Patent Publication Number: US-6706618-B2

Title: Substrate processing apparatus, substrate support apparatus, substrate processing method, and substrate fabrication method

Description:
This is divisional of application Ser. No. 09/138,802, filed Aug. 24, 1998 now U.S. Pat. No. 6,451,670. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a substrate processing apparatus, a substrate support apparatus, a substrate processing method, and a substrate fabrication method and, more particularly, to a substrate processing apparatus, a substrate support apparatus, and a substrate processing method by which two substrates are overlapped and contacted, and a substrate fabrication method using any of these apparatuses and method. 
     2. Description of the Related Art 
     A method of contacting two wafers (substrates) and bonding the contacted wafers by, e.g., anodization, pressing, or heat treatment is suited to the fabrication of a wafer having an SOI structure or the like. 
     FIGS. 25A and 25B are schematic views showing a part of a process of bonding wafers. In this bonding process, as shown in FIG. 25A, a first wafer  1  is set on a wafer support jig  201  with the bonding surface of the first wafer  1  turned up, and a second wafer  2  is gently overlapped on the first wafer  1  with the bonding surface of the second wafer  2  turned down. In this state, as shown in FIG. 25A, the upper wafer  2  floats by the gas (e.g., air or inert gas) between the wafers. 
     Next, as shown in FIG. 25B, a press pin  202  presses the center and its vicinity of the upper wafer  2  before the gas between the wafers  1  and  2  is completely discharged. Consequently, air between the central portions of the wafers is pushed toward the periphery to first contact the central portions of the wafers  1  and  2 . Thereafter, air between the wafers is gradually pushed toward the periphery to increase the area of the contact portion. Finally, the two wafers are entirely contacted. 
     This method is useful as a method of contacting two wafers by a simple jig and is also considered as a fundamental technique readily applicable to large-diameter wafers. 
     Generally, the in-plane uniformity of a wafer becomes difficult to ensure as the diameter of the wafer increases. The above method is expected to be able to solve the problem of this in-plane uniformity even when the method is applied to the fabrication of large-diameter wafers. However, a technique suited to larger diameters is being desired. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an apparatus and a method of easily contacting large-diameter substrates. 
     A substrate processing apparatus according to the present invention is a substrate processing apparatus for overlapping and contacting two substrates comprising a support unit for supporting a first substrate, and a press unit for pressing a second substrate opposed to the first substrate supported by the support unit, wherein the support unit can support the first substrate while a predetermined portion of the first substrate is so curved as to separate from the second substrate and is not curved. 
     In the above substrate processing apparatus, the press unit preferably presses a back surface of the second substrate opposed to the first substrate supported by the support unit. 
     In the above substrate processing apparatus, it is preferable that the support unit can support the first substrate while a peripheral portion of the first substrate is so curved as to separate from the second substrate and is not curved, and the press unit press a substantially central portion of the second substrate opposed to the first substrate supported by the support unit. 
     In the above substrate processing apparatus, it is preferable that the supports unit support the first substrate in a curved state until a predetermined time elapses after the press unit starts pressing a back surface of the second substrate, and support the first substrate in an uncurved state thereafter. 
     In the above substrate processing apparatus, it is preferable that the support unit supports the first substrate in a curved state until a contact portion of the first and second substrates spreads to a predetermined region after the press unit starts pressing a back surface of the second substrate, and supports the first substrate in an uncurved state thereafter. 
     In the above substrate processing apparatus, the support unit preferably comprises an adjusting unit for adjusting the curve of the first substrate to spread a contact portion of the first and second substrates step by step from a center to a periphery thereof. 
     In the above substrate processing apparatus, the adjusting unit preferably adjusts the curve of the first substrate to spread the contact portion of the first and second substrates concentrically step by step. 
     In the above substrate processing apparatus, the support unit preferably comprises a chuck unit for curving the first substrate by chucking a back surface of the first substrate. 
     In the above substrate processing apparatus, it is preferable that the support unit comprises a support table having a peripheral portion lower than a central portion, and the support table chuck the first substrate. 
     In the above substrate processing apparatus, the support table preferably comprises a disk-like central portion and one or a plurality of annular peripheral portions. 
     In the above substrate processing apparatus, it is preferable that the support table comprises a disk-like central portion and an annular peripheral portion, and a chuck surface of the peripheral portion be inclined to a chuck surface of the central portion. 
     In the above substrate processing apparatus, a chuck surface of the central portion and a chuck surface of the peripheral portion preferably have chuck mechanisms for chucking the first substrate. 
     In the above substrate processing apparatus, it is preferable that frog-like pins for supporting the first substrate be formed on the chuck surfaces of the central portion and/or the peripheral portion, and the first substrate be chucked by evacuating a space between the pins. 
     In the above substrate processing apparatus, only a chuck surface of the peripheral portion of the support table preferably has a chuck mechanism for chucking the first substrate. 
     In the above substrate processing apparatus, the support table of the support unit preferably chucks only a periphery of the first substrate to separate a central portion of the first substrate from a central portion of the support table. 
     In the above substrate processing apparatus, it is preferable that the apparatus further comprises a substrate manipulation unit for supporting the second substrate with the second substrate opposing the first substrate supported by the support unit, and releasing the second substrate, and the press unit press the second substrate when the substrate manipulation unit releases the second substrate. 
     In the above substrate processing apparatus, it is preferable that the support unit substantially horizontally support the first substrate, and the substrate manipulation unit substantially horizontally support the second substrate above the first substrate and release the second substrate. 
     A substrate support apparatus according to the present invention is a substrate support apparatus for supporting one of two substrates when the two substrates are overlapped and contacted comprising a support unit capable of supporting the substrate in a curved state and an uncurved state. 
     In the above substrate support apparatus, the support unit preferably can support the substrate while a periphery of the substrate is curved and is not curved with respect to a central portion of the substrate. 
     In the above substrate support apparatus, the support unit preferably supports the substrate first in a curved state and then in an uncurved state. 
     In the above substrate support apparatus, the support unit preferably comprises an adjusting unit for adjusting the curve of the substrate to spread a contact portion of the supported substrate and the other substrate to be contacted to the substrate step by step from a center to a periphery thereof. 
     In the above substrate support apparatus, the adjusting unit preferably adjusts the curve of the supported substrate to spread the contact portion concentrically step by step. 
     In the above substrate support apparatus, the support unit preferably curves the substrate by chucking a back surface of the substrate. 
     In the above substrate support apparatus, it is preferable that the support unit comprises a support table having a peripheral portion lower than a central portion, and the support table chuck the first substrate. 
     In the above substrate support apparatus, the support table preferably comprises a disk-like central portion and one or a plurality of annular peripheral portions. 
     In the above substrate support apparatus, it is preferable that the support table comprises a disk-like central portion and an annular peripheral portion, and a chuck surface of the peripheral portion be inclined to a chuck surface of the central portion. 
     In the above substrate support apparatus, a chuck surface of the central portion and a chuck surface of the peripheral portion preferably have chuck mechanisms for chucking the substrate. 
     In the above substrate support apparatus, only a chuck surface of the peripheral surface of the support table preferably has a chuck mechanism for chucking the substrate. 
     In the above substrate support apparatus, the support table of the support unit preferably chucks only a periphery of the substrate to separate a central portion of the substrate from a central portion of the support table. 
     A substrate processing method according to the present invention is a substrate processing method of overlapping and contacting two substrates comprising the steps of opposing first and second substrates to each other, pressing the second substrate while a periphery of the first substrate is so curved as to separate from a periphery of the second substrate, and setting the first substrate in an uncurved state after a predetermined time elapses. 
     A substrate processing method according to the present invention is a substrate processing method of overlapping and contacting two substrates comprising the steps of opposing first and second substrates to each other, pressing the second substrate while a periphery of the first substrate is so curved as to separate from a periphery of the second substrate, and setting the first substrate in an uncurved state after a contact portion of the first and second substrates spreads to a predetermined region. 
     A substrate processing method according to the present invention is a substrate processing method of overlapping and contacting two substrates comprising the steps of opposing first and second substrates to each other, pressing the second substrate while a periphery of the first substrate is so curved as to separate from a periphery of the second substrate, and adjusting the curve of the first substrate to spread a contact portion of the first and second substrates step by step from a center to a periphery thereof. 
     A substrate processing method according to the present invention is a substrate processing method of overlapping and contacting two substrates comprising the steps of opposing first and second substrates to each other, pressing the second substrate while a periphery of the first substrate is so curved as to separate from a periphery of the second substrate, and adjusting the curve of the first substrate to spread a contact portion of the first and second substrates concentrically step by step from a center to a periphery thereof. 
     A substrate processing method according to the present invention is a substrate processing method of overlapping and contacting two substrates comprising the steps of opposing first and second substrates to each other, pressing the second substrate while a predetermined portion of the first substrate is so curved as to separate from the second substrate, and setting the first substrate in an uncurved state after a predetermined time elapses. 
     A substrate processing method according to the present invention is a substrate processing method of overlapping and contacting two substrates comprising the steps of opposing first and second substrates to each other, pressing the second substrate while a predetermined portion of the first substrate is so curved as to separate from the second substrate, and setting the first substrate in an uncurved state after a contact portion of the first and second substrates spreads to a predetermined region. 
     A substrate processing method according to the present invention is a substrate processing method of overlapping and contacting two substrates comprising the steps of opposing first and second substrates to each other, pressing the second substrate while a predetermined portion of the first substrate is so curved as to separate from the second substrate, and adjusting the curve of the first substrate to spread a contact portion of the first and second substrates step by step from a center to a periphery thereof. 
     In each of the above substrate processing methods, a back surface of the second substrate is preferably pressed. 
     Further objects, features, and advantages of the present invention will become apparent from the following detailed description of embodiments of the present invention with reference to the accompanying chucks. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view schematically showing the overall arrangement of a wafer processing apparatus according to one preferred embodiment of the present invention; 
     FIG. 2 is an enlarged view of a part of FIG. 1; 
     FIG. 3 is a view showing an example of a vacuum chuck mechanism formed in a wafer support table; 
     FIG. 4 is a view showing the example of the vacuum chuck mechanism formed in the wafer support table; 
     FIGS. 5A to  5 F are views conceptually showing the way a contact portion spreads when two wafers are contacted by the wafer processing apparatus; 
     FIGS. 6A to  6 F are views conceptually showing the way a contact portion spreads when two wafers are contacted by a conventional wafer processing apparatus; 
     FIGS. 7 to  12  are sectional views taken along a line A-A′ of the wafer processing apparatus shown in FIGS. 1 and 2; 
     FIG. 13 is a block diagram showing the configuration of a control system of the wafer processing apparatus; 
     FIG. 14 is a flow chart showing a control procedure of the wafer processing apparatus; 
     FIG. 15 is a sectional view showing the arrangement of a wafer support table according to the second embodiment; 
     FIG. 16 is a sectional view showing the arrangement of a wafer support table according to the third embodiment; 
     FIG. 17 is a sectional view showing the arrangement of a wafer support table according to the fourth embodiment; 
     FIG. 18 is a sectional view showing the arrangement of the wafer support table according to the fourth embodiment; 
     FIG. 19 is a view showing the state wherein preparations for a process of contacting two wafers are complete; 
     FIG. 20 is a schematic view showing the state wherein the contact portion has reached a contact boundary b 1 ; 
     FIG. 21 is a schematic view showing the state wherein the contact portion has reached a contact boundary b 2 ; 
     FIG. 22 is a schematic view showing the state wherein the contact portion has reached a contact boundary b 3 ; 
     FIG. 23 is a schematic view showing the state wherein the entire surfaces of two wafers are contacted; 
     FIGS. 24A to  24 F are sectional views showing an example of a process of fabricating a wafer having an SOI structure or the like; and 
     FIGS. 25A and 25B are schematic views showing a part of a process of bonding wafers. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail below with reference to the accompanying chucks. 
     [First Embodiment] 
     FIG. 1 is a perspective view schematically showing the overall arrangement of a wafer processing apparatus  100  according to this embodiment. FIG. 2 is an enlarged view of a part of FIG.  1 . FIGS. 3 and 4 are sectional views showing the arrangement of a wafer support unit of the wafer processing apparatus  100  shown in FIGS. 1 and 2. FIGS. 7 to  12  are sectional views taken along a line A-A′ of the wafer processing apparatus shown in FIGS. 1 and 2. FIGS. 7 to  12  show operation of contacting two wafers. 
     This wafer processing apparatus  100  is an apparatus for overlapping and contacting two wafers and suited to carry out a method of fabricating a wafer having an SOI structure or the like by bonding two wafers. 
     The wafer processing apparatus  100  comprises a wafer support table  3  and a wafer moving mechanism  4 . The wafer support table  3  supports the back surface of a first wafer  1  (FIG.  3 ). The wafer moving mechanism  4  chucks the back surface of a second wafer  2  (FIG. 3) and opposes the second wafer  2  substantially parallel to the first wafer  1 . 
     The wafer support table  3  is preferably so constructed as to come in contact only with the back surface of the first wafer  1 . This prevents contamination of the first wafer  1  by particles and break of the periphery of the first wafer  1 . This wafer support table  3  has a vacuum chuck mechanism for chucking the first wafer  1 . FIGS. 3 and 4 show a wafer support mechanism which is pressing the central portion of the wafer  2  with a press pin  6   a  (FIGS.  11  and  12 ). 
     The wafer moving mechanism  4  is preferably so constructed as to come in contact only with the back surface of the second wafer  2 . In this embodiment, the wafer moving mechanism  4  has a groove  4   a  for vacuum-chucking a wafer. To chuck the second wafer  2 , it is only necessary to evacuate the space in the groove  4   a . While a wafer chuck unit  4   c  is chucking the back surface of the second wafer  2 , the wafer moving mechanism  4  rotates the second wafer  2  through about 180° around a shaft  4   b  and opposes the second wafer  2  substantially parallel to the first wafer  1 . The shaft  4   b  is positioned substantially halfway between the wafer support table  3  and the wafer chuck unit  4   c.    
     This wafer processing apparatus  100  also comprises displacement detectors  12  and  15  and a Z-axis stage  5  (FIG. 7) as a mechanism for adjusting the gap between the two opposing wafers  1  and  2 . The displacement detector  15  measures the thickness of the first wafer  1  after the first wafer  1  is placed on the wafer support table  3 . The displacement detector  12  measures the thickness of the second wafer  2  after the second wafer  2  is chucked by the wafer chuck unit  4   c . On the basis of the measurement results from the two displacement detectors  12  and  15 , the Z-axis stage  5  (FIG. 7) vertically moves the wafer support table  3  to adjust the gap between the wafers  1  and  2  to a set value. 
     The wafer processing apparatus  100  further comprises a press mechanism  6  for pressing substantially the center of the upper wafer  2  while the two wafers  1  and  2  are so supported as to face each other. After the two wafers  1  and  2  are so supported as to face each other, the press pin  6   a  of the press mechanism  6  pivots on a shaft  6   b  to the vicinity of the back surface of the upper wafer  2 . When the wafer chuck unit  4   c  of the wafer moving mechanism  4  releases the upper wafer  2 , the press mechanism  6  pushes the press pin  6   a  against the back surface of the upper wafer  2  to press the back surface. The two wafers  1  and  2  are gradually contacted from the pressed portion toward the periphery. Accordingly, the gas between the wafers  1  and  2  is discharged toward the periphery and prevented from remaining between the wafers  1  and  2 . 
     The wafer  2  is preferably pressed by the press pin  6   a  at substantially the same time the wafer chuck unit  4   c  releases the wafer  2 . If this is the case, the press operation can be started while the gap between the two wafers  1  and  2  is kept adjusted to a set value. Consequently, the contacted wafers can be given uniform quality. It is also possible to effectively prevent any gas from remaining between the wafers  1  and  2  and prevent misalignment between the wafers  1  and  2 . 
     The press mechanism  6  incorporates a vibrator (e.g., a piezoelectric element) for vibrating the press pin  6   a . The press mechanism  6  can efficiently discharge the gas between the wafers  1  and  2  by vibrating the press pin  6   a  when pressing the wafer  2 . 
     Pressing of the wafer  2  by the press pin  6   a  can also be controlled at another timing. For example, after the wafer  2  is released, it can be pressed at a predetermined timing before a predetermined amount or more of gas between the wafers  1  and  2  is discharged, when a predetermined time is counted, or at a predetermined timing before the distance between the wafers  1  and  2  is decreased to a predetermined distance or less by the weight of the wafer  2  and the like. 
     The wafer processing apparatus  100  also comprises a wafer transfer robot  10  and a wafer alignment unit  11 . The wafer transfer robot  10  sets the wafers  1  and  2  on the wafer support table  3  and the wafer chuck unit  4   c , respectively, and receives the contacted wafers from the wafer support table  3 . 
     In this wafer processing apparatus  100 , wafer cassettes  7  and  8  containing unprocessed wafers  1  and  2 , respectively, and a wafer cassette  9  for accommodating processed wafers are arranged in each individual predetermined positions. In this embodiment, the unprocessed wafers  1  and  2  are accommodated in the wafer cassettes  7  and  8  with the back surfaces of these wafers turned down. 
     When the start of a process of contacting wafers is designated by an operation switch  16   b  of an operation panel  16 , the wafer transfer robot  10  chucks the back surface of an unprocessed wafer  1  contained in the wafer cassette  7  and transfers the wafer  1  to the wafer alignment unit  11 . The wafer alignment unit  11  detects the central position and the direction (e.g., the position of the orientation flat or notch) of the transferred wafer  1  by using a sensor and adjusts the detected central position and direction. The wafer alignment unit  11  is preferably so constructed as to come in contact only with the back surface of the wafer  1 . 
     After that, the wafer transfer robot  10  receives the wafer  1  thus completely aligned and loads the wafer  1  in a predetermined position on load pins  13  projecting upward from the wafer support table  3 . After the wafer  1  is loaded on the load pins  13  in this manner, the wafer support table  3  moves up and supports the wafer  1 . The wafer  1  is already aligned by the wafer alignment unit  11  and transferred onto the wafer support table  3  while being kept aligned. Therefore, it is unnecessary to again adjust the central position and direction of the wafer  1  on the wafer support table  3 . However, an arrangement by which the wafer  1  is aligned on the wafer support table  3  can also be used. 
     Next, the wafer transfer robot  10  picks up an unprocessed wafer  2  from the wafer cassette  8 . Following the same procedure as above, the wafer alignment unit  11  adjusts the central position and direction of the wafer  2  and loads the wafer  2  in a predetermined position on load pins  14  projecting upward from the wafer chuck unit  4   c . After the wafer  2  is thus loaded on the load pins  14 , the wafer chuck unit  4   c  pivots on the shaft  4   c  and comes in contact with the back surface of the wafer  2 . The wafer chuck unit  4   c  chucks the wafer  2  by evacuating the space in the groove  4   a . As in the above procedure, the wafer  2  is already aligned by the wafer alignment unit  11  and chucked by the wafer chuck unit  4   c  while being kept aligned. This obviates the need for readjusting the central position and direction of the wafer  2  when the wafer  2  is chucked. When the wafer  2  is chucked, it is also effective to retract the load pins  14  downward, instead of pivoting the wafer chuck unit  4   c.    
     While the wafers  1  and  2  are supported by the wafer support table  3  and the wafer chuck unit  4   c  as described above, the displacement detectors  15  and  12  measure the thicknesses of the wafers  1  and  2 . More specifically, the displacement detectors  15  and  12  move sensors  15   a  and  12   a  to positions above the wafers  1  and  2 , irradiate light onto the wafers  1  and  2 , and measure the thicknesses of the wafers  1  and  2  on the basis of the reflected light. 
     When the measurements of the thicknesses of the wafers  1  and  2  are complete, the wafer chuck unit  4   c  pivots through around 180° on the shaft  4   b  as described above and opposes the wafer  2  substantially parallel to the wafer  1 . Then, the Z-axis stage  5  adjusts the gap between the wafers  1  and  2 , and the press pin  6   a  presses the wafer  2  to complete the contacting process. 
     When the contacting process is finished, the wafer support table  3  is moved down by the Z-axis stage  5  to allow the load pints  13  to support the processed wafer. The wafer transfer robot  10  receives the processed wafers and places the wafers into the wafer cassette  9 . 
     By repeatedly executing the above procedures, a plurality of wafers contained in the wafer cassettes  7  and  8  can be successively processed. 
     The arrangement of the wafer support table  3  will be described below. The wafer support table  3  has a disk-like central portion  3   c  and an annular peripheral portion  3   d . Chuck grooves  3   a  and  3   b  for vacuum-chucking the wafer  1  are formed in the chuck surfaces (the surfaces for chucking the wafer  1 ) of the central portion  3   c  and the peripheral portion  3   d , respectively. In the embodiment shown in FIGS. 3 and 4, the chuck grooves  3   a  are nine concentric annular grooves, and the chuck grooves  3   b  are four concentric annular grooves. 
     The chuck grooves  3   a  and  3   b  are connected to chuck holes  18   a  and  18   b , respectively, and further connected to valves  19   a  and  19   b , respectively. These valves  19   a  and  19   b  are connected to a vacuum pump  20  through a pipe  18 . Wafer chuck operations by the chuck grooves  3   a  and  3   b  can be independently controlled by the valves  19   a  and  19   b  which can be separately opened and closed. 
     The chuck mechanism for the wafer  1  can take various forms. One preferable example is a mechanism which supports the wafer  1  by pins arranged in the form of a frog and evacuates the space between these pins to chuck the wafer  1  to the chuck surface of the wafer support table  3 . This chuck mechanism alleviates the influence (e.g., poor planarity of the wafer  1 ) of particles and the like between the wafer  1  and the wafer support table  3 . One example of this chuck mechanism when the size of a wafer is  8  inches is a chuck mechanism in which the diameter of each pin is about 0.2 mm and the distance (pitch) between pins is about 2.2 mm. In this mechanism, the contact ratio between the wafer and the wafer support table is approximately 1.2%. 
     To press the wafer  2  by the press pin  6   a , both of the valves  19   a  and  19   b  are opened to chuck the wafer  1  by both of the chuck grooves  3   a  and  3   b . Consequently, as shown in FIG. 3, the peripheral portion  3   d  of the wafer  1  curves as to separate from the wafer  2 , i.e., the wafer  1  warps into the shape of an umbrella. 
     In this state, the central portion of the wafer  2  is pressed as shown in FIG.  3 . Consequently, the central portions of the wafers land  2  contact, and the contact portion gradually spreads toward the periphery. The spread of the contact portion stops when it spreads substantially uniformly to the vicinity of the periphery of the central portion  3   c  throughout the entire circumference. This is so because the wafer  1  is supported while being warped into the shape of an umbrella. 
     The spread of the contact portion from the center to the periphery of the wafer does not always proceed at the same rate throughout the entire circumference. This is so because, e.g., the bonding surfaces of the wafers  1  and  2  are uneven, and the force acting in the direction in which the wafers  1  and  2  are contacted decreases from the center to the periphery, which increases the influence of the nonuniformity of wafers in the direction from the center to the periphery. 
     If the spread of the contact portion does not proceed at the same rate, i.e., if the contact portion does not concentrically spread, a gas (e.g., air) can remain between the wafers. This phenomenon is considered to become more pronounced as the diameter of wafers increases. 
     When, however, the wafer  1  is pressed while it is warped into the shape of an umbrella as shown in FIG. 3A, the spread of the contact portion of the two wafers  1  and  2  once stops near the periphery of the central portion  3   c . Accordingly, the nonuniformity of the spread rate of the contact portion is absorbed near the periphery of the central portion  3   c . As a consequence, the contact portion is formed into a substantially circular shape. 
     After the contact portion takes a substantially circular shape (e.g., when a predetermined time elapses after the start of pressing), the valve  19   b  is closed to stop chucking by the chuck grooves  3   b . Consequently, the wafer  1  that had warped into the shape of an umbrella flattens as shown in FIG.  4 . Accordingly, the contact portion again spreads from the central portion  3   c  toward the periphery. 
     As described above, the difference between the spread rates of the contact portion in all directions (entire periphery) from the center to the periphery is absorbed in the middle of the wafer contacting process. After the contact portion is formed into a substantially circular shape in this manner, the process is continued. Therefore, even large-diameter wafers can be contacted more uniformly. 
     FIGS. 5A to  5 F are views conceptually showing the way the contact portion spreads when the wafer processing apparatus  100  contacts two wafers. Referring to FIGS. 5A to  5 F, the spread of the contact portion proceeds in the order of  5 A→ 5 B→ 5 C→ 5 D→ 5 E→ 5 F. In FIGS. 5A to  5 F, each hatched region indicates the contact portion of the two wafers. 
     When the press pin  6   a  starts pressing the center of the wafer  2  while the entire surface of the lower wafer (wafer  1 ) is chucked as shown in FIG. 3, a contact portion is first formed in the center of the wafers as shown in FIG.  5 A and gradually spreads as shown in FIGS. 5B and 5C. The shape of this contact portion is not a perfect circle. That is, as shown in FIG. 5C, the periphery of the contact portion is often uneven. 
     The spread of the contact portion stops in the state shown in FIG. 5D, i.e., when the spread proceeds to the vicinity of the periphery of the central portion  3   c  of the wafer support table  3  (a position where the spread of the contact portion stops will be referred to as a contact boundary hereinafter). Hence, the contact portion can be formed into a substantially circular shape by continuously chucking the wafer by the chuck grooves  3   b  in the peripheral portion  3   c  until the contact portion reaches the contact boundary throughout the entire circumference. 
     Next, as shown in FIG. 4, the valve  19   b  connected to the chuck grooves  3   b  in the peripheral portion is closed to release the periphery of the wafer. Consequently, the lower wafer (wafer  1 ) flattens. Accordingly, as shown in FIG. 5E, the spread of the contact portion again proceeds. Finally, as shown in FIG. 5F, the entire surfaces of the two wafers are contacted. 
     FIGS. 6A to  6 F are views conceptually showing the way the contact portion spreads when a wafer support jig  201  shown in FIGS. 25A and 25B contacts two wafers. Referring to FIGS. 6A to  6 F, the spread of the contact portion proceeds in the order of  6 A→ 6 B→ 6 C→ 6 D→ 6 E→ 6 F. 
     In this process, the difference between the spread rates in all directions when the contact portion radially spreads is not absorbed in the middle of the process. The influence of the rate difference becomes conspicuous particularly near the peripheries of the wafers. Consequently, as shown in FIG. 6F, any gas between the wafers may remain without being discharged to form non-contact portions  601 . However, the example shown in FIGS. 6A to  6 F shows the result when wafers are contacted under the same conditions as the wafer processing apparatus  100  except for the method of supporting the wafer  1 . Therefore, the formation of the non-contact portions  601  may be suppressed by changing the conditions of the contacting process. 
     This non-contact portion  601  naturally has an adverse effect on subsequent steps. For example, when the wafer processing apparatus  100  is applied to SOI wafer fabrication steps (to be described later), defects occur in the non-contact portions  601 , so these regions cannot be used. This unavoidably lowers the yield. 
     The operation of the wafer processing apparatus  100  when two wafers are contacted will be described below with reference to FIGS. 7 to  12 . 
     When the wafer transfer robot  10  loads wafers  1  and  2  on the load pints  13  and  14 , respectively, as shown in FIG. 7, the Z-axis stage  5  moves the wafer support table  3  up to a predetermined position where the wafer  1  is supported. Also, the wafer moving mechanism  4  pivots the wafer chuck unit  4   c  on the shaft  4   b  to a predetermined position where the wafer  2  can be chucked. 
     Next, as shown in FIG. 8, the sensors  15   a  and  12   a  of the displacement detectors  15  and  12  move to positions above the wafers  1  and  2  and measure the thicknesses of the wafers  1  and  2 , respectively. After the thicknesses of the wafers  1  and  2  are measured, the sensors  15   a  and  12   a  return to the initial positions shown in FIG.  7 . 
     As shown in FIG. 9, the wafer moving mechanism  4  pivots the wafer chuck unit  4   c  through about 180° on the shaft  4   b  to oppose the wafers  1  and  2  to each other substantially horizontally. On the basis of the previously measured thicknesses of the wafers  1  and  2 , the Z-axis stage  5  adjusts the height of the wafer support table  3  to adjust the gap between the wafers  1  and  2  to a set value. This gap is preferably about 20 to 100 μm, and more preferably, about 30 to 60 μm in the center of a wafer. Also, the valves  19   a  and  19   b  are opened to chuck the entire surface of the wafer  1  to the chuck surface of the wafer support table  3 . Consequently, the wafer  1  warps into the shape of an umbrella as mentioned earlier. 
     As shown in FIG. 10, the press pin  6   a  is pivoted on the shaft  6   b  to the vicinity of the back surface of the wafer  2  (e.g., a position where the press pin  6   a  essentially comes in contact with the back surface of the wafer  2 ). 
     Next, as shown in FIG. 11, the press pin  6   a  presses the back surface of the wafer  2  at the same time the wafer chuck unit  4   c  releases the wafer  2 . Consequently, the wafers  1  and  2  are gradually contacted from the center to the contact boundary to result in the state shown in FIG. 5D, i.e., the state wherein the contact portion is formed into a substantially circular shape. During pressing, the gas between the wafers  1  and  2  can be efficiently discharged by vibrating the press pin  6   a.    
     As shown in FIG. 12, the periphery of the wafer  1  is released. Consequently, the contact portion of the wafers  1  and  2  spreads beyond the contact boundary to the periphery. Note that the periphery of the wafer  1  can be released by setting the atmospheric pressure in the chuck grooves  3   b  by closing the valve  19   b . This release needs to be performed after the contact portion spreads to the contact boundary and takes a substantially circular shape after the press pin  6   a  starts the pressing operation. More specifically, the chuck is stopped when a predetermined time (long enough to allow the contact portion to spread to the contact boundary throughout the entire circumference) elapses after the pressing operation is started. Alternatively, the chuck is stopped after the contact portion is found to have spread to the contact boundary by measuring the warp of the wafer  2  by the sensor  15   a.    
     After the press mechanism  6  is returned to the original state (shown in FIG.  2 ), the wafer chuck unit  4   c  is returned to the original state (shown in FIG.  2 ). The valve  19   a  is closed to set the pressure in the chuck grooves  3   a  at the atmospheric pressure (release the wafer  1 ). The wafer support table  3  is moved down to allow the load pins  13  to support the contacted wafers. In this state the wafer transfer robot  10  chucks the lower portion of the contacted wafers, transfers the wafers to the wafer cassette  9 , and places the wafers in the wafer cassette  9 . 
     FIG. 13 is a block diagram showing the configuration of a control system of the wafer processing apparatus  100 . A CPU  17   a  operating on the basis of a program  17   b  of a controller  17  controls the wafer transfer robot  10 , the wafer alignment unit  11 , the displacement detectors  12  and  15 , the Z-axis stage  5 , the wafer moving mechanism  4 , the press mechanism  6 , the operation panel  16 , and a valve controller  19 . 
     FIG. 14 is a flow chart showing a control procedure performed on the basis of the program  17   b . The operation of the control system of the wafer processing apparatus  100  will be described below with reference to this flow chart. 
     When the start of a contacting process is designated by operating the operation panel  16 , the CPU  17   a  initializes the individual components connected to the controller  17  in step S 101 . In this initialization step, the CPU  17   a  also checks the presence and positions of the wafer cassettes  7 ,  8 , and  9 . If preparations are not complete, the CPU  17   a  displays information indicating this on a display panel  16   a  to inform the operator of the information. 
     In step S 102 , the CPU  17   a  controls the wafer transfer robot  10  to chuck the wafer  1  in the wafer cassette  7 . In step S 103 , the CPU  17   a  transfers the chucked wafer  1  to the wafer alignment unit  11  and aligns (adjusts the central position and direction of) the wafer  1 . In step S 104 , the CPU  17   a  controls the wafer transfer robot  10  to load the wafer  1  in a predetermined position on the load pins  13  projecting upward from the wafer support table  3 . The CPU  17   a  also controls the Z-axis stage  5  to move the wafer support table  3  up to a predetermined position. The CPU  17   a  opens the valve  19   a  to chuck the central portion of the wafer  1 . 
     In step S 105 , the CPU  17   a  controls the wafer transfer robot  10  to chuck the wafer  2  in the wafer cassette  8 . In step S 106 , the CPU  17   a  transfers the wafer  2  to the wafer alignment unit  11  and aligns (adjusts the central position and direction of) the wafer  2 . In step S 107 , the CPU  17   a  controls the wafer transfer robot  10  to load the wafer  2  in a predetermined position on the load pins  14  projecting upward from the wafer chuck unit  4   c . The CPU  17   a  also controls a pivoting motor  4   d  of the wafer moving mechanism  4  to pivot the wafer chuck unit  4   c  through a predetermined angle on the shaft  4   b  and allow the wafer chuck unit  4   c  to chuck the wafer  2 . 
     In step S 108 , the CPU  17   a  controls a driver  15   b  of the displacement detector  15  to move the sensor  15   a  to a predetermined position above the wafer  1  and measure the thickness of the wafer  1  by the sensor  15   a.    
     In step S 109 , the CPU  17   a  controls a driver  12   b  of the displacement detector  12  to move the sensor  12   a  to a predetermined position above the wafer  2  and measure the thickness of the wafer  2  by the sensor  12   a.    
     In step S 110 , the CPU  17   a  controls the pivoting motor  4   d  of the wafer moving mechanism  4  to pivot the wafer chuck unit  4   c  through approximately 180° on the shaft  4   b  to oppose the wafers  1  and  2  to each other substantially horizontally. 
     In step S 111 , on the basis of the measured thicknesses of the wafers  1  and  2  the CPU  17   a  forms data for adjusting the gap between the wafers  1  and  2  to a set value. On the basis of this data, the CPU  17   a  controls the Z-axis stage  5  to adjust the gap between the wafers  1  and  2 . 
     In step S 112 , the CPU  17   a  controls a pivoting motor  6   d  of the press mechanism  6  to pivot the press pin  6   a  on the shaft  6   b  to, e.g., bring the tip of the press pin  6   a  essentially into contact with the back surface of the wafer  2 . 
     In step S 113 , the CPU  17   a  opens the valve  19   b  to chuck the entire surface of the wafer  1  to the chuck surface of the wafer support table  3 . Consequently, the wafer  1  warps into the shape of an umbrella. 
     In step S 114 , the CPU  17   a  releases the wafer  2  by the wafer chuck unit  4   c . In step S 115 , the CPU  17   a  controls the pivoting motor  6   d  and a vibrator  6   c  of the press mechanism  6  to press the back surface of the wafer  2  by the press pin  6   a  and vibrate the press pin  6   a . By executing step S 115  immediately after the execution of step S 114 , the wafer  2  can be released and pressed at substantially the same time. However, the press can also be started when a predetermined time is counted after the execution of step S 114 . 
     When a predetermined time elapses after the execution of step S 115  is started, the flow advances to step S 116 . In step S 116 , the CPU  17   a  closes the valve  19   b  to return the pressure in the chuck grooves  3   b  to the atmospheric pressure and release the periphery of the wafer  1 . Consequently, the wafer  1  flattens, and the contact portion spreads toward the periphery. 
     When the wafers  1  and  2  are completely contacted, the flow advances to step S 117 , and the CPU  17   a  controls the pivoting motor  6   d  of the press mechanism  6  to return the press pin  6   a  to the initial position. In step S 118 , the CPU  17   a  controls the pivoting motor  4   d  of the wafer moving mechanism  4  to return the wafer chuck unit  4   c  to the initial position. 
     In step S 119 , the CPU  17   a  closes the valve  19   a  to completely release the wafer  1 . In step S 120 , the CPU  17   a  controls the Z-axis stage  5  to move the wafer support table  3  down to the initial position. Consequently, the contacted wafers are supported by the load pins  13 . 
     In step S 121 , the CPU  17   a  controls the wafer transfer robot  10  to transfer the contacted wafers to the wafer cassette  9  and places the wafers in the wafer cassette  9 . 
     In step S 122 , the CPU  17   a  checks whether the contacting process is completely performed for all wafers contained in the wafer cassettes  7  and  8 . If unprocessed wafers remain, the flow returns to step S 102  to repeat the process. If the CPU  17   a  determines in step S 122  that the contacting process is completely performed for all wafers, the CPU  17   a  completes the whole process procedure. The CPU  17   a  preferably informs the operator of the completion by, e.g., displaying information to this effect on the display panel  16   a  or by using a buzzer. 
     In this wafer processing apparatus as described above, the peripheries of two wafers are separated in the form of a ring, and substantially the center of one wafer is pressed. When the contact portion of the two wafers takes a substantially circular shape, the spread of the contact portion is once stopped. This absorbs the nonuniformity of the spread rate of the contact portion. Accordingly, the gas between the two wafers can be efficiently and reliably discharged. 
     This wafer processing apparatus  100  also achieves the following effects. 1) Since the press is started at the same time the upper wafer  2  is released, gas between the wafers  1  and  2  can be reliably discharged toward the periphery. 2) The two wafers  1  and  2  can be accurately aligned because the upper wafer  2  does not slide when the wafers  1  and  2  are opposed to each other. 3) The gap between the wafers  1  and  2  can be adjusted to an appropriate distance. Consequently, it is possible to give uniform quality to the fabricated wafers and obviate the need for sorting the wafers  1  and  2  in advance. 4) The surfaces of the wafers  1  and  2  can be prevented from being contaminated with particles. 5) Break of the periphery of a wafer can be prevented. 6) By giving vibrations to wafers when the wafers are pressed, any gas that remains between the wafers can be further reduced. 
     [Second Embodiment] 
     This embodiment provides another arrangement of the wafer support table  3  according to the first embodiment. FIG. 15 is a sectional view showing the arrangement of a wafer support table  3 ′ according to this embodiment. Note that the same reference numerals as in the first embodiment denote essentially the same parts in the second embodiment. 
     In this wafer support table  3 ′, a central portion of the wafer chuck surface is a circular central surface  3   f  perpendicular to the axial direction (in a direction z in FIG.  15 ). The peripheral portion of the wafer chuck surface is an annular peripheral surface  3   e  inclined to the central surface  3   f.    
     A wafer processing apparatus incorporating this wafer support table  3 ′ in place of the wafer support table  3  also achieves the same effects as in the first embodiment. 
     [Third Embodiment] 
     This embodiment provides still another arrangement of the wafer support table  3  according to the first embodiment. FIG. 16 is a sectional view showing the arrangement of a wafer support table  3 ″ according to this embodiment. Note that the same reference numerals as in the first embodiment denote essentially the same parts in the third embodiment. 
     This wafer support table  3 ″ is substantially the same as the wafer support table  3  according to the first embodiment except that the chuck grooves in the central portion are omitted. When this wafer support table  3 ″ chucks a wafer, as shown in FIG. 16, the central portion of the wafer floats surface. Therefore, even if a foreign matter exists between the chuck surface of the wafer support table  3 ″ and the wafer, the operation is not easily affected by this foreign matter. 
     [Fourth Embodiment] 
     This embodiment provides another example of the wafer processing apparatus. In the above embodiments, the contact portion is shaped only once in the process of contacting two wafers. However, to properly contact wafers having larger diameters, it is effective to shape the contact portion twice or more. This embodiment relates to a wafer processing apparatus for adjusting the shape of the contact portion three times. 
     FIGS. 17 and 18 are sectional views showing a part of the wafer processing apparatus according to this embodiment. The same reference numerals as in the first embodiment denote essentially the same parts in FIGS. 17 and 18. Components such as chuck holes and valves for vacuum-chucking wafers are omitted from FIGS. 17 and 18. Portions not shown in FIGS. 17 and 18 are the same as in the wafer processing apparatus  100  according to the first embodiment. That is, the wafer processing apparatus according to this embodiment is obtained by replacing the structure (wafer support table  3 ) on the Z-axis stage  5  in the wafer processing apparatus  100  according to the first embodiment. 
     In this embodiment, a wafer support table  300  comprises a first support member  31 , a second support member  32 , a third support member  33 , and a fourth support member  34 . The first support member  31  has a disk-like shape and is connected to a support table  51  by a connecting member  35 . This support table  51  is fixed to a Z-axis stage  5 . The second support member  32  has an annular shape surrounding the perimeter of the first support member  31  and is connected to a sliding member  36 . The third support member  33  has an annular-shape surrounding the perimeter of the second support member  32  and is connected to a sliding member  37 . The fourth support member  34  has an annular shape surrounding the perimeter of the third support member  33  and is connected to the first support member  31  by the connecting member  35 . 
     The sliding member  36  connected to the second support member  32  has sliding pins  36   a  and  36   b  engaged with guide grooves  38   a  and  39   a  of racks  38  and  39 , respectively. The sliding member  37  connected to the third support member  33  has sliding pins  37   a  and  37   b  engaged with the guide grooves  38   a  and  39   a  of the racks  38  and  39 , respectively. 
     The racks  38  and  39  are moved in opposite directions (A and B) by rotations of motors  40  and  41 , respectively. When the racks  38  and  39  move in the directions of arrows A and B, respectively, in the state shown in FIG. 18, the second and third support members  32  and  33  move up in this order. More specifically, the rotations of the motors  40  and  41  horizontally move the racks  38  and  39  in the directions A and B, respectively. Accordingly, the sliding pins  36   a  and  36   b  engaged with the guide grooves  38   a  and  39   a  move up to push the second support member  32  upward. Subsequently, the sliding pins  37   a  and  37   b  engaged with the guide grooves  38   a  and  39   a  move up to push the third support member  33  upward. 
     When the second and third support members  32  and  33  are pushed upward as described above, the chuck surfaces of the first to third support members  31  to  33  form a substantially flat surface. 
     On the other hand, by horizontally moving the racks  38  and  39  in opposite directions of A and B, respectively, it is possible to move the second and third support members  32  and  33  down to the state shown in FIGS. 17 and 18. 
     A procedure of contacting two wafers by using this wafer processing apparatus will be described below. First, a wafer transfer robot  10  (FIG. 1) loads a wafer  1  on load pins  13 . Next, the Z-axis stage  5  is driven to bring the chuck surface of the first support member  31  into contact with the back surface of the wafer  1 . In this step, the positions of the second and third support members  32  and  33  are so controlled that the chuck surfaces of the second to fourth support members  32  to  34  form a substantially plane surface. 
     In this state, a displacement detector  15  measures the thickness of the wafer  1 . Also, a displacement detector  12  measures the thickness of a wafer  2  chucked by a wafer chuck unit  4   c . Next, a wafer moving mechanism  41  is pivoted through about 180° on a shaft  4   b  to oppose the wafers  1  and  2  to each other substantially horizontally. On the basis of the previously measured thicknesses of the wafers  1  and  2 , the Z-axis stage  5  adjusts the height of a wafer support table  3  to adjust the gap between the wafers  1  and  2  to a set value. 
     Subsequently, the spaces in chuck grooves formed in the chuck surfaces of the first to fourth support members  31  to  34  are evacuated to chuck the entire surface of the wafer  1 . A press pin  6   a  is then pivoted on a shaft  6   b  to the vicinity of the back surface of the wafer  2  (e.g., a position where the press pin  6   a  essentially comes in contact with the back surface of the wafer  2 ). In this manner preparations for the process of contacting the two wafers are complete. FIG. 19 shows the state wherein preparations for the process of contacting the two wafers are complete. 
     When the press pin  6   a  starts pressing, the central portions of the wafers  1  and  2  first contact. The contact portion gradually spreads toward the periphery. This spread stops when reaching a first contact boundary b 1 , i.e., the vicinity of the periphery of the first support member  31 . The first contact boundary b 1  absorbs any nonuniformity of the spread rate of the contact portion. As a consequence, the contact portion takes a substantially circular shape. FIG. 20 is a view schematically showing the state wherein the contact portion has reached the first contact boundary b 1 . 
     Next, the second support member  32  is moved up to further spreads the contact portion toward the periphery. The spread stops when reaching a second contact boundary b 2 , i.e., the vicinity of the outer periphery of the second support member  32 . The second contact boundary b 2  absorbs any nonuniformity of the spread rate of the contact portion. As a consequence, the contact portion takes a substantially circular shape. FIG. 21 is a view schematically showing the state wherein the contact portion has reached the second contact boundary b 2 . 
     The third support member  33  is then moved up to further spreads the contact portion toward the periphery. The spread stops when reaching a third contact boundary b 3 , i.e., the vicinity of the outer periphery of the third support member  33 . The third contact boundary b 3  absorbs any nonuniformity of the spread rate of the contact portion. As a consequence, the contact portion takes a substantially circular shape. FIG. 22 is a view schematically showing the state wherein the contact portion has reached the third contact boundary b 3 . 
     Subsequently, the wafer  1  is released by the chuck grooves formed in the chuck surface of the fourth support member  34 . Consequently, the contact portion further spreads toward the periphery. Finally, the entire surface of the wafer  1  contacts to the wafer  2 . FIG. 23 is a view schematically showing the state wherein the entire surfaces of the wafers  1  and  2  contact each other. 
     After a press mechanism  6  is returned to the original state, the wafer chuck unit  4   c  is returned to the original state. The chuck by the chuck grooves formed in the first to third support members  31  to  33  is stopped. After that, the wafer support table  30  is moved down to allow the load pins  13  to support the contacted wafers. The wafer transfer robot  10  chucks the lower portion of the contacted wafers, transfers the wafers to a wafer cassette  9 , and places the wafers in the wafer cassette  9 . 
     As described above, even large-diameter wafers can be contacted with high quality by shaping the contact portion a plurality of number of times. 
     [Another Embodiment] 
     Each of the above embodiments relates to an apparatus and a method of contacting two wafers step by step from the center to the periphery. However, the present invention is also applicable to an apparatus and a method by which arbitrary portions (e.g., portions of the peripheries) of wafers are first contacted, and the area of the contact portion is increased step by step to finally contact the entire surfaces of the two wafers. 
     [Application of Wafer Processing Apparatus] 
     An application of each of the above wafer processing apparatuses will be described below. FIGS. 24A to  24 F are views showing an example of a process of fabricating a wafer having an SOI structure. 
     First, a single-crystal Si wafer  501  for forming a first wafer  1  is prepared, and a porous Si layer  502  is formed on the major surface of the wafer  501  (FIG.  24 A). Next, at least one non-porous layer  503  is formed on the porous Si layer  502  (FIG.  24 B). Preferable examples of the non-porous layer  503  are a single-crystal Si layer, a porous Si layer, an amorphous Si layer, a metal film layer, a compound semiconductor layer, and a superconductor layer. A device such as a MOSFET can also be formed in the non-porous layer  503 . 
     An SiO 2  layer  504  is formed on the non-porous layer  503  to obtain a first wafer  1  (FIG.  24 C). The first wafer  1  is placed in the wafer cassette  7  such that the SiO 2  layer  504  is turned up. 
     A second wafer  2  is separately prepared and placed in the wafer cassette  8  such that the front surface is turned up. 
     When the wafer processing apparatus is operated in this state, the first and second wafers  1  and  2  are so contacted as to sandwich the SiO 2  layer  504  between them on the wafer support table (FIG.  24 D). The contacted wafers are placed in the wafer cassette  9 . 
     After this process, anodization, pressing, or, if necessary, heat treatment, or a combination of these processes can also be performed for the contacted wafers (FIG. 24D) to strengthen the bonding. 
     Preferable examples of the second wafer  2  are an Si wafer, a wafer obtained by forming an SiO 2  layer on an Si wafer, a light-transmitting wafer such as a quartz wafer, and a sapphire wafer. However, the second wafer  2  can also be still another kind of wafer provided that the surface to be bonded is sufficiently flat. 
     Subsequently, the first wafer  1  is removed from the porous Si layer  502  of the second wafer  2  (FIG.  24 E), and the porous Si layer  502  is selectively etched away. FIG. 24F schematically shows the wafer obtained by the above fabrication method. 
     This fabrication method can fabricate high-quality wafers because two wafers are contacted while any gas between the wafers is appropriately discharged. 
     In the present invention, large-diameter substrates can be easily contacted. 
     The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention. Therefore, to apprise the public of the scope of the present invention the following claims are made.