Patent Publication Number: US-11640912-B2

Title: Apparatus for bonding substrates having a substrate holder with holding fingers and method of bonding substrates

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     Korean Patent Application No. 10-2018-0103029, filed on Aug. 30, 2018, in the Korean Intellectual Property Office, and entitled: “Apparatus for Bonding Substrates and Method of Bonding Substrates,” is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     Embodiments relate to a substrate bonding apparatus and a substrate bonding method, and more particularly, to a substrate bonding apparatus and a substrate bonding method, by which a semiconductor device is manufactured at low costs and with improved precision. 
     2. Description of the Related Art 
     For the development of a semiconductor device having a three-dimensional (3D) connection structure, two semiconductor wafers need to be precisely bonded with each other. When the two semiconductor wafers are very precisely bonded together, semiconductor devices providing high performance, while having small sizes, can be manufactured at low costs and with high reliability. Although various methods to bond two semiconductor wafers together are being developed, further bonding precision is necessary. 
     SUMMARY 
     According to an aspect, there is provided a substrate bonding apparatus including a substrate susceptor configured to support a first substrate downwards, a substrate holder located over the substrate susceptor and configured to hold a second substrate, and a chamber housing configured to accommodate the substrate susceptor and the substrate holder, wherein the substrate holder includes a plurality of holding fingers that are operable independently. 
     According to another aspect, there is provided a substrate bonding apparatus including a substrate susceptor configured to support a first substrate downwards, a substrate holder located over the substrate susceptor and configured to hold a second substrate, and a pressing finger arranged at a location corresponding to a center of the second substrate and configured to press the second substrate toward the first substrate, wherein the substrate holder is configured to release holding of the second substrate after a spontaneous bonding propagation between the first substrate and the second substrate is sufficiently conducted and an edge of the second substrate approaches an edge of the first substrate. 
     According to yet another aspect, there is provided a substrate bonding method including arranging a substrate holder holding a second substrate disposed thereon, on a substrate susceptor having a first substrate disposed thereon, such that the first and second substrates are aligned with each other, bringing a center of the second substrate to contact the first substrate, allowing a spontaneous bonding propagation between the first substrate and the second substrate to be sufficiently conducted, bringing the second substrate close to the first substrate while maintaining holding of the second substrate, and releasing the holding of the second substrate. 
     According to yet another aspect, there is provided a substrate bonding apparatus, including a substrate susceptor to support a first substrate, a substrate holder over the substrate susceptor to hold a second substrate, and a pressing finger at a center of the substrate holder, the pressing finger being moveable toward the substrate susceptor to press the second substrate toward the first substrate, wherein the substrate holder is to release holding the second substrate after a spontaneous bonding propagation between the first substrate and the second substrate is sufficiently conducted and an edge of the second substrate approaches an edge of the first substrate, and wherein the substrate holder is to release holding of the second substrate after moving toward the first substrate until a distance between the edge of the second substrate and the edge of the first substrate is about 1 μm to about 50 μm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features will become apparent to those of skill in the art by describing in detail exemplary embodiments with reference to the attached drawings, in which: 
         FIG.  1    illustrates a side view of a substrate bonding apparatus according to an embodiment; 
         FIG.  2    illustrates a bottom view of a substrate holder included in the substrate bonding apparatus of  FIG.  1   ; 
         FIGS.  3 A through  3 E  illustrate conceptual views of stages in a method of bonding a first substrate and a second substrate together, according to an embodiment; 
         FIGS.  4 A and  4 B  illustrate conceptual views of stages in a method of bonding a first substrate and a second substrate together, according to another embodiment; 
         FIGS.  5 A through  5 E  illustrate conceptual views of stages in a method of bonding a first substrate and a second substrate together, according to another embodiment; 
         FIGS.  6 A through  6 E  illustrate conceptual views of stages in a method of bonding a first substrate and a second substrate together, according to still another embodiment; 
         FIG.  7    illustrates a conceptual view of distortion on a substrate when holding of a second substrate is released without bringing the edge of the second substrate close to a first substrate by using holding fingers; 
         FIG.  8    illustrates a schematic diagram of a method of aligning a first substrate with a second substrate, according to an embodiment; 
         FIG.  9    illustrates a side view of a holding finger according to an embodiment; 
         FIG.  10    illustrates a side view of a holding finger according to another embodiment; 
         FIG.  11    illustrates a side view of a holding finger according to another embodiment; 
         FIG.  12    illustrates a side view of a substrate bonding apparatus according to another embodiment; 
         FIG.  13    illustrates a schematic diagram of a first substrate and a second substrate bonded with each other, according to an embodiment; and 
         FIG.  14    illustrates a magnified cross-sectional view of portion XII of  FIG.  13   . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments will be described more fully with reference to the accompanying drawings, in which exemplary embodiments are shown. 
       FIG.  1    is a side view of a substrate bonding apparatus  100  according to an embodiment. 
     Referring to  FIG.  1   , the substrate bonding apparatus  100  may include a substrate susceptor  120 , a substrate holder  110  located over the substrate susceptor  120 , and a chamber housing  130  that accommodates the substrate susceptor  120  and the substrate holder  110 . 
     The chamber housing  130  may surround the substrate susceptor  120  and the substrate holder  110 . According to some embodiments, a vacuum pressure or an atmospheric pressure may be formed within the chamber housing  130 . A first substrate W 1  and a second substrate W 2  respectively supported or held by the substrate susceptor  120  and the substrate holder  110  may be protected by the chamber housing  130 . 
     The chamber housing  130  may have a wall  132  and an opening  134 , e.g., the opening  134  may extend through the wall  132 . The first and second substrates W 1  and W 2  may be carried into or carried out of the chamber housing  130  via the opening  134 . The opening  134  may be sealed as necessary to protect the inside of the chamber housing  130  from an external environment. 
     The substrate susceptor  120  may support the first substrate W 1  seated thereon. According to some embodiments, the first substrate W 1  may be a single crystal substrate. According to some embodiments, the first substrate W 1  may be a silicon wafer. 
     The substrate susceptor  120  may include a susceptor body  122  and a first substrate fixing unit  124 . For example, as illustrated in  FIG.  1   , the first substrate fixing unit  124  may be within the susceptor body  122 , e.g., a top surface of the first substrate fixing unit  124  may be exposed by and substantially level with a top surface of the susceptor body  122 . The substrate susceptor  120  may be configured to fix the first substrate W 1  by using, e.g., a vacuum or an electrostatic force. When the substrate susceptor  120  fixes the first substrate W 1  by using a vacuum, the first substrate fixing unit  124  may be configured to form a lower pressure than an ambient pressure at a bottom surface of the first substrate W 1 . When the substrate susceptor  120  fixes the first substrate W 1  by using an electrostatic force, the first substrate fixing unit  124  may be configured to generate an electrostatic force allowing the first substrate W 1  to be fixed. 
     The substrate holder  110  may be provided to face the substrate susceptor  120 , e.g., the first substrate W 1  may be between the substrate susceptor  120  and the substrate holder  110 . The substrate holder  110  may hold the second substrate W 2 . The substrate holder  110  may hold the second substrate W 2  from the top of the second substrate W 2 , e.g., the second substrate W 2  may be between the first substrate W 1  and the substrate holder  110 . In other words, the substrate holder  110  may contact an upper surface of the second substrate W 2  and hold the second substrate W 2 . According to some embodiments, the second substrate W 2  may be a single crystal substrate. According to some embodiments, the second substrate W 2  may be a silicon wafer. 
       FIG.  2    is a bottom view of the substrate holder  110 . 
     Referring to  FIGS.  1  and  2   , an outer circumference of a substrate holder body  112  may be generally a circle. A plurality of holding fingers  114  may be arranged substantially at regular intervals along the outer circumference of the substrate holder body  112 . Angles θ formed between each two adjacent holding fingers  114  from a center of a bottom of the substrate holder  110  may be substantially uniform. 
     The substrate holder  110  may have more than four holding fingers  114 . According to some embodiments, the substrate holder  110  may have eight holding fingers  114 . According to some embodiments, the substrate holder  110  may have an odd number of holding fingers  114 , e.g., five, seven, nine, eleven, thirteen, or fifteen holding fingers  114 . According to some embodiments, the number of holding fingers  114  may be thirty or less. 
     The holding fingers  114  may extend and be configured to reciprocate, e.g., move, in a direction substantially perpendicular to the second substrate W 2  (i.e., in the z direction). The reciprocation, e.g., movement, of the holding fingers  114  may be implemented by holding finger actuators  114   a . Operations of the holding finger actuators  114   a  may be controlled by a controller  140 . 
     As shown in  FIG.  2   , the holding finger actuators  114   a  may be independently electrically connected to the controller  140  and may be independently controlled by the controller  140 , e.g., the controller  140  may control each finger actuator  114   a  independently of the other finger actuators  114   a . In other words, respective motions of, e.g., each of, the holding fingers  114  according to motions of the respective holding finger actuators  114   a  may be independent from each other. This will be described later in more detail. For example, multilayer piezoelectric actuators, voice coil motors (VCMs), or lag and pinion coupled with a motor may be used as the holding finger actuators  114   a , but embodiments are not limited thereto. 
     A pressing finger  116  for pressing the second substrate W 2  may be provided on the center of the substrate holder  110 , e.g., the holding fingers  114  may surround the pressing finger  116 . The pressing finger  116  may be configured to reciprocate, e.g., move, in the direction substantially perpendicular to the second substrate W 2  (i.e., in the z direction). The reciprocation of the pressing finger  116  may be implemented by a pressing finger actuator  116   a . An operation of the pressing finger actuator  116   a  may be controlled by the controller  140 . For example, a multilayer piezoelectric actuator, a VCM, or a lag and pinion coupled with a motor may be used as the pressing finger actuator  116   a , but embodiments are not limited thereto. 
     A relative distance between the first substrate W 1  and the second substrate W 2 , e.g., along the z direction, may be sensed by a distance sensor  150 , e.g., the distance sensor  150  may be positioned within the chamber housing  130  to face a space between the substrate holder  110  and the substrate susceptor  120 . According to some embodiments, the distance sensor  150  may measure a distance between the first substrate W 1  and the second substrate W 2  by perceiving images of the first substrate W 1  and the second substrate W 2 . According to some embodiments, the distance sensor  150  may measure the distance between the first substrate W 1  and the second substrate W 2  by radiating electromagnetic waves to the first substrate W 1  and the second substrate W 2  and then analyzing electromagnetic waves reflected from the first substrate W 1  and the second substrate W 2 . The distance sensor  150  recognizes the relative distance between the first substrate W 1  and the second substrate W 2  according to various methods, and embodiments are not limited to the above-described methods. 
     According to some embodiments, each of the holding finger actuators  114   a  may have an arm shape. When the holding finger actuator  114   a  is in the shape of an arm, each of the holding fingers  114  is coupled to one side of the arm. As the arm operates, the holding finger  114  may move in the z direction. 
       FIGS.  3 A through  3 E  are conceptual views of stages in a method of bonding the first substrate W 1  and the second substrate W 2  together, according to an embodiment. 
     Referring to  FIG.  3 A , the substrate holder  110  having the second substrate W 2  held thereon may be arranged over the substrate susceptor  120  having the first substrate W 1  disposed thereon, so that the first substrate W 1  and the second substrate W 2  are aligned with each other. For example, as illustrated in  FIG.  3 A , the first and second substrates W 1  and W 2  may be positioned to face each other with a predetermined distance, e.g., a first distance d 1 , therebetween along the z direction. For example, as further illustrated in  FIG.  3 A , the distance sensor  150  may be peripheral to, e.g., and between, the substrate holder  110  and the substrate susceptor  120 , and may face the distance between the first and second substrates W 1  and W 2 . 
     The substrate holder  110  may be moveable on an x-y plane to achieve the alignment, e.g., the controller  140  may move the substrate holder  110  in the x direction and/or the y direction until the first and second substrates W 1  and W 2  are aligned. According to some embodiments, an alignment mark may be marked on the first substrate W 1  and/or the second substrate W 2  in order to achieve the alignment, e.g., achieve alignment of respective edges of the first and second substrates W 1  and W 2 . 
     Moreover, the substrate holder  110  may be moveable in the z direction to achieve the alignment, e.g., the controller  140  may move the substrate holder  110  in the z direction until the first and second substrates W 1  and W 2  are aligned and are parallel at the first distance d 1 . The first distance d 1  between the first substrate W 1  and the second substrate W 2  may be about 30 μm to about 100 μm. In this alignment step, when the first distance d 1  is larger than about 100 μm, a bonding propagation (which will be described later) may be insufficient. On the other hand, when the first distance d 1  is smaller than about 30 μm, the bonding propagation may be excessive, and thus voids may be generated in bonding-propagated portions of the first and second substrates W 1  and W 2 . 
     The alignment between the substrate holder  110  and the substrate susceptor  120  may be feedback-controlled, e.g., by a sensor system including the distance sensor  150  and the controller  140 . In another example, an additional controller (other than the controller  140 ) may be used to control movement of the substrate holder  110  in the x direction, y direction, and z direction. 
     Referring to  FIG.  3 B , i.e., a first phase, the second substrate W 2  may be pressed toward the first substrate W 1  by using the pressing finger  116 , e.g., only the pressing finger  116  may move toward the first substrate W 1  (along the arrow in  FIG.  3 B ). At this time, the holding fingers  114  of the substrate holder  110  may continuously hold the second substrate W 2 , e.g., the holding fingers  114  may continuously maintain edges of the second substrate W 2  at a stationary state while the pressing finger  116  pushes the center of the second substrate W 2 . As the pressing finger  116  presses the second substrate W 2 , the center of the second substrate W 2  may contact the first substrate W 1 . 
     Referring to  FIG.  3 C , i.e., a second phase, a sufficient spontaneous bonding propagation between the first substrate W 1  and the second substrate W 2  may be allowed. For example, referring to  FIG.  3 C , the spontaneous bonding propagation may spread from the centers of the contacting first and second substrates W 1  and W 2 , i.e., from the contact point between the first and second substrates W 1  and W 2  that overlaps the pressing finger  116  (point O in  FIG.  3 C ), towards edges of the first and second substrates W 1  and W 2 , e.g., along a second distance d 2  in a radial direction. 
     In detail, according to some embodiments, respective surfaces of the first substrate W 1  and the second substrate W 2  that face each other may have been plasma-processed, e.g., before the beginning of the bonding process. In this case, electrostatic attraction occurs between the facing surfaces of the first and second substrates W 1  and W 2 , e.g., when the distance between the first and second substrates W 1  and W 2  is sufficiently small. Accordingly, a bonding propagation may occur, so the first and second substrates W 1  and W 2  may gradually bond together along the second distance d 2  due to the electrostatic attraction therebetween (e.g., from a center O to a bonding front BF in  FIG.  3 C ). That is, due to the bonding propagation, the respective surfaces of the two first and second substrates W 1  and W 2  may gradually bond together from initial contact portions thereof even without special, e.g., further, application of an external force. 
     In this case, a boundary may be defined between the bonded and un-bonded portions of the first and second substrates W 1  and W 2 . That is, the boundary, i.e., the bonding front BF, may be defined between a first portion (a portion between points O and BF of  FIG.  3 C ) where the respective surfaces of the two first and second substrates W 1  and W 2  are bonded to each other and a second portion (a portion to the right of point BF of  FIG.  3 C ) where the respective surfaces of the two first and second substrates W 1  and W 2  are not bond with each other. 
     If the holding fingers  114  were to stop holding the second substrate W 2 , and an edge of the second substrate W 2  were to freely fall and contact an edge of the first substrate W 1 , the bonding front BF would have propagated up to the edge of the first substrate W 1  (or the second substrate W 2 ), e.g., too quickly and non-uniformly, thereby generating voids in bonding interface between the first and second substrates W 1  and W 2 . In contrast, referring to  FIG.  3 C , since the holding fingers  114 , according to embodiments, hold the second substrate W 2 , the bonding front BF may not be propagated up to the edge of the first substrate W 1  (or the second substrate W 2 ). The bonding front BF may be propagated only up to a position where attraction between the respective surfaces of the two first and second substrates W 1  and W 2  and an elastic restoring force of the second substrate W 2  balance. 
     If the first distance d 1  between the edge of the second substrate W 2  and the edge of the first substrate W 1  increases, the second distance d 2  between the center O and the bonding front BF may decrease. If the first distance d 1  between the edge of the second substrate W 2  and the edge of the first substrate W 1  decreases, the second distance d 2  between the center O and the bonding front BF may increase. 
     Referring to  FIG.  3 D , i.e., a third phase, the holding fingers  114  may descend toward the first substrate W 1  such that the edge of the second substrate W 2  bonds with the edge of the first substrate W 1 . Consequently, the first substrate W 1  and the second substrate W 2  may be completely bonded with each other. 
     At this time, not all of the holding fingers  114  may move at the same speed. Moreover, not all of the holding fingers  114  may move, e.g., displaced by the same amount. In other words, the holding fingers  114  may be independently controlled and independently operate, e.g., each of the holding fingers  114  may be independently controlled to move a predetermined distance along the z direction at a predetermined speed to achieve proper bonding between the first and second substrates W 1  and W 2 . 
     In detail, a plurality of distance sensors  150  may be arranged around the first substrate W 1  and the second substrate W 2 . The plurality of distance sensors  150  measure distances between the first substrate W 1  and the second substrate W 2  at their locations, respectively, and transmit the measured distances to the controller  140  of  FIG.  1   . 
     The controller  140  may calculate a distance between the first substrate W 1  and the second substrate W 2  at a location of each of the holding fingers  114 , by using distance information corresponding to the distances between the first substrate W 1  and the second substrate W 2  respectively measured by the distance sensors  150 . The distance between the first and second substrates W 1  and W 2  at the location of each of the holding fingers  114  may be calculated, via interpolation, from the distance between the first and second substrates W 1  and W 2  at the location of each of the distance sensors  150 . However, embodiments are not limited to this. 
     The controller  140  may calculate a target location of each holding finger  114  for a next moment, e.g., a distance by which and a speed at which the holding finger  114  needs to move in the z direction, based on the distance between the first and second substrates W 1  and W 2  calculated at the location of the holding finger  114 . Next, the controller  140  may transmit, to each of the holding finger actuators  114   a  of  FIG.  1   , a signal for allowing each of the holding fingers  114  to move to the calculated target location. In other words, the controller  140  may be configured to calculate, in real time, the distance between the first and second substrates W 1  and W 2  at the location of each holding finger  114  and to control, in real time, a target position and an operating speed of the holding finger  114 , based on the calculated distance. 
     According to some embodiments, the holding fingers  114  may descend toward the first substrate W 1  at different speeds. For example, four holding fingers  114  located in two intersecting directions from among the eight holding fingers  114  may descend at constant speeds, and the remaining four holding fingers  114  may descend at lower speeds than the descending speeds of the former holding fingers  114 . 
     According to some embodiments, the holding fingers  114  may descend toward the first substrate W 1  to different heights. For example, the four holding fingers  114  located in two intersecting directions from among the eight holding fingers  114  may have distances of H from the first substrate W 1  at an arbitrary moment when the four holding fingers  114  descend, and the remaining four holding fingers  114  may have distances of (H+δH) from the first substrate W 1 . In other words, while the eight holding fingers  114  are descending, a height difference of δH between the height of the four holding fingers  114  and the height of the remaining four holding fingers  114  may be kept constant. 
     Referring to  FIG.  3 E , i.e., a fourth phase, the holding of the second substrate W 2  may be released by the holding fingers  114 . A method of releasing the holding of the second substrate W 2  may vary according to the method of holding the second substrate W 2 . If a suction unit is provided to the holding fingers  114  and the second substrate W 2  is held by the suction unit, the holding may be released by equalizing an internal pressure of the suction unit with a pressure of the outside. If power supplied to the holding fingers  114  generates an electrostatic force and the second substrate W 2  is held by the electrostatic force, the holding may be released by interrupting the supply of power to the holding fingers  114 . The holding-released holding fingers  114  may move in a direction away from the first substrate W 1 . 
       FIGS.  4 A and  4 B  are conceptual views of stages in a method of bonding the first substrate W 1  and the second substrate W 2  together, according to another embodiment. The bonding method according to the present embodiment is common to the embodiment described above with reference to  FIGS.  3 A through  3 E  in terms of the operations of  FIGS.  3 A through  3 C . Therefore,  FIGS.  4 A and  4 B  refer to operations performed after those described with reference to  FIG.  3 C . 
     Referring to  FIG.  4 A , the holding fingers  114  may move toward the first substrate W 1  until a third distance d 3  between the respective edges of the first and second substrates W 2  and W 1  is about 1 μm to about 50 μm. The moving of the holding fingers  114  toward the first substrate W 1  may be accomplished by a cooperation between the distance sensors  150 , the controller  140 , and the holding finger actuators  114   a , as described above with reference to  FIG.  3 D . During this process, the bonding front BF may be closer to the edge of the first substrate W 1  than the bonding front BF of  FIG.  3 C . A fourth distance d 4  between the center O and the bonding front BF may also be greater than the second distance d 2  of  FIG.  3 C . 
     Referring to  FIG.  4 B , even when the edge of the second substrate W 2  is apart from the edge of the first substrate W 1 , the holding of the second substrate W 2  may be released by the holding fingers  114 . The method of releasing the holding of the second substrate W 2  has been described above with reference to  FIG.  3 E , and thus a description thereof will be omitted. 
     When the holding of the second substrate W 2  by the holding fingers  114  is released, the bonding front BF spreads in a radial direction, i.e., in a direction toward the edge of the first substrate W 1 , until the edge of the second substrate W 2  bonds to the edge of the first substrate W 1 . If, after the center of the second substrate W 2  is pressed by the pressing finger  116  to contact the first substrate W 1 , the holding of the second substrate W 2  were to be released without bringing the edge of the second substrate W 2  close to the first substrate W 1  by using the holding fingers  114 , bonding between the first substrate W 1  and the second substrate W 2  would not have been sufficient, e.g., due to voids therebetween. 
       FIGS.  5 A through  5 E  are conceptual views of stages in a method of bonding the first substrate W 1  and the second substrate W 2  together, according to another embodiment. 
     Referring to  FIG.  5 A , a substrate holder  110   a  having the second substrate W 2  held thereon may be arranged on a substrate susceptor  120   a  having the first substrate W 1  held thereon such as to be aligned with the substrate susceptor  120   a.    
     The substrate susceptor  120   a  may have susceptor holding fingers  114   s  in order to hold the first substrate W 1 . The substrate susceptor  120   a  may include a susceptor pressing finger  116   s  in order to press the first substrate W 1 . The susceptor holding fingers  114   s  and the susceptor pressing finger  116   s  may be arranged at locations facing the holding fingers  114  and the pressing finger  116  of the substrate holder  110   a , respectively. In other words, the number of susceptor holding fingers  114   s  may be equal to that of holding fingers  114 . 
     The substrate holder  110   a  and the substrate susceptor  120   a  may move relative to each other on the x-y plane in order to align the first substrate W 1  with the second substrate W 2  and/or to align the susceptor holding fingers  114   s  with the holding fingers  114 . 
     The substrate holder  110   a  and the substrate susceptor  120   a  may move relative to each other in the z direction in order to adjust a fifth distance d 5  between the first substrate W 1  and the second substrate W 2 . The fifth distance d 5  between the first substrate W 1  and the second substrate W 2  may be about 30 μm to about 100 μm. When the fifth distance d 5  is too big, a bonding propagation may be insufficient. On the other hand, when the fifth distance d 5  is too small, the bonding propagation may be excessive, and thus voids may be generated in bonding-propagated portions of the first and second substrates W 1  and W 2 . 
     The alignment between the substrate holder  110   a  and the substrate susceptor  120   a  and the adjustment of the distance therebetween may be feedback-controlled by the controller  140  connected to the sensor system including the distance sensors  150 . 
     Referring to  FIG.  5 B , the pressing fingers  116  and  116   s  of the substrate holder  110   a  and the substrate susceptor  120   a  may approach each other to press the first substrate W 1  and the second substrate W 2  toward each other. In other words, the second substrate W 2  may be pressed toward the first substrate W 1  by using the pressing finger  116  of the substrate holder  110   a . The first substrate W 1  may be pressed toward the second substrate W 2  by using the susceptor pressing finger  116   s  of the substrate susceptor  120   a . Respective centers of the first substrate W 1  and the second substrate W 2  may contact each other due to the pressing. 
     According to some embodiments, a distance by which each of the pressing finger  116  and  116   s  moves may be a half of the fifth distance d 5  between the two first and second substrates W 1  and W 2 . However, embodiments are not limited thereto, and the distance by which the pressing finger  116  moves downward may be greater than the distance by which the susceptor pressing finger  116   s  moves upward. According to another embodiment, the distance by which the pressing finger  116  moves downward may be less than the distance by which the susceptor pressing finger  116   s  moves upward. 
     Referring to  FIG.  5 C , a sufficient spontaneous bonding propagation between the first substrate W 1  and the second substrate W 2  may be allowed. For example, the bonding propagation may be conducted until a distance between the centers of the first and second substrates W 1  and W 2  and the bonding front BF becomes d 6 . This has been described above with reference to  FIG.  3 C , and thus a redundant description thereof will be omitted. 
     Referring to  FIG.  5 D , the holding fingers  114  and the susceptor holding fingers  114   s  may approach each other such that the edge of the second substrate W 2  and the edge of the first substrate W 1  bond together. At this time, the movements of the holding fingers  114  and the susceptor holding fingers  114   s  may be controlled by the controller  140 . A control scheme of the controller  140  has been described above with reference to  FIG.  3 D , and thus a description thereof will not be repeated. 
     The holding fingers  114  and the susceptor holding fingers  114   s  may maintain holding the second substrate W 2  and the first substrate W 1 , respectively, until the edge of the second substrate W 2  and the edge of the first substrate W 1  bond together. 
     Referring to  FIG.  5 E , after bonding between the second substrate W 2  and the first substrate W 1  is completed, the holding fingers  114  may release the holding of the second substrate W 2 . This has been described above with reference to  FIG.  3 E , and thus a description thereof will not be repeated. The holding-released holding fingers  114  may be moved in a direction away from the first substrate W 1 . 
     According to the embodiment of  FIGS.  5 A through  5 E , because the amounts by which the first and second substrates W 1  and W 2  are distorted, i.e., pushed along the z direction, by the pressing fingers  116  and  116   s  are small, more precise bonding is possible. Moreover, because both the holding fingers  114  holding the second substrate W 2  and the susceptor holding fingers  114   s  holding the first substrate W 1  are actively controlled, the first substrate W 1  and the second substrate W 2  may bond with each other more precisely. 
       FIGS.  6 A through  6 E  are conceptual views of stages in a method of bonding the first substrate W 1  and the second substrate W 2  together, according to another embodiment. 
     Referring to  FIG.  6 A , the substrate holder  110   a  having the second substrate W 2  held thereon may be arranged on a substrate susceptor  120   b  having the first substrate W 1  held thereon, such as to be aligned with the substrate susceptor  120   b  with a seventh distance d 7  therebetween. This structure is the same as the embodiment described above with reference to  FIG.  5 A , and thus a description thereof will not be repeated. 
     Referring to  FIG.  6 B , the susceptor pressing finger  116   s  of the substrate susceptor  120   b  may contact the first substrate W 1  without substantially pressing the first substrate W 1  upward. This is to support downward pressing of the second substrate W 2  as will be described later. 
     The pressing finger  116  of the substrate holder  110   a  may press the second substrate W 2  downward. The second substrate W 2  may be pressed downward by the pressing finger  116  until the second substrate W 2  contacts the first substrate W 1 . Because the first substrate W 1  is supported by the susceptor pressing finger  116   s , even when a pressure of the pressing finger  116  is applied to the first substrate W 1 , the first substrate W 1  may not be substantially distorted. 
     Referring to  FIG.  6 C , a sufficient spontaneous bonding propagation between the first substrate W 1  and the second substrate W 2  may be allowed. For example, the bonding propagation may be conducted until a distance between the center of the first and second substrates W 1  and W 2  and the bonding front BF becomes an eight distance d 8 . This has been described above with reference to  FIG.  3 C , and thus a description thereof will not be repeated. 
     Referring to  FIG.  6 D , the holding fingers  114  may be brought close to the susceptor holding fingers  114   s  such that the respective edges of the first and second substrates W 1  and W 2  bond to each other. The movement of the holding fingers  114  may be controlled by the controller  140 . The control scheme of the controller  140  has been described above with reference to  FIG.  3 D , and thus a description thereof will not be repeated. The holding fingers  114  may continue holding the second substrate W 2  until the respective edges of the first and second substrates W 1  and W 2  bond to each other. 
     Referring to  FIG.  6 E , after boning between the second substrate W 2  and the first substrate W 1  is completed, the holding fingers  114  may release the holding of the second substrate W 2 . This has been described above with reference to  FIG.  3 E , and thus a description thereof will not be repeated. The holding-released holding fingers  114  may be moved in a direction away from the first substrate W 1 . 
       FIG.  7    is a conceptual view illustrating occurrence of distortion on a substrate when, after a bonding front is formed, holding of the second substrate W 2  is released without bringing the edge of the second substrate W 2  close to the first substrate W 1  by using the holding fingers  114 . 
     As shown in  FIG.  7   , the entire substrate is distorted, and, in particular, is distorted with specific directivity and in a (+)-character shape. This distortion may occur because of a crystal structure of the substrate and may occur when a single crystal substrate, e.g., a silicon substrate, is used. In detail, a [100] or [010] crystal direction (i.e., crystallographic direction) may be a direction of 45° from an [110] crystal direction. A stress in the [110] crystal direction may be less than that of each of the [100] and [010] crystal directions. Accordingly, distortion in the [110] crystal direction may be less than distortion in each of the [100] and [010] crystal directions. 
     On the other hand, according to embodiments, when, after a bonding front is formed, the edge of the second substrate W 2  is brought close to the first substrate W 1  by the holding fingers  114  and then the holding of the second substrate W 2  is released, distortion in the (+)-character shape may be significantly reduced. 
     When the number of holding fingers  114  is greater than 4, an influence according to a crystal direction may be reduced, and thus distortion in the (+)-character shape may be reduced. According to some embodiments, when the number of holding fingers  114  is a multiple of 4, such as 8, 12, or 16, a deviation of an azimuthal direction may be reduced. Thus, these embodiments may be favorable. According to some embodiments, when the number of holding fingers  114  is an odd number, such as 5, 7, or 9, a tendency toward occurrence of distortion that is symmetrical about the center of the substrate may be reduced. Thus, these embodiments may be favorable. 
       FIG.  8    is a schematic diagram illustrating a method of aligning the first substrate W 1  with the second substrate W 2 , according to an embodiment. 
     Referring to  FIG.  8   , the second substrate W 2  may be disposed over the first substrate W 1 , while the first substrate W 1  and the second substrate W 2  are not parallel to each other. In this case, to bond the first substrate W 1  and the second substrate W 2  together, arrangement of the first substrate W 1  and the second substrate W 2  in parallel to each other may be first needed. In other words, before the first substrate W 1  and the second substrate W 2  contact each other to bond together, they may be arranged in parallel to each other according to a method which will now be described. 
     A plurality of distance sensors  150  may be arranged around the first substrate W 1 . Although three distance sensors  150  are disposed in  FIG.  8   , four or more distance sensors  150  may be disposed. 
     As described above with reference to  FIG.  3 D , each distance sensor  150  may measure a distance between the first substrate W 1  and the second substrate W 2  at its location. Because the first substrate W 1  and the second substrate W 2  are not parallel to each other, distances between the first substrate W 1  and the second substrate W 2  respectively measured by the distance sensors  150  may be different from each other. The controller  140  may estimate a distance between the first substrate W 1  and the second substrate W 2  at a location of each of the holding fingers  114 , from the distances between the first substrate W 1  and the second substrate W 2  respectively measured by the distance sensors  150 . 
     The controller  140  may calculate how much each holding finger  114  is to move in the z direction to arrange the first substrate W 1  and the second substrate W 2  in parallel to each other, by using the estimated distance between the first substrate W 1  and the second substrate W 2 . The controller  140  transmits the calculated values to the holding finger actuators  114   a  of the holding fingers  114 , respectively, and the holding finger actuators  114   a  operate according to the received calculated values and thus the first substrate W 1  and the second substrate W 2  may be arranged in parallel to each other. 
     In  FIG.  8   , an arrow shown beside each of the holding fingers  114  indicates an exemplary distance, e.g., and direction, by which each of the holding fingers  114  needs to move in order to arrange the first substrate W 1  and the second substrate W 2  in parallel to each other. Bigger arrows among the shown arrows indicate that the holding fingers  114  need to move longer distances. Although an orientation of the second substrate W 2  is adjusted in  FIG.  8   , one of ordinary skill in the art may understand that the same result may be obtained due to adjustment of an orientation of the first substrate W 1  or orientations of the first substrate W 1  and the second substrate W 2 . 
     When a comparative substrate bonding apparatus, i.e., without holding fingers  114 , is used and the first substrate and the second substrate are not parallel to each other, a special, e.g., separate, unit for making them parallel to each other is needed. In contrast, when substrate bonding apparatuses according to embodiments are used and the first substrate W 1  and the second substrate W 2  are not parallel to each other, the plurality of holding fingers  114 , e.g., which are within the substrate bonding apparatuses, may adjust positions of the first and second substrates W 1  and W 2  to make them parallel to each other. Thus, the aforementioned special unit employed in the comparative substrate bonding apparatus is not needed, and the substrate bonding apparatuses according to embodiments may have simplified structures. 
       FIG.  9    is a side view of a holding finger  114   v  according to an embodiment. The holding finger  114   v  may be substantially similar to the holding finger  114  in  FIGS.  1 - 8   . 
     Referring to  FIG.  9   , the holding finger  114   v  may have a fluid passage  114   h  that communicates with an end  114   t . When the end  114   t  contacts the second substrate W 2 , the fluid passage  114   h  may be isolated from the outside of the holding finger  114   v . When an internal pressure of the fluid passage  114   h  is lower than an external pressure of the holding finger  114   v , the second substrate W 2  may be attached to the holding finger  114   v  with a strength that is proportional to a difference between the two pressures. 
     When holding of the second substrate W 2  is intended to be released, the holding may be released by making the internal pressure of the fluid passage  114   h  equal to or almost equal to the external pressure of the holding finger  114   v . To this end, a fluid may be supplied to the fluid passage  114   h  via a special passage. 
     The end  114   t  may be formed of an elastic material such that the holding finger  114   v  and the second substrate W 2  may more smoothly adhere to each other and thus may be more tightly attached to each other. For example, the end  114   t  may be formed of any suitable polymer material having elasticity. When the end  114   t  is formed of an elastic material, damage of the second substrate W 2  may be reduced. 
       FIG.  10    is a side view of a holding finger  114   e  according to another embodiment. Any of the embodiments in  FIGS.  1 - 8    may employ fingers  114   e  instead of fingers  114  or  114   v.    
     Referring to  FIG.  10   , the holding finger  114   e  may include an electrostatic device  114   es  capable of generating an electrostatic force, around an end  114   t  of the holding finger  114   e . The electrostatic device  114   es  may generate the electrostatic force by applying power, and the second substrate W 2  may be attached to the holding finger  114   v  by the electrostatic force. Accordingly, when supply of the power to the electrostatic device  114   es  is interrupted, the holding of the second substrate W 2  may be immediately released. 
       FIG.  11    is a side view of a holding finger  114   f  according to another embodiment. Any of the embodiments in  FIGS.  1 - 8    may employ fingers  114   f  instead of fingers  114 ,  114   v , or  114   e.    
     Referring to  FIG.  11   , the holding finger  114   f  may include a first portion  114 _ 1  extending in a direction perpendicular to a main surface of the second substrate W 2 , and a second portion  114 _ 2  extending in a direction parallel to the main surface of the second substrate W 2 . The second portion  114 _ 2  may include the end  114   t  of the holding finger  114   f . In other words, the second portion  114 _ 2  may directly contact the second substrate W 2 . 
     According to some embodiments, the fluid passage  114   h  described above with reference to  FIG.  9    may be provided within the holding finger  114   f . In this case, a plurality of through holes may be provided on a bottom surface of the second portion  114 _ 2  and may communicate with the fluid passage  114   h.    
     According to some embodiments, the electrostatic device  114   es  described above with reference to  FIG.  10    may be provided within the holding finger  114   f . In this case, the electrostatic device  114   es  may be provided within the second portion  114 _ 2 . 
     Because the second portion  114 _ 2  of the holding finger  114   f  extends in a horizontal direction (i.e., the direction parallel to the main surface of the second substrate W 2 ), a contact area between the holding finger  114   f  and the second substrate W 2  may be increased, and the second substrate W 2  may be held with a more uniform pressure. 
       FIG.  12    is a side view of a substrate bonding apparatus  100   h  according to another embodiment. The substrate bonding apparatus  100   h  of  FIG.  12    is the same as the substrate bonding apparatus  100  of  FIG.  1   , except that a pressing finger  116   h  is the same as the holding fingers  114 . Accordingly, a description of  FIG.  12    that is the same as given above with reference to  FIG.  1    will not be repeated herein, and a difference therebetween will now be mainly described. 
     Referring to  FIG.  12   , the pressing finger  116   h  may be configured to hold the second substrate W 2 . A principle that the pressing finger  116   h  holds the second substrate W 2  may be the same as the holding fingers  114 . 
     As substrates have become recently larger and thinner, when only the edge of a substrate is held, a center portion of the substrate may sag downward due to a load of the substrate. This may cause the second substrate W 2  to be, e.g., damaged while being handled. Accordingly, when the pressing finger  116   h  located at the center of the second substrate W 2  holds the center portion of the second substrate W 2 , sagging of the substrate during substrate handling may be greatly reduced. 
     In a substrate bonding apparatus and a substrate bonding method according to embodiments, precision of bonding substrates together may be increased, and thus a semiconductor device may be manufactured with higher precision. Moreover, the yield of a product is improved due to the higher precision, and thus manufacturing costs thereof are reduced. 
       FIG.  13    is a schematic diagram illustrating the first substrate W 1  and the second substrate W 2  bonded with each other, according to an embodiment.  FIG.  14    is a magnified cross-sectional view of a portion XII of  FIG.  13   . 
     Referring to  FIG.  13   , the first substrate W 1  may include first semiconductor devices  20  on a surface thereof, and the second substrate W 2  may include second semiconductor devices  30  on a surface thereof. The first semiconductor devices  20  and the second semiconductor devices  30  may bond with each other to constitute an integrated semiconductor device, e.g., a CMOS image sensor (CIS). 
     Referring to  FIG.  14   , a first semiconductor device  20  may include a transistor Tr having a gate electrode  255  and source and drain regions  251  on a semiconductor substrate  201 , e.g., single crystal silicon. The gate electrode  255  may be electrically connected to a layer over the gate electrode  255 , via first buried wires  259 . The first buried wires  259  may each include a first barrier metal layer  259   a  and a first metal layer  259   b , and may be insulated from each other by a first interlayer insulation layer  257 . 
     Other intermediate wiring layers may include buried wires  265  and  268 , each including a barrier metal layer and a metal layer, and interlayer insulation layers  263  and  266  that surround the buried wires  265  and  268  to insulate them from each other. The wiring layers may be separated from each other by diffusion prevention layers  261 ,  281 , and  269 , except for portions of the buried wires in each wiring layer that contact each other. 
     A second semiconductor device  30  may include a transistor Tr having a gate electrode  327  and source and drain regions  323  on a semiconductor substrate  301 , e.g., single crystal silicon. The gate electrode  327  may be electrically connected to a layer below the gate electrode  327 , via second buried wires  331 . The second buried wires  331  may each include a second barrier metal layer  331   a  and a second metal layer  331   b , and may be insulated from each other by a second interlayer insulation layer  329 . 
     Other intermediate wiring layers may include buried wires  338 , each including a barrier metal layer and a metal layer, and an interlayer insulation layer  335  that surrounds the buried wires  338  to insulate them from each other. Optoelectric conversion units  321  may be provided within the semiconductor substrate  301 . 
     As shown in  FIG.  14   , the first substrate W 1  and the second substrate W 2  may bond together such that the first semiconductor device  20  and the second semiconductor device  30  are connected to each other with the buried wires  268  of the first semiconductor device  20  and the buried wires  338  of the second semiconductor device  30  facing each other. 
     Line widths of buried wires have become greatly thinner due to recent trends toward miniaturization of semiconductor devices. To bond the semiconductor devices together to have appropriate electrical characteristics, precision of several nanometers or less, e.g., about 40 nm or less, is needed. Therefore, when the substrate bonding apparatus and the substrate bonding method described above with reference to  FIGS.  1  through  4 B ,  FIG.  8   , and  FIG.  12    are used, a semiconductor device may be manufactured with higher precision. 
     The methods and processes described herein may be performed by code or instructions to be executed by a computer, processor, manager, or controller. Because the algorithms that form the basis of the methods (or operations of the computer, processor, or controller) are described in detail, the code or instructions for implementing the operations of the method embodiments may transform the computer, processor, or controller into a special-purpose processor for performing the methods described herein. 
     Also, another embodiment may include a computer-readable medium, e.g., a non-transitory computer-readable medium, for storing the code or instructions described above. The computer-readable medium may be a volatile or non-volatile memory or other storage device, which may be removably or fixedly coupled to the computer, processor, or controller which is to execute the code or instructions for performing the method embodiments described herein. 
     By way of summation and review, embodiments provide a substrate bonding apparatus and a substrate bonding method capable of manufacturing a semiconductor device at low costs and with improved precision. That is, embodiments provide independently controlled holding fingers as part of a substrate holder for holding a substrate. The holding fingers move toward a lower substrate until an edge of an upper substrate contacts the lower substrate, and, after the upper substrate and the lower substrate bond together, holding is released, e.g., as opposed to a comparative apparatus that causes an edge of the substrate to freely fall due to release of the holding when edges of substrates are far apart from each other. Moreover, while the holding fingers are being independently controlled, a bonding front situation (distance between substrates) of a substrate is ascertained in real time, and a moving speed, a displacement, and the like of the holding fingers are controlled by reflecting the bonding front situation. 
     Example embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. In some instances, as would be apparent to one of ordinary skill in the art as of the filing of the present application, features, characteristics, and/or elements described in connection with a particular embodiment may be used singly or in combination with features, characteristics, and/or elements described in connection with other embodiments unless otherwise specifically indicated. Accordingly, it will be understood by those of skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.