Patent Publication Number: US-2023162982-A1

Title: Bonding apparatus and bonding method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Japanese Patent Application No. 2021-191029 filed on Nov. 25, 2021, the entire disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The various aspects and embodiments described herein pertain generally to a bonding apparatus and a bonding method. 
     BACKGROUND 
     In a bonding apparatus described in Patent Document 1, when an upper wafer and a lower wafer are bonded, a central portion of the upper wafer is transformed downwards so that the central portion of the upper wafer and a central portion of the lower wafer come into contact with each other, and contact regions of the upper wafer and the lower wafer that are in contact with each other are enlarged from the central portions toward peripheral portions of the wafers.
     Patent Document 1: Japanese Patent Laid-open Publication No. 2014-229787   

     SUMMARY 
     In one exemplary embodiment, a bonding apparatus is configured to bond a first substrate and a second substrate. The bonding apparatus includes a first holder, a second holder, a moving unit, a first transforming unit, a second transforming unit and a controller. The first holder is configured to attract and hold the first substrate from above. The second holder is provided below the first holder, and is configured to attract and hold the second substrate from below. The moving unit is configured to move the first holder and the second holder relative to each other. The first transforming unit is configured to make a central portion of the first substrate held by the first holder protruded downwards. The second transforming unit is configured to make a central portion of the second substrate held by the second holder protruded upwards. The controller is configured to control the moving unit, the first transforming unit, and the second transforming unit to perform a control of bringing the central portions of the first substrate and the second substrate into contact with each other. The controller performs a control of changing a protruding amount of the central portion of the first substrate according to a protruding amount of the central portion of the second substrate. 
     The foregoing summary is illustrative only and is not intended to be any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description that follows, embodiments are described as illustrations only since various changes and modifications will become apparent to those skilled in the art from the following detailed description. The use of the same reference numbers in different figures indicates similar or identical items. 
         FIG.  1    is a plan view illustrating a bonding system according to an exemplary embodiment; 
         FIG.  2    is a side view of the bonding system of  FIG.  1   ; 
         FIG.  3    is a side view illustrating an example of a first substrate and a second substrate; 
         FIG.  4    is a flowchart showing a bonding method according to the exemplary embodiment; 
         FIG.  5    is a side view illustrating an example of a bonding apparatus; 
         FIG.  6    is a cross sectional view illustrating an example of an upper chuck and a lower chuck; 
         FIG.  7    is a flowchart illustrating details of a process S 109  of  FIG.  4   ; 
         FIG.  8 A  is a side view illustrating an example of an operation in a process S 112 ,  FIG.  8 B  is a side view illustrating an operation following the operation of  FIG.  8 A , and  FIG.  8 C  is a side view illustrating an operation following the operation of  FIG.  8 B ; 
         FIG.  9    is a cross sectional view illustrating an example of a state upon the completion of the process S 112 ; 
         FIG.  10    is a cross sectional view illustrating an example of a state upon the completion of a process S 113 ; 
         FIG.  11    is a cross sectional view illustrating an example of a state upon the completion of a process S 114 ; 
         FIG.  12    is a cross sectional view illustrating an example of a state between the process S 114  and a process S 115 ; 
         FIG.  13    is a cross sectional view illustrating an example of a state upon the completion of the process S 115 ; 
         FIG.  14    is a diagram illustrating an example of a relationship between ΔZ 1  and ΔZ 2  and ΔZ 3 ; 
         FIG.  15    is a perspective view illustrating an example of bending of an upper wafer; and 
         FIG.  16    is a plan view illustrating an example of an attraction surface of the lower chuck. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part of the description. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Furthermore, unless otherwise noted, the description of each successive drawing may reference features from one or more of the previous drawings to provide clearer context and a more substantive explanation of the current exemplary embodiment. Still, the exemplary embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings, may be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the various drawings, same or corresponding parts will be assigned same reference numerals, and redundant description will be omitted. Further, the X-axis direction, the Y-axis direction and the Z-axis direction are orthogonal to each other. The X-axis and Y-axis directions are horizontal directions, and the Z-axis direction is a vertical direction 
     First, a configuration of a boding system  1  according to an exemplary embodiment will be described with reference to  FIG.  1    and  FIG.  2   . The bonding system  1  forms a combined substrate T by bonding a first substrate W 1  and a second substrate W 2  shown in  FIG.  3   . At least one of the first substrate W 1  and the second substrate W 2  is, for example, a semiconductor substrate such as a silicon wafer or a compound semiconductor wafer on which a plurality of devices is formed. The devices include electronic circuits. Either one of the first substrate W 1  and the second substrate W 2  may be a bare wafer on which no device is formed. The first substrate W 1  and the second substrate W 2  have substantially the same diameter. Although not particularly limited, the compound semiconductor wafer may be a GaAs wafer, a SiC wafer, a GaN wafer, or an InP wafer. Further, instead of the semiconductor substrate, a glass substrate may be used. 
     In the following, the first substrate W 1  may sometimes be referred to as “upper wafer W 1 ”; the second substrate W 2 , “lower wafer W 2 ”; and the combined substrate T, “combined wafer T.” As shown in  FIG.  3   , among plate surfaces of the upper wafer W 1 , a plate surface to be bonded to the lower wafer W 2  will be referred to as “bonding surface W 1   j ”, and a plate surface opposite to the bonding surface W 1   j  will be referred to as “non-bonding surface W 1   n ”. Further, among plate surfaces of the lower wafer W 2 , a plate surface to be bonded to the upper wafer W 1  will be referred to as “bonding surface W 2   j ”, and a plate surface opposite to the bonding surface W 2   j  will be referred to as “non-bonding surface W 2   n.”   
     As depicted in  FIG.  1   , the bonding system  1  includes a carry-in/out station  2  and a processing station  3 . The carry-in/out station  2  and the processing station  3  are arranged in the order of the carry-in/out station  2  and the processing station  3  along the negative Y-axis direction. Further, the carry-in/out station  2  and the processing station  3  are connected as one body. 
     The carry-in/out station  2  includes a placing table  10  and a transfer section  20 . The placing table  10  is equipped with a multiple number of placing plates  11 . Respectively provided on the placing plates  11  are cassettes C 1 , C 2 , and C 3  each of which accommodates therein a plurality (e.g., 25 sheets) of substrates horizontally. The cassette C 1  accommodates therein a plurality of upper wafers W 1 ; the cassette C 2 , a plurality of lower wafers W 2 ; and the cassette C 3 , a plurality of combined wafers T. In the cassette C 1  (C 2 ), the upper wafers W 1  (lower wafers W 2 ) are accommodated while being aligned in direction with their bonding surfaces W 1   j  (W 2   j ) facing upwards. 
     The transfer section  20  is provided adjacent to the negative Y-axis side of the placing table  10 . Provided in this transfer section  20  are a transfer path  21  extending in the X-axis direction and a transfer device  22  configured to be movable along the transfer path  21 . The transfer device  22  is configured to be movable in the Y-axis direction as well as in the X-axis direction and pivotable around the Z-axis, and serves to transfer the upper wafer W 1 , the lower wafer W 2 , and the combined wafer T between the cassettes C 1  to C 3  placed on the placing table  10  and a third processing block G 3  of the processing station  3  to be described later. 
     In addition, the number of the cassettes C 1  to C 3  placed on the placing table  10  is not limited to the illustrated example. Moreover, a cassette for collecting a defective substrate may also be provided on the placing table  10  in addition to the cassettes C 1 , C 2 , and C 3 . 
     The processing station  3  is equipped with, for example, three processing blocks G 1 , G 2  and G 3 . By way of example, the first processing block G 1  is disposed on the rear side (positive X-axis side of  FIG.  1   ) of the processing station  3 , and the second processing block G 2  is provided on the front side (negative X-axis side of  FIG.  1   ) of the processing station  3 . Further, the third processing block G 3  is disposed on the carry-in/out station  2  side (positive Y-axis side of  FIG.  1   ) of the processing station  3 . 
     Further, a transfer region  60  is formed in an area surrounded by the first to third processing blocks G 1  to G 3 . In the transfer region  60 , a transfer device  61  is disposed. The transfer device  61  has a transfer arm configured to be movable in a vertical direction and a horizontal direction and pivotable around a vertical axis, for example. 
     The transfer device  61  moves within the transfer region  60  and transfers the upper wafer W 1 , the lower wafer W 2  and the combined wafer T to preset apparatuses within the first to third processing blocks G 1  to G 3  adjacent to the transfer region  60 . 
     Provided in the first processing block G 1  are, for example, a surface modifying apparatus  33  and a surface hydrophilizing apparatus  34 . The surface modifying apparatus  33  is configured to modify the bonding surface W 1   j  of the upper wafer W 1  and the bonding surface W 2   j  of the lower wafer W 2 . The surface hydrophilizing apparatus  34  is configured to hydrophilize the modified bonding surfaces W 1   j  and W 2   j  of the upper and lower wafers W 1  and W 2 . 
     For example, the surface modifying apparatus  33  cuts a SiO 2  bond on the bonding surfaces W 1   j  and W 2   j  to form a dangling bond of Si, thus enabling the bonding surfaces W 1   j  and W 2   j  to be hydrophilized afterwards. In the surface modifying apparatus  33 , an oxygen gas as a processing gas is excited into plasma to be ionized under, for example, a decompressed atmosphere. As oxygen ions are radiated to the bonding surface W 1   j  of the upper wafer W 1  and the bonding surface W 2   j  of the lower wafer W 2 , the bonding surfaces W 1   j  and W 2   j  are modified by being plasma-processed. The processing gas is not limited to the oxygen gas and may be, by way of non-limiting example, a nitrogen gas or the like. 
     The surface hydrophilizing apparatus  34  hydrophilizes the bonding surface W 1   j  of the upper wafer W 1  and the bonding surface W 2   j  of the lower wafer W 2  with, for example, a hydrophilizing liquid such as pure water. To elaborate, the surface hydrophilizing apparatus  34  supplies the pure water onto the upper wafer W 1  or the lower wafer W 2  while rotating the upper wafer W 1  or the lower wafer W 2  held by, for example, a spin chuck. The pure water is diffused on the bonding surfaces W 1   j  and W 2   j  by a centrifugal force, and imparts an OH group to the dangling bond of Si, thus allowing the bonding surfaces W 1   j  and W 2   j  to be hydrophilized. The surface hydrophilizing apparatus  34  also has a function of cleaning the bonding surfaces W 1   j  and W 2   j.    
     In the second processing block G 2 , a bonding apparatus  41 , a first temperature adjusting apparatus  42 , and a second temperature adjusting apparatus  43  are disposed, for example. The boning apparatus  41  is configured to form the combined wafer T by bonding the hydrophilized upper and lower wafers W 1  and W 2 . The first temperature adjusting apparatus  42  is configured to adjust a temperature distribution of the upper wafer W 1  before the upper wafer W 1  is bonded, that is, before it is brought into contact with the lower wafer W 2 . The second temperature adjusting apparatus  43  is configured to adjust a temperature distribution of the lower wafer W 2  before the lower wafer W 2  is bonded, that is, before it is brought into contact with the upper wafer W 1 . In addition, in the present exemplary embodiment, although the first temperature adjusting apparatus  42  and the second temperature adjusting apparatus  43  are provided separately from the bonding apparatus  41 , they may be configured as a part of the bonding apparatus  41 . 
     In the third processing block G 3 , a first position adjusting apparatus  51 , a second position adjusting apparatus  52 , and transition apparatuses  53  and  54  are stacked in this order from top to bottom, for example (see  FIG.  2   ). The location of the individual apparatuses in the third processing block G 3  is not limited to the example shown in  FIG.  2   . The first position adjusting apparatus  51  adjusts the direction of the upper wafer W 1  in the horizontal direction by rotating the upper wafer W 1  about a vertical axis, and vertically inverts the upper wafer W 1  so that the bonding surface W 1   j  of the upper wafer W 1  faces down. The second position adjusting apparatus  52  adjusts the direction of the lower wafer W 2  in the horizontal direction by rotating the lower wafer W 2  about a vertical axis. In the transition apparatus  53 , the upper wafer W 1  is temporarily disposed. Further, in the transition apparatus  54 , the lower wafer W 2  and the combined wafer T are temporarily disposed. In addition, in the present exemplary embodiment, although the first position adjusting apparatus  51  and the second position adjusting apparatus  52  are provided separately from the bonding apparatus  41 , they may be configured as a part of the bonding apparatus  41 . 
     The bonding system  1  is equipped with a control device  90 . The control device  90  is, for example, a computer, and includes a CPU (Central Processing Unit)  91  and a recording medium  92  such as a memory. The recording medium  92  stores therein a program for controlling various kinds of processings performed in the bonding system  1 . The control device  90  controls the operation of the bonding system  1  by causing the CPU  91  to execute the program stored in the recording medium  92 . 
     Now, referring to  FIG.  4   , a bonding method according to the present exemplary embodiment will be described. The bonding method includes, for example, processes S 101  to S 109 . The processes S 101  to S 109  are performed under the control of the control device  90 . Further, the bonding method may not include all of the processes S 101  to S 109 . For example, the processes S 104  and S 108  may be omitted. Furthermore, the bonding method may include other processes in addition to the processes S 101  to S 109 . 
     First, the cassette C 1  accommodating the plurality of upper wafers W 1 , the cassette C 2  accommodating the plurality of lower wafers W 2 , and the empty cassette C 3  are placed on the placing table  10  of the carry-in/out station  2 . 
     Subsequently, the transfer device  22  takes out the upper wafer W 1  from the cassette C 1 , and transfers it to the transition apparatus  53  of the third processing block G 3  of the processing station  3 . Thereafter, the transfer device  61  takes out the upper wafer W 1  from the transition apparatus  53 , and transfers it to the surface modifying apparatus  33  of the first processing block G 1 . 
     Next, the surface modifying apparatus  33  modifies the bonding surface W 1   j  of the upper wafer W 1  (process S 101 ). The modification of the bonding surface W 1   j  is performed in the state that the bonding surface W 1   j  faces upwards. Thereafter, the transfer device  61  takes out the upper wafer W 1  from the surface modifying apparatus  33 , and transfers it to the surface hydrophilizing apparatus  34 . 
     Afterwards, the surface hydrophilizing apparatus  34  hydrophilizes the bonding surface W 1   j  of the upper wafer W 1  (process S 102 ). The hydrophilization of the bonding surface W 1   j  is performed in the state that the bonding surface W 1   j  faces upwards. Thereafter, the transfer device  61  takes out the upper wafer W 1  from the surface hydrophilizing apparatus  34 , and transfers it to the first position adjusting apparatus  51  of the third processing block G 3 . 
     Next, the first position adjusting apparatus  51  adjusts the direction of the upper wafer W 1  in the horizontal direction by rotating the upper wafer W 1  about the vertical axis, and inverts the upper wafer W 1  upside down (process S 103 ). As a result, a notch of the upper wafer W 1  is directed toward a predetermined direction, and the bonding surface W 1   j  of the upper wafer W 1  is directed downwards. Thereafter, the transfer device  61  takes out the upper wafer W 1  from the first position adjusting apparatus  51 , and transfers it to the first temperature adjusting apparatus  42  of the second processing block G 2 . 
     Then, the first temperature adjusting apparatus  42  adjusts the temperature of the upper wafer W 1  (process S 104 ). The temperature adjustment of the upper wafer W 1  is performed in the state that the bonding surface W 1   j  of the upper wafer W 1  faces downwards. Thereafter, the transfer device  61  takes out the upper wafer W 1  from the first temperature adjusting apparatus  42 , and transfers it to the bonding apparatus  41 . 
     In parallel with the above-described processing on the upper wafer W 1 , the following processing for the lower wafer W 2  is performed. First, the transfer device  22  takes out the lower wafer W 2  from the cassette C 2 , and transfers it to the transition apparatus  54  of the third processing block G 3  of the processing station  3 . Thereafter, the transfer device  61  takes out the lower wafer W 2  from the transition apparatus  54 , and transfers it to the surface modifying apparatus  33  of the first processing block G 1 . 
     Then, the surface modifying apparatus  33  modifies the bonding surface W 2   j  of the lower wafer W 2  (process S 105 ). The modification of the bonding surface W 2   j  is performed in the state that the bonding surface W 2   j  faces upwards. Thereafter, the transfer device  61  takes out the lower wafer W 2  from the surface modifying apparatus  33 , and transfers it to the surface hydrophilizing apparatus  34 . 
     Subsequently, the surface hydrophilizing apparatus  34  hydrophilizes the bonding surface W 2   j  of the lower wafer W 2  (process S 106 ). The hydrophilization of the bonding surface W 2   j  is performed in the state that the bonding surface W 2   j  faces upwards. Thereafter, the transfer device  61  takes out the lower wafer W 2  from the surface hydrophilizing apparatus  34 , and transfers it to the second position adjusting apparatus  52  of the third processing block G 3 . 
     Next, the second position adjusting apparatus  52  adjusts the direction of the lower wafer W 2  in the horizontal direction by rotating the lower wafer W 2  about the vertical axis (process S 107 ). As a result, a notch of the lower wafer W 2  is directed toward a predetermined direction. Thereafter, the transfer device  61  takes out the lower wafer W 2  from the second position adjusting apparatus  52 , and transfers it to the second temperature adjusting apparatus  43  of the second processing block G 2 . 
     Afterwards, the second temperature adjusting apparatus  43  adjusts the temperature of the lower wafer W 2  (process S 108 ). The temperature adjustment of the lower wafer W 2  is performed in the state that the bonding surface W 2   j  of the lower wafer W 2  faces upwards. Thereafter, the transfer device  61  takes out the lower wafer W 2  from the second temperature adjusting apparatus  43 , and transfers it to the bonding apparatus  41 . 
     Next, the bonding apparatus  41  bonds the upper wafer W 1  and the lower wafer W 2  to produce the combined wafer T (process S 109 ). Thereafter, the transfer device  61  takes out the combined wafer T from the bonding apparatus  41 , and transfers it to the transition apparatus  54  of the third processing block G 3 . 
     Finally, the transfer device  22  takes out the combined wafer T from the transition apparatus  54 , and transfers it to the cassette C 3  on the placing table  10 . Accordingly, the series of processes are ended. 
     Now, referring to  FIG.  5   , an example of the bonding apparatus  41  will be described. As depicted in  FIG.  5   , the bonding apparatus  41  is equipped with, for example, a support frame  101 , an upper chuck  110 , a lower chuck  120 , and a moving unit  130 . The upper chuck  110  corresponds to a first holder described in the claims, and the lower chuck  120  corresponds to a second holder described in the claims. 
     The support frame  101  supports, for example, the upper chuck  110 , the lower chuck  120 , and the moving unit  130 . The support frame  101  includes a placing table  102 , a plurality of supporting columns  103  uprightly disposed on a top surface of the placing table  102 , and an upper frame  104  fixed to upper ends of the plurality of supporting columns  103 . 
     The upper frame  104  supports the upper chuck  110  from above. The upper chuck  110  attracts and holds the upper wafer W 1  from above. Meanwhile, the lower chuck  120  is provided below the upper chuck  110 , and attracts and holds the lower wafer W 2  from below. 
     The moving unit  130  moves the upper chuck  110  and the lower chuck  120  relative to each other. By way of example, the moving unit  130  includes a first moving unit  131  for moving the lower chuck  120  in the X-axis direction. In addition, the moving unit  130  includes a second moving unit  132  for moving the lower chuck  120  in the Y-axis direction. 
     The first moving unit  131  is configured to be moved along a pair of first rails  131   a  extending in the X-axis direction. The pair of first rails  131   a  are provided on a top surface of the second moving unit  132 . The moving unit  130  moves the lower chuck  120  in the X-axis direction by moving the first moving unit  131  in the X-axis direction. 
     The second moving unit  132  is configured to be moved along a pair of second rails  132   a  extending in the Y-axis direction. A pair of second rails  132   a  are provided on a top surface of the placing table  102 . The moving unit  130  moves the first moving unit  131  and the lower chuck  120  in the Y-axis direction by moving the second moving unit  132  in the Y-axis direction. 
     The lower chuck  120  is mounted to the first moving unit  131 , and is moved in the X-axis direction and the Y-axis direction along with the first moving unit  131 . Further, the first moving unit  131  may be configured to move the lower chuck  120  in a vertical direction. Also, the first moving unit  131  may be configured to rotate the lower chuck  120  around a vertical axis. The rotation direction around the vertical axis will sometimes be referred to as θ direction. 
     The moving unit  130  moves the lower chuck  120  in the X-axis direction, the Y-axis direction and the θ direction, thus allowing the upper wafer W 1  held by the upper chuck  110  and the lower wafer W 2  held by the lower chuck  120  to be aligned in the horizontal direction. In addition, the moving unit  130  moves the lower chuck  120  in the Z-axis direction, thus allowing the upper wafer W 1  held by the upper chuck  110  and the lower wafer W 2  held by the lower chuck  120  to be aligned in the vertical direction. 
     Further, the moving unit  130  just needs to move the upper chuck  110  and the lower chuck  120  relative to each other in the X-axis direction, the Y-axis direction, and the θ direction. By way of example, the moving unit  130  may move the upper chuck  110  in the X-axis direction, the Y-axis direction, and the θ direction. Alternatively, the moving unit  130  may move the lower chuck  120  in the X-axis and Y-axis directions, while moving the upper chuck  110  in the θ direction. 
     The moving unit  130  moves the relative positions of the upper chuck  110  and the lower chuck  120  between a substrate transfer position and a bonding position. The substrate transfer position is a position where the upper chuck  110  receives the upper wafer W 1  from the transfer device  61 , the lower chuck  120  receives the lower wafer W 2  from the transfer device  61 , and the lower chuck  120  hands the combined wafer T over to the transfer device  61 . The substrate transfer position is a position where a carry-out of the combined wafer T produced by the n th  (n is a natural number equal or larger than 1) bonding and a carry-in of the upper wafer W 1  and the lower wafer W 2  to be bonded by the (n+1) th  bonding are performed in succession. 
     The transfer device  61  advances into a space directly under the upper chuck  110  when it passes the upper wafer W 1  to the upper chuck  110 . Further, when the transfer device  61  receives the combined wafer T from the lower chuck  120  and passes the lower wafer W 2  to the lower chuck  120 , the transfer device  61  advances into a space directly above the lower chuck  120 . The upper chuck  110  and the lower chuck  120  are set aside so that the transfer device  61  can easily advance into the space therebetween, and, further, the distance between the upper chuck  110  and the lower chuck  120  in the vertical direction is set to be large. 
     Meanwhile, the bonding position is a position where the upper wafer W 1  and the lower wafer W 2  are made to face each other at a predetermined distance therebetween to be bonded to each other. For example, the bonding position is a position shown in  FIG.  9   . At the bonding position, a distance G between the upper wafer W 1  and the lower wafer W 2  in the vertical direction is narrow, as compared to that at the substrate transfer position. Further, unlike at the substrate transfer position, the upper wafer W 1  and the lower wafer W 2  are overlapped at the bonding position when viewed from the vertical direction. 
     Now, referring to  FIG.  6   , the upper chuck  110  and the lower chuck  120  will be described. The upper chuck  110  is divided into a plurality of (e.g., two) regions  110   a  and  110   b  in the diametrical direction thereof. These regions  110   a  and  110   b  are provided in this order from the center toward the periphery of the upper chuck  110 . When viewed from the top, the region  110   a  has a circular shape, and the region  110   b  has an annular shape. Separate vacuum pumps  112   a  and  112   b  are connected to the regions  110   a  and  110   b , respectively. The upper chuck  110  is capable of vacuum-attracting the upper wafer W 1  for each of the regions  110   a  and  110   b . The upper chuck  110  vacuum-attracts the upper wafer W 1  horizontally by the operation of the vacuum pumps  112   a  and  112   b.    
     The bonding apparatus  41  includes a first transforming unit  180  configured to transform the upper wafer W 1  held by the upper chuck  110  so that the central portion of the upper wafer W 1  is protruded downwards. The first transforming unit  180  includes, by way of example, a push pin  181 , and a driving unit  182  configured to move the push pin  181  up and down. The push pin  181  is inserted through a through hole  113  which is vertically formed through the central portion of the upper chuck  110 . The driving unit  182  lowers the push pin  181 , thus allowing the central portion of the upper wafer W 1  to be protruded downwards. The central portion of the upper wafer W 1  is protruded below a peripheral portion of the upper wafer W 1 . A protruding amount ΔZ 1  (see  FIG.  11   ) of the central portion of the upper wafer W 1  can be adjusted by controlling the position of the push pin  181 . 
     The lower chuck  120  is divided into a plurality of (e.g., three) regions  120   a ,  120   b , and  120   c  in the diametrical direction thereof. These regions  120   a ,  120   b  and  120   c  are provided in this order from the center toward the periphery of the lower chuck  120 . When viewed from the top, the region  120   a  has a circular shape, and the regions  120   b  and  120   c  have an annular shape. Separate vacuum pumps  122   a ,  122   b  and  122   c  are connected to the regions  120   a ,  120   b , and  120   c , respectively. The lower chuck  120  is capable of vacuum-attracting the lower wafer W 2  for each of the regions  120   a ,  120   b  and  120   c . The lower chuck  120  vacuum-attracts the lower wafer W 2  horizontally by the operation of the vacuum pumps  122   a ,  122   b  and  122   c.    
     The lower chuck  120  includes, for example, a base  123  and an attraction unit  124 . The attraction unit  124  is provided on the base  123 , and attracts and holds the lower wafer W 2  from below. When viewed from above, the attraction unit  124  has, for example, a circular shape. A fastening ring  126  is provided around the attraction unit  124 . The periphery of the attraction unit  124  is fixed to the base  123  by the fastening ring  126 . A pressure-variable space  125  is formed between a top surface of the base  123  and a bottom surface of the attraction unit  124 . This pressure-variable space  125  is hermetically sealed. 
     The attraction unit  124  has a circular top surface whose diameter is larger than that of the lower wafer W 2 . Ribs  127  are provided on the top surface of the attraction unit  124 . The ribs  127  serve to separate the plurality of regions  120   a ,  120   b  and  120   c . The ribs  127  may divide the annular regions  120   b  and  120   c  into a plurality of (e.g., eight) sector-shaped sub-regions along the circumferential direction (see  FIG.  16   ). An attracting pressure for the lower wafer W 2  can be varied for each sub-region. In addition, the number and the layout of the ribs  127  are not limited to those shown in  FIG.  16   . 
     The bonding apparatus  41  is equipped with a second transforming unit  190  configured to transform the lower wafer W 2  held by the lower chuck  120 , thus allowing the central portion of the lower wafer W 2  to be protruded upwards. The second transforming unit  190  elastically transforms the attraction unit  124  by changing a pressure in the pressure-variable space  125 . The attraction unit  124  is made of, for example, ceramic such as alumina or silicon carbide. The second transforming unit  190  includes a vacuum pump  191  and an electro-pneumatic regulator  192 . The second transforming unit  190  may also be provided with a switching valve  193 . 
     The vacuum pump  191  decompresses the pressure-variable space  125  by exhausting a gas in the pressure-variable space  125 . Due to the decompression of the pressure-variable space  125 , the top surface of the attraction unit  124  becomes a horizontal plane, so that the lower wafer W 2  attracted by the attraction unit  124  becomes horizontal. The electro-pneumatic regulator  192  pressurizes the pressure-variable space  125  by supplying a gas into the pressure-variable space  125 . Due to the pressurization of the pressure-variable space  125 , the top surface of the attraction unit  124  becomes an upwardly protruding curved surface, so that the lower wafer W 2  attracted by the attraction unit  124  is protruded upwards. The switching valve  193  switches the pressure-variable space  125  between a state in which it is connected to the vacuum pump  191  and a state in which it is connected to the electro-pneumatic regulator  192 . 
     The second transforming unit  190  pressurizes the pressure-variable space  125  to allow the central portion of the lower wafer W 2  held by the lower chuck  120  to be protruded upwards. The central portion of the lower wafer W 2  is protruded above a peripheral portion of the lower wafer W 2 . A protruding amount ΔZ 2  (see  FIG.  10   ) of the central portion of the lower wafer W 2  can be adjusted by controlling the pressure of the pressure-variable space  125 . 
     A measuring unit  140  measures the protruding amount ΔZ 2  of the central portion of the lower wafer W 2 . A measurement target  141  of the measuring unit  140  is moved up and down along with the central portion of the lower wafer W 2 . The measuring unit  140  is, for example, an electrostatic capacitance sensor. The electrostatic capacitance sensor measures the protruding amount ΔZ 2  by detecting electrostatic capacitance that varies according to a distance from the measurement target  141 . The base  123  has an accommodation space  123   a  in which the measuring unit  140  is accommodated, and an insertion hole  123   b  into which the measurement target  141  is inserted. The accommodation space  123   a  and the insertion hole  123   b  are formed in the center of the base  123 . The measurement target  141  is fixed to the center of the bottom surface of the attraction unit  124 , and is moved up and down within the insertion hole  123   b.    
     Now, referring to  FIG.  7    to  FIG.  14   , details of the process S 109  in  FIG.  4    will be explained. First, the transfer device  61  carries the upper wafer W 1  and the lower wafer W 2  into the bonding apparatus  41  (process S 111 ). In the process S 111 , the relative positions of the upper chuck  110  and the lower chuck  120  are set at the substrate transfer position shown in  FIG.  5   . The upper chuck  110  attracts and holds the upper wafer W 1  horizontally from above, and the lower chuck  120  attracts and holds the lower wafer W 2  horizontally from below. 
     Then, the moving unit  130  moves the relative positions of the upper chuck  110  and the lower chuck  120  from the substrate transfer position shown in  FIG.  5    to the bonding position shown in  FIG.  9    (process S 112 ). In the process S 112 , the upper wafer W 1  and the lower wafer W 2  are aligned. For this alignment, a first camera S 1  and a second camera S 2  are used, as illustrated in  FIG.  8 A  to  FIG.  8 C . 
     The first camera S 1  is fixed to the upper chuck  110 , and serves to image the lower wafer W 2  held by the lower chuck  120 . A plurality of reference points P 21  to P 23  are previously formed on the bonding surface W 2   j  of the lower wafer W 2 . As these reference points P 21  to P 23 , patterns such as electronic circuits are used. The number of the reference points can be set as required. 
     Meanwhile, the second camera S 2  is fixed to the lower chuck  120 , and serves to image the upper wafer W 1  held by the upper chuck  110 . A plurality of reference points P 11  to P 13  are previously formed on the bonding surface W 1   j  of the upper wafer W 1 . As these reference points P 11  to P 13 , patterns such as an electronic circuits are used. The number of the reference points can be set as required. 
     First, as shown in  FIG.  8 A , the moving unit  130  adjusts the relative positions of the first camera S 1  and the second camera S 2  in the horizontal direction. To elaborate, the moving unit  130  moves the lower chuck  120  in the horizontal direction so that the second camera S 2  is located approximately directly under the first camera S 1 . Then, the first camera S 1  and the second camera S 2  image a common target X, and the moving unit  130  finely adjusts the position of the second camera S 2  in the horizontal direction so that the positions of the first camera S 1  and the second camera S 2  in the horizontal direction are coincident. Accordingly, the alignment of the first camera S 1  and the second camera S 2  is completed. 
     Subsequently, as shown in  FIG.  8 B , the moving unit  130  moves the lower chuck  120  vertically upwards, and, then, adjusts the positions of the upper chuck  110  and the lower chuck  120  in the horizontal direction. To elaborate, while the moving unit  130  is moving the lower chuck  120  in the horizontal direction, the first camera S 1  images the reference points P 21  to P 23  of the lower wafer W 2  in sequence, and the second camera S 2  images the reference points P 11  to P 13  of the upper wafer W 1  in sequence.  FIG.  8 B  shows a state in which the first camera S 1  is imaging the reference point P 21  of the lower wafer W 2  and the second camera S 2  is imaging the reference point P 11  of the upper wafer W 1 . 
     The first camera S 1  and the second camera S 2  transmit the obtained image data to the control device  90 . The control device  90  controls the moving unit  130  based on the image data obtained by the first camera S 1  and the image data obtained by the second camera S 2 , and adjusts the position of the lower chuck  120  in the horizontal direction so that the reference points P 11  to P 13  of the upper wafer W 1  and the reference points P 21  to P 23  of the lower wafer W 2  are matched. 
     Thereafter, as depicted in  FIG.  8 C , the moving unit  130  moves the lower chuck  120  vertically upwards. As a result, the distance G (see  FIG.  9   ) between the bonding surface W 2   j  of the lower wafer W 2  and the bonding surface W 1   j  of the upper wafer W 1  becomes a predetermined distance of, for example, 80 μm to 200 μm. For the adjustment of the distance G, a first displacement meter S 3  and a second displacement meter S 4  are used. 
     Like the first camera S 1 , the first displacement meter S 3  is fixed to the upper chuck  110  and measures the thickness of the lower wafer W 2  held by the lower chuck  120 . By way of example, the first displacement meter S 3  radiates light to the lower wafer W 2 , receives reflected light from the top and bottom surfaces of the lower wafer W 2 , and measures the thickness of the lower wafer W 2 . This thickness measurement is performed when the moving unit  130  moves the lower chuck  120  in the horizontal direction, for example. The method whereby the first displacement meter S 3  measures the thickness is, by way of non-limiting example, a confocal method, a spectral interference method, a triangulation method, or the like. A light source of the first displacement meter S 3  is an LED or a laser. 
     Meanwhile, the second displacement meter S 4  is fixed to the lower chuck  120 , the same as the second camera S 2 , and measures the thickness of the upper wafer W 1  held by the upper chuck  110 . For example, the second displacement meter S 4  radiates light to the upper wafer W 1 , receives reflected light from the top and bottom surfaces of the upper wafer W 1 , and measures the thickness of the upper wafer W 1 . This thickness measurement is performed when the moving unit  130  moves the lower chuck  120  in the horizontal direction, for example. The method whereby the second displacement meter S 4  measures the thickness is, for example, a confocal method, a spectral interference method, a triangulation method, or the like. A light source of the second displacement meter S 4  is an LED or a laser. 
     The first displacement meter S 3  and the second displacement meter S 4  transmit the measured data to the control device  90 . The control device  90  controls the moving unit  130  based on the data measured by the first displacement meter S 3  and the data measured by the second displacement meter S 4 , and adjusts the position of the lower chuck  120  in the vertical direction so that the distance G becomes a set value ΔZ 3  (ΔZ 3 =ΔZ 1 +ΔZ 2 ). Here, ΔZ 1  is a protruding amount of the central portion of the upper wafer W 1  in a process S 114 , and ΔZ 2  is a protruding amount of the central portion of the lower wafer W 2  in a process S 113 . 
     Subsequently, as shown in  FIG.  10   , the control device  90  controls the second transforming unit  190  to make the central portion of the lower wafer W 2  protruded upwards (process S 113 ). The control device  90  pressurizes the pressure-variable space  125 , thus allowing the central portion of the lower wafer W 2  held by the attraction unit  124  to be protruded upwards. The protruding amount ΔZ 2  is measured by the measuring unit  140 , and the pressure of the pressure-variable space  125  is controlled so that the measurement value becomes a set value. 
     Further, in the present exemplary embodiment, although the lower wafer W 2  is transformed after it is attracted to the lower chuck  120 , the lower chuck  120  may be first transformed and then the lower wafer W 2  may be attracted to the lower chuck  120 . In the latter case, bending precision of the lower wafer W 2  can be improved, as compared to the former case, so that an unintended position deviation of the reference points P 21  to P 23  of the lower wafer W 2  can be suppressed. Therefore, the bonding precision can be bettered. 
     Thereafter, as illustrated in  FIG.  11   , the control device  90  stops the operation of the vacuum pump  112   a  to cancel the vacuum attraction of the upper wafer W 1  in the region  110   a . Thereafter, the control device  90  lowers the push pin  181  of the first transforming unit  180 , thus allowing the central portion of the upper wafer W 1  to be protruded downwards (process S 114 ). As a result, the central portions of the upper wafer W 1  and the lower wafer W 2  come into contact with each other and are bonded to each other. Here, the order of the processes S 113  and S 114  may be reversed. 
     Since the bonding surface W 1   j  of the upper wafer W 1  and the bonding surface W 2   j  of the lower wafer W 2  are modified, a van der Waals force (intermolecular force) is first generated between the bonding surfaces W 1   j  and W 2   j , so that the bonding surfaces W 1   j  and W 2   j  are bonded. Further, since the bonding surface W 1   j  of the upper wafer W 1  and the bonding surface W 2   j  of the lower wafer W 2  are hydrophilized, hydrophilic groups (for example, OH groups) are hydrogen-bonded, so that the bonding surfaces W 1   j  and W 2   j  are strongly bonded to each other. 
     The van der Waals force is generated even when a gap exists between the upper wafer W 1  and the lower wafer W 2 . The narrower the gap is, the greater the van der Waals force may be. The van der Waals force makes the upper wafer W 1  and the lower wafer W 2  attracted to each other. Therefore, after the central portions of the upper wafer W 1  and the lower wafer W 2  come into contact with each other as shown in  FIG.  11   , a region A where they are in contact with each other is widened, as illustrated in  FIG.  12   . Since, however, the peripheral portion of the upper wafer W 1  is attracted to and held by the upper chuck  110 , the expansion of the region A temporarily stops at a certain point between the central portion and the peripheral portion of the upper wafer W 1 . 
     Next, as depicted in  FIG.  13   , the control device  90  stops the operation of the vacuum pump  112   b  to release the vacuum attraction of the upper wafer W 1  in the region  110   b , thus allowing the peripheral portion of the upper wafer W 1  to fall down (process S 115 ). As a result, the region A is expanded up to the peripheral portion, so that the entire bonding surface W 1   j  of the upper wafer W 1  and the entire bonding surface W 2   j  of the lower wafer W 2  come into contact with each other to produce the combined wafer T. 
     According to the present exemplary embodiment, as shown in  FIG.  11   , by making the central portion of the upper wafer W 1  protruded below the peripheral portion of the upper wafer W 1 , the upper wafer W 1  is bent into a downwardly protruding shape, and, also, by making the central portion of the lower wafer W 2  protruded above the peripheral portion of the lower wafer W 2 , the lower wafer W 2  is bent into an upwardly protruding shape. By bending the upper wafer W 1  and the lower wafer W 2  into vertically symmetrical shapes, a difference in elongation rates between the upper wafer W 1  and the lower wafer W 2  can be reduced. As a result, after the bonding, a deviation between the reference points P 11  to P 13  of the upper wafer W 1  and the reference points P 21  to P 23  of the lower wafer W 2  when viewed from the vertical direction can be reduced. 
     After the combined wafer T is obtained, the control device  90  raises the push pin  181  of the first transforming unit  180 . Further, the control device  90  decompresses the pressure-variable space  125  of the lower chuck  120 , and cancels the transformation of the lower wafer W 2 . The lower chuck  120  attracts and holds the combined wafer T horizontally from below. 
     Next, the moving unit  130  moves the relative positions of the upper chuck  110  and the lower chuck  120  from the bonding position to the substrate transfer position (process S 116 ). For example, the moving unit  130  first lowers the lower chuck  120  to widen the distance between the lower chuck  120  and the upper chuck  110  in the vertical direction. Then, the moving unit  130  moves the lower chuck  120  sideways, and sets the lower chuck  120  and the upper chuck  110  aside. 
     Next, the transfer device  61  performs a carry-out of the combined wafer T with respect to the bonding apparatus  41  (process S 117 ). Specifically, the lower chuck  120  releases the attraction and holding of the combined wafer T. Then, the transfer device  61  receives the combined wafer T from the lower chuck  120  and carries it out of the bonding apparatus  41 . 
     By the way, the protruding amount ΔZ 2  of the central portion of the lower wafer W 2  may be changed for the purpose of changing the elongation rate of the lower wafer W 2  or the like. Here, the purpose of changing the protruding amount ΔZ 2  is not particularly limited. When changing the protruding amount ΔZ 2 , if the protruding amount ΔZ 1  of the central portion of the upper wafer W 1  is kept constant without being changed, the behavior of the expansion of the region A is changed. 
     For example, if ΔZ 2  is made smaller while ΔZ 1  remains the same, the sum ΔZ 3  of ΔZ 1  and ΔZ 2  becomes smaller. Accordingly, as shown in  FIG.  11   , when the central portions of the upper wafer W 1  and the lower wafer W 2  come into contact with each other, a gap between the peripheral portions of the upper wafer W 1  and the lower wafer W 2  is narrowed. The narrower the gap is, the larger the van der Waals force gets, making it easier for the region A to be expanded. Therefore, as shown in  FIG.  12   , the size of the region A when the expansion of the region A is temporarily stopped increases. 
     In this way, if ΔZ 1  is not changed but kept constant when ΔZ 2  is changed, the behavior of the expansion of the region A is changed. If the behavior of the expansion of the region A is changed, the bonding precision decreases, which may cause an increase of a deviation between the reference points P 11  to P 13  of the upper wafer W 1  and the reference points P 21  to P 23  of the lower wafer W 2  after the bonding, when viewed from the vertical direction. 
     The control device  90  of the present exemplary embodiment changes the protruding amount ΔZ 1  of the central portion of the upper wafer W 1  according to the protruding amount ΔZ 2  of the central portion of the lower wafer W 2 . Accordingly, the behavior of the expansion of the region A can be kept constant, so that the bonding precision can be improved. The control device  90  may change the sum ΔZ 3  of ΔZ 1  and ΔZ 2  (ΔZ 3 =ΔZ 1 +ΔZ 2 ) according to ΔZ 2 . 
     Now, referring to  FIG.  14   , an example of the relationship between ΔZ 1 , ΔZ 2  and ΔZ 3  will be discussed. The control device  90  performs, for example, a control of decreasing ΔZ 1  when ΔZ 2  increases, as shown in  FIG.  14   . Accordingly, it is possible to suppress a change in the sum ΔZ 3  of ΔZ 1  and ΔZ 2 , so that the size of the region A when the expansion of the region A is temporarily stopped can be maintained constant, as illustrated in  FIG.  12   . Since the behavior of the expansion of the region A can be kept constant, the bonding precision can be improved. 
     The control device  90  may also perform a control of increasing the sum ΔZ 3  of ΔZ 1  and ΔZ 2  when the protruding amount ΔZ 2  of the central portion of the lower wafer W 2  increases. When the decrease rate of ΔZ 1  is small as compared to the increase rate of ΔZ 2 , ΔZ 3  becomes large. Thus, a change in the van der Waals force caused by the gap between the upper and lower wafers can be suppressed, so that the size of the region A when the expansion of the region A is temporarily stopped as shown in  FIG.  12    can be maintained constant. Since the behavior of the expansion of the region A can be kept constant, the bonding precision can be improved. 
     The control device  90  may have a function of changing the setting of the protruding amount ΔZ 2  of the central portion of the lower wafer W 2 . By way of example, the control device  90  may acquire bending data of the upper wafer W 1  under no load as shown in  FIG.  15   , and may change the protruding amount ΔZ 2  of the central portion of the lower wafer W 2  according to the bending of the upper wafer W 1  under no load. Here, no load means a state in which a stress on the surface of the substrate is substantially zero, for example, a state in which no attracting pressure is generated. Further, in  FIG.  15   , a gray scale represents a height difference. The bending of the upper wafer W 1  under no load is not limited to the bending shown in  FIG.  15   . 
     A cross-sectional shape from the central portion to the peripheral portion of the upper wafer W 1  when the central portions of the upper wafer W 1  and the lower wafer W 2  are in contact with each other as shown in  FIG.  11    changes according to the bending of the upper wafer W 1  under no load. Therefore, if the bending of the upper wafer W 1  under no load is changed, the behavior of the expansion of the region A may be changed. 
     If the control device  90  changes the protruding amount ΔZ 2  of the central portion of the lower wafer W 2  according to the bending of the upper wafer W 1  under no load, the behavior of the expansion of the region A can be maintained constant, so that the bonding precision can be improved. In addition, the relationship between the bending of the upper wafer W 1  under no load and the protruding amount ΔZ 2  of the central portion of the lower wafer W 2  is determined in advance by experiments or the like and stored in the recording medium  92  in advance to be read and used later. 
     The bending of the upper wafer W 1  under no load may be measured with a measuring device (not shown). As the measuring device, a commercially available three-dimensional shape measuring device or the like may be used. The measuring device may be provided inside the bonding system  1 , or outside the bonding system  1 . The measuring device transmits the measurement data of the bending to the control device  90 , and the control device  90  receives the measurement data of the bending sent by the measuring device. 
     The control device  90  may acquire data on a position deviation of the reference points P 11  to P 13  formed on the bonding surface W 1   j  of the upper wafer W 1 , and may vary the protruding amount ΔZ 2  of the central portion of the lower wafer W 2  according to the position deviation of the reference points P 11  to P 13 . The position deviation of the reference points P 11  to P 13  may be a position deviation from target positions when the reference points P 11  to P 13  are formed on the bonding surface W 1   j , or may be a position deviation from the reference points P 21  to P 23  formed on the bonding surface W 2   j  of the lower wafer W 2 . In the former case, the data of the position deviation is acquired from an apparatus that forms the reference points P 11  to P 13 , for example, an apparatus that forms an electronic circuit. In the latter case, the data of the position deviation can be acquired by using the first camera S 1  and the second camera S 2 . 
     Likewise, the control device  90  may acquire data on a position deviation of the reference points P 21  to P 23  formed on the bonding surface W 2   j  of the lower wafer W 2 , and may vary the protruding amount ΔZ 2  of the central portion of the lower wafer W 2  according to the position deviation of the reference points P 21  to P 23 . The position deviation of the reference points P 21  to P 23  may be a position deviation from target positions when the reference points P 21  to P 23  are formed on the bonding surface W 2   j , or may be a position deviation from the reference points P 11  to P 13  formed on the bonding surface W 1   j  of the upper wafer W 1 . In the former case, the data of the position deviation is acquired from an apparatus that forms the reference points P 21  to P 23 , for example, an apparatus that forms an electronic circuit. In the latter case, the data of the position deviation can be acquired by using the first camera S 1  and the second camera S 2 . 
     Moreover, in order to perform the control of maintaining the behavior of the expansion of the region A constant, the control device  90  may control, besides (A) the protruding amount ΔZ 1  of the central portion of the upper wafer W 1  or (B) the protruding amount ΔZ 2  of the central portion of the lower wafer W 2 , (C) an attracting force of the upper chuck  110 , (D) an attracting force of the lower chuck  120 , (E) a driving force for lowering the push pin  181 , or the like. (D) The attracting force of the lower chuck  120  includes a distribution of the attracting pressure of the lower chuck  120 . 
     So far, the exemplary embodiment of the substrate processing apparatus and the substrate processing method according to the present disclosure have been described. However, the present disclosure is not limited to the above-described exemplary embodiment and the like. Various changes, modifications, substitutions, additions, deletions and combinations may be made within the scope of the claims, which are all incorporated within a technical scope of the present disclosure. 
     According to the exemplary embodiment, it is possible to improve the bonding precision between the substrates. 
     From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting. The scope of the inventive concept is defined by the following claims and their equivalents rather than by the detailed description of the exemplary embodiments. It shall be understood that all modifications and embodiments conceived from the meaning and scope of the claims and their equivalents are included in the scope of the inventive concept.