action point) of the holding claw  218 . 
     Next, the holding force of the clamp part  206  will be described. Functions required for holding the wafer  205  by the clamp part  206  are mainly an alignment function in a state of mounting the wafer  205 , a slip prevention function in a rotation start-up state, and a function against a centrifugal force caused by eccentricity of the wafer  205  in a steady rotating state. The alignment function at the time of mounting the wafer  205  is covered by a static holding force. This is a holding force generated by the compression spring  215  of the clamp part  206 . Regarding the prevention of slip and the centrifugal force of the wafer  205 , since the centrifugal force generated by each component of the clamp part  206  is added to the holding force, the centrifugal force is considered. In the rotation start-up state, a holding force equal to or larger than an inertial force for stopping the wafer  205  is required. In the steady rotating state, a holding force equal to or larger than a centrifugal force obtained by integrating the eccentricity amount, the mass, and the rotation speed of the wafer  205  is required. The holding force against these forces can be adjusted by the shape and mass of components such as the bearing  213 , the bearing holding unit  214 , the rod  216 , and the link  217 . 
     &lt;Wafer Loading Operation&gt; 
       FIG.  8    is a flowchart illustrating an operation of loading the wafer  205  into the wafer chuck  200 . Each operation in  FIG.  8    is executed by driving each device by the control device  14  of the wafer processing system  10  illustrated in  FIG.  1   , but each device of the wafer processing system  10  will be described below as a main body of the operation. 
     In step S 1 , the wafer chuck  200  descends and retracts. 
     In step S 2 , the conveyance device  12  conveys the wafer  205  until the center of the rotary stage  201  of the wafer chuck  200  in the inspection chamber  13  coincides with the center of the wafer  205 . 
     In step S 3 , the wafer chuck  200  is raised to a height at which the wafer  205  can be loaded. 
       FIG.  9    is a cross-sectional view illustrating a state of the wafer chuck  200  in step S 4  of  FIG.  8   . As illustrated in  FIG.  9   , the rotary stage  201  is provided with a gas supply source  401  connected to each of the gas supply openings  203  of all the air bearing pads  300  by a pipe  403  and a gas exhaust source  402  connected to each of the gas exhaust openings  204  by a pipe  404 . The gas supply source  401  and the gas exhaust source  402  can be configured with, for example, a pump. The driving of the gas supply source  401  and the gas exhaust source  402  is controlled by the above-described control device  14  ( FIG.  1   ). 
     In step S 4 , the control device  14  drives the gas supply source  401  to supply positive pressure gas to the gas supply openings  203  of all the air bearing pads  300 . 
     Since the outer peripheral portion of the wafer  205  is supported by the conveyance device  12 , the wafer has deformation due to self-weight sinking in which the central portion bends in the direction of gravity during conveyance or when moving onto the wafer chuck  200  (wafer  205   b ). When the plurality of air bearing pads  300  on the wafer chuck  200  are formed on the same plane, it is likely that the central portion of the wafer  205  comes into contact with the wafer chuck  200  due to self-weight sinking of the wafer  205 . In order to avoid the contact, the gas is supplied from the gas supply source  401 , and the positive pressure gas is discharged from the air bearing pad  300 . As a result, since the deformation of the wafer  205  due to self-weight sinking is corrected, it is possible to avoid the contact of the wafer  205  with the wafer chuck  200 . 
     Returning to  FIG.  8   , in step S 5 , the wafer chuck  200  ascends until the wafer  205  is placed on the wafer support unit  202 . 
     In step S 6 , the control device  14  drives the gas supply source  401  and the gas exhaust source  402  to hold the lower surface of the wafer  205  in the out-of-plane direction. 
     In step S 7 , the control device  14  drives the clamp part  206  to hold the wafer  205  in the in-plane direction while centering the wafer  205  by the holding claw  218 , and the loading of the wafer  205  into the wafer chuck  200  is completed. 
       FIG.  10    is a cross-sectional view illustrating a state of the wafer chuck  200  in step S 7 . As illustrated in  FIG.  10   , the wafer  205  is held in the out-of-plane direction by the air bearing pad  300  and held in the in-plane direction by the holding claw  218 , so that the wafer is held by the wafer chuck  200  with high flatness. 
     The amount and coordinates of the surface dust of the wafer  205  held as described above are inspected by the optical measurement unit  131  ( FIG.  1   ). 
     Modification 1 
       FIG.  11    is a cross-sectional view illustrating a configuration of the wafer chuck  200  according to Modification 1. As illustrated in  FIG.  11   , the pipe  403  branches so as to be able to supply the gas only to the air bearing pad  300  positioned at the central portion of the rotary stage  201 , and is provided with a valve  450  (switching mechanism) that controls supply and interruption of gas to the air bearing pad  300  positioned at the peripheral portion. The opening and closing of the valve  450  can be controlled by the control device  14 . 
     In step S 4  described above, by closing the valve  450  and supplying the positive pressure gas only to the air bearing pad  300  positioned at the central portion of the rotary stage  201 , the self-weight sinking of the wafer  205  can be corrected more efficiently. In step S 6 , the valve  450  is opened. In  FIG.  11   , the valve  450  is built in the wafer chuck  200 , but of course, the effect is similar even if the valve  450  is installed outside the wafer chuck  200 . 
     Modification 2 
       FIG.  12 A  is a plan view illustrating an air bearing pad  301  according to Modification 2.  FIG.  12 B  is a cross-sectional view taken along line E-E of  FIG.  12 A . 
     As illustrated in  FIGS.  12 A and  12 B , a groove  204   a  in an annular shape is provided on the upper surface of the air bearing pad  301  so as to surround the gas supply opening  203 , and a plurality of gas exhaust openings  204   b  communicating with the groove  204   a  are provided. The number of the gas exhaust openings  204  can be changed according to the arrangement pattern of the air bearing pad  301 . In the example illustrated in  FIG.  12 A , four gas exhaust openings  204   b  are provided every 90°. The distance between the center of the gas supply opening  203  and the center of each gas exhaust opening  204   b  in a plan view of the air bearing pad  301  is the same. 
     With such a structure, the inside of the groove  204   a  becomes a negative pressure region, and all the gas supplied from the gas supply opening  203  can be exhausted from the gas exhaust openings  204   b  formed in the groove  204   a . As described above, the flow path is completed for each air bearing pad  301 , and the air flow does not interfere with other air bearing pads  301 . 
     The depth of the groove  204   a  is not limited, but may be, for example, equal to the depth of the space  203   a  or deeper than the space  203   a . By setting the depth of the groove  204   a  to be equal to or greater than the depth of the space  203   a , the gas can be exhausted more efficiently. 
     Also in Modification 2, similarly to the first embodiment, the gas supply opening  203  of a certain (first) air bearing pad  301  is set as the first gas supply opening, and the gas exhaust opening  204   b  arranged around the gas supply opening is set as the first gas exhaust opening. Similarly, the gas supply opening  203  of another (second) air bearing pad  301  is defined as the second gas supply opening, and the gas exhaust opening  204   b  arranged around the gas supply opening is defined as the second gas exhaust opening. In addition, a circle centered on the first gas supply opening and having a radius equal to a distance between the first gas supply opening and the first gas exhaust opening is defined as a first virtual circumference. Similarly, a circle centered the second gas supply opening and having a radius equal to a distance between the second gas supply opening and the second gas exhaust opening is defined as a second virtual circumference. At this time, a part of the first virtual circumference and a part of the second virtual circumference exist between the first gas supply opening and the second gas supply opening. 
     Modification 3 
       FIG.  13 A  is a plan view illustrating an air bearing pad  302  according to Modification 3.  FIG.  13 B  is a cross-sectional view taken along line F-F of  FIG.  13 A . 
     As illustrated in  FIGS.  13 A and  13 B , the air bearing pad  302  does not include the groove  204   a  described above but is provided with the plurality of gas exhaust openings  204   c  so as to surround the gas supply opening  203 . In the example illustrated in  FIG.  13 A , eight gas exhaust openings  204   c  are provided every 45° on the virtual circumference C. 
     Even with such a structure, without releasing the gas supplied from the gas supply opening  203 , all the gas can be collected at the gas exhaust openings  204   c  and exhausted. As described above, the flow path is completed for each air bearing pad  302  and the air flow does not interfere with other air bearing pads  302 . 
     Technical Effects 
     As described above, in the wafer chuck  200  of the first embodiment, the plurality of gas supply openings  203  are provided in the rotary stage  201 , and one or more gas exhaust openings  204  are provided so as to surround the periphery of each of the plurality of gas supply openings  203 . As a result, since the flow path of the gas is locally formed, the fluctuation of the distribution of the holding pressure in the out-of-plane direction of the wafer  205  can be minimized even if the centrifugal force acts. As a result, the flatness of the wafer  205  can be maintained with high accuracy. 
     The mechanical restraint of the wafer  205  is only the pressing in the in-plane direction using the holding claw  218  of the clamp part  206 , the factor causing the deformation of the wafer  205  is eliminated, and thus high flatness can be achieved. 
     As described above, since the wafer chuck  200  can hold the wafer  205  with high flatness, foreign matters can be detected with high accuracy when the wafer chuck is mounted on the foreign matter inspection device. 
     Second Embodiment 
     In the first embodiment described above, an example in which one gas supply source and one gas exhaust source are provided for all the air bearing pads  300  is described, but the number of gas supply sources and gas exhaust sources may be plural. Therefore, in a second embodiment, an example in which the plurality of gas supply sources and the plurality of gas exhaust sources are provided is suggested. 
       FIG.  14    is a cross-sectional view illustrating a configuration of the wafer chuck  200  according to the second embodiment. As illustrated in  FIG.  14   , a first gas supply source  405  and a first gas exhaust source  406  are connected to the air bearing pads  300  positioned in the central portion of the rotary stage  201 , and a second gas supply source  407  and a second gas exhaust source  408  are connected to the air bearing pads  300  positioned in the peripheral portion of the rotary stage  201 . 
     The first gas supply source  405  and the second gas supply source  407  can supply gases having different pressures (supply pressures). The first gas exhaust source  406  and the second gas exhaust source  408  can exhaust gases at different pressures (exhaust pressures). By making the ratio between the supply pressure of the first gas supply source  405  and the exhaust pressure of the first gas exhaust source  406  and the ratio between the supply pressure of the second gas supply source  407  and the exhaust pressure of the second gas exhaust source  408  constant, the gap G between the air bearing pad  300  and the wafer  205  can be made constant. For example, by making the pressures (values of the supply pressure and the exhaust pressure) of the first gas supply source  405  and the first gas exhaust source  406  higher than the pressures (values of the supply pressure and the exhaust pressure) of the second gas supply source  407  and the second gas exhaust source  408  while maintaining the ratio of the supply pressure and the exhaust pressure, the holding force of the wafer  205  at high flatness can be strengthened against disturbance. 
     In the example illustrated in  FIG.  14   , the air bearing pads  300  positioned in the central portion and the air bearing pads  300  positioned in the peripheral portion can have different gas supply and exhaust pressures, but the connection destination of the first gas supply source  405  and the first gas exhaust source  406  and the connection destination of the second gas supply source  407  and the second gas exhaust source  408  can be arbitrarily changed. 
     In addition, the plurality of gas supply sources and the plurality of gas exhaust sources can be connected to one air bearing pad  300 . 
     Technical Effects 
     As described above, in the second embodiment, two sets (a plurality of sets) of the gas supply sources and the gas exhaust sources which are independently controlled are provided. As a result, the supply pressure and the exhaust pressure of the gas can be efficiently controlled according to the arrangement of the air bearing pad  300 . 
     Third Embodiment 
     In the first and second embodiments described above, the example in which the upper surface of the rotary stage  201  is flat is described, but the rotary stage  201  may have a shape in which the height is lower toward the central portion and is higher toward the outside in the radial direction according to the deformation shape due to the self-weight sinking of the wafer. 
       FIG.  15    is a cross-sectional view illustrating a structure of the wafer chuck  200  according to a third embodiment. As illustrated in  FIG.  15   , the upper surface of the rotary stage  201  is formed in a stepped structure in which the height is lower toward the central portion along the shape (wafer  205   b ) when the wafer  205  is deformed by the self-weight sinking. In the example illustrated in  FIG.  15   , the steps are three steps, but may be two steps or more. Further, the shape of the rotary stage  201  may be a structure in which the height changes continuously instead of the stepped structure. 
     By connecting the plurality of sets of the gas supply sources and the gas exhaust sources to the rotary stage  201  of the present embodiment as in the second embodiment, the value of the supply pressure of the gas supplied to the air bearing pad  300  in the central portion can be made larger than the value of the supply pressure of the gas supplied to the air bearing pad  300  in the peripheral portion while maintaining the ratio of the supply pressure and the exhaust pressure, and the value of the exhaust pressure of the gas exhausted from the air bearing pad  300  in the central portion can be made larger than the value of the exhaust pressure of the gas exhausted from the air bearing pad  300  in the peripheral portion. 
     Technical Effects 
     As described above, in the third embodiment, the rotary stage  201  has a shape in which the height is lower toward the central portion and is higher toward the outside in the radial direction. As a result, when the wafer chuck  200  is raised to bring the wafer  205  and the rotary stage  201  close to each other in steps S 3  to S 4  described above, it is possible to avoid contact between the wafer  205  and the rotary stage  201  without supplying gas from the gas supply source  401 . Therefore, the energy required for the operation of loading the wafer  205  into the wafer chuck  200  can be reduced. 
     Modification 
     The present disclosure is not limited to the above-described embodiments, and includes various modifications. For example, the above-described embodiments have been described in detail in order to describe the present disclosure in an easy-to-understand manner and are not required to necessarily have all the described configurations. Further, a part of one embodiment can be replaced with a configuration of another embodiment. Further, a part of one embodiment can be added to a configuration of another embodiment. In addition, for a part of the configuration of each embodiment, a part of the configuration of another embodiment can be added, deleted, or replaced. 
     As an example of a modification, for example, in order to secure high flatness of the wafer, a desired structure and performance of the air bearing pad can be freely selected according to a position such as a radius to be mounted on the rotary stage. 
     REFERENCE SIGNS LIST 
     
         
           10  wafer processing system 
           11  introduction unit 
           12  conveyance device 
           13  inspection chamber 
           14  control device 
           131  optical measurement unit 
           132  motor 
           133  linear motion moving unit 
           200  wafer chuck 
           201  rotary stage 
           202  wafer support unit 
           203  gas supply opening 
           203   a  space 
           204  gas exhaust opening 
           204   a  groove 
           205  wafer 
           206  clamp part 
           211  cam 
           212  air cylinder 
           213  bearing 
           214  bearing holding unit 
           215  compression spring 
           216  rod 
           217  link 
           218  holding claw 
           300  to  302  air bearing pad 
           350  gas streamline 
           401  gas supply source 
           402  gas exhaust source 
           403  pipe 
           404  pipe 
           405  first gas supply source 
           406  first gas exhaust source 
           407  second gas supply source 
           408  second gas exhaust source 
           450  valve