Abstract:
A mounting apparatus for mounting a device onto a substrate includes a table that holds the substrate, a mounting head that carries a device to be mounted on the substrate, a camera that is movable to a position between the table and the mounting head and includes a first imager that captures an image of the substrate on the table and a second imager that captures images of the device carried by the mounting head, a third imager that captures an image of a first device mounted on the substrate, and a controller that controls the mounting head to position a second device to be mounted on the substrate and being carried by the mounting head based on a position of the first device that is determined based on the image captured by the third imager.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is based upon and claims the benefit of priority of Japanese Patent Application No. 2015-174084, filed on Sep. 3, 2015, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    An aspect of this disclosure relates to a device mounting apparatus and a device mounting method. 
         [0004]    2. Description of the Related Art 
         [0005]    An optical module used in the field of optical communication includes a light-emitting device and a light-receiving device mounted on an optical waveguide. For example, an optical module is used for high-speed optical communications performed by, for example, supercomputers and high-end servers using high-speed interfaces. 
         [0006]    A certain type of optical module is formed by sequentially stacking a lens sheet and a flexible substrate (“substrate”) on which a light-emitting device and a light-receiving device are mounted on an optical waveguide. In such an optical module, the light-emitting device and the light-receiving device are aligned with the optical waveguide so that light from the light-emitting device can enter the optical waveguide and light from the optical waveguide can enter the light-receiving device (see, for example, Japanese Laid-Open Patent Publication No. 2009-69360). 
         [0007]    Because even slight misalignment of the light-emitting device and the light-receiving device results in the loss of light emitted from the light-emitting device and received by the light-receiving device, it is necessary to accurately align the light-emitting device and the light-receiving device with the optical waveguide. However, it is difficult to perform accurate alignment, and the light-emitting device and the light-receiving device may be mounted on positions slightly different from desired positions. Such slight misalignment may reduce the characteristics and the yield of optical modules. 
       SUMMARY OF THE INVENTION 
       [0008]    In an aspect of this disclosure, there is provided a mounting apparatus for mounting a device onto a substrate. The mounting apparatus includes a table that holds the substrate, a mounting head that carries a device to be mounted on the substrate, a camera that is movable to a position between the table and the mounting head and includes a first imager that captures an image of the substrate on the table and a second imager that captures images of the device carried by the mounting head, a third imager that captures an image of a first device mounted on the substrate, and a controller that controls the mounting head to position a second device to be mounted on the substrate and being carried by the mounting head based on a position of the first device that is determined based on the image captured by the third imager. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a top view of an optical module; 
           [0010]      FIG. 2  is a cut-away side view of an optical module; 
           [0011]      FIGS. 3A and 3B  are drawings illustrating a device mounting method; 
           [0012]      FIGS. 4A and 4B  are drawings illustrating a device mounting method; 
           [0013]      FIGS. 5A through 5C  are drawings illustrating positional relationships between light-emitting and light-receiving devices and through holes; 
           [0014]      FIG. 6  is a drawing illustrating an exemplary configuration of a device mounting apparatus according to a first embodiment; 
           [0015]      FIGS. 7A and 7B  are drawings illustrating a device mounting method according to the first embodiment; 
           [0016]      FIGS. 8A and 8B  are drawings illustrating a device mounting method according to the first embodiment; 
           [0017]      FIG. 9  is a drawing illustrating an exemplary configuration of a device mounting apparatus according to a second embodiment; 
           [0018]      FIG. 10  is a drawing illustrating an exemplary configuration of a device mounting apparatus according to a third embodiment; 
           [0019]      FIGS. 11A and 11B  are drawings illustrating an exemplary configuration of a device mounting apparatus according to a fourth embodiment; 
           [0020]      FIGS. 12A and 12B  are drawings illustrating an exemplary configuration of a device mounting apparatus according to a fifth embodiment; 
           [0021]      FIGS. 13A and 13B  are drawings illustrating a device mounting method according to a sixth embodiment; 
           [0022]      FIGS. 14A and 14B  are drawings illustrating a device mounting method according to the sixth embodiment; 
           [0023]      FIG. 15  is a drawing illustrating a device mounting method according to the sixth embodiment; and 
           [0024]      FIG. 16  is a flowchart illustrating a device mounting method according to the sixth embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0025]    Embodiments of the present invention are described below with reference to the accompanying drawings. Below, the same reference number is assigned to the same components, and repeated descriptions of those components are omitted. 
       &lt;Optical Module&gt; 
       [0026]    First, an optical module according to an embodiment is described.  FIG. 1  is a top view of the optical module, and  FIG. 2  is a cut-away side view of the optical module. The optical module is formed by stacking a lens sheet  30  and a flexible substrate (“substrate”)  40  on a sheet-shaped optical waveguide  20 . The optical waveguide  20  has a structure where a center core  21   a  is sandwiched between and surrounded by clads  21   b . A lens ferrule  22  is attached to one end of the optical waveguide  20 , and a mirror  23  is formed near the other end of the optical waveguide  20 . The lens sheet  30  includes multiple lenses  31  formed on a first surface  30   a , and a second surface  30   b  that is bonded to the optical waveguide  20  via an adhesive sheet  91 . 
         [0027]    Wiring (not shown) is formed on the substrate  40 . A light-emitting device  50 , a light-receiving device  60 , a driver  70 , and a transimpedance amplifier (TIA)  80  are mounted on a first surface  40   a  of the substrate  40 . The light-emitting device  50  may be, for example, a vertical-cavity surface-emitting laser (VCSEL), and the light-receiving device  60  may be, for example, a photodiode. The driver  70  is an integrated circuit (IC) for driving the light-emitting device  50 . The TIA  80  is an IC for converting a current which is generated by the light-receiving device  60  based on detected light into a voltage. One of the light-emitting device  50  and the light-receiving device  60  may be referred to as a “first device” and the other one of the light-emitting device  50  and the light-receiving device  60  may be referred to as a “second device”. 
         [0028]    Through holes  41  are formed in the substrate  40  as optical paths for light emitted from the light-emitting device  50  and light entering the light-receiving device  60 . The first surface  30   a  of the lens sheet  30  is bonded to a second surface  40   b  of the substrate  40  via an adhesive sheet  92 . The light-emitting device  50 , the light-receiving device  60 , the driver  70 , and the TIA  80  mounted on the substrate  40  are connected to the wiring formed on the substrate  40  via bumps  42 . The bumps  42  are made of a metal such as gold. Also, spaces between the bumps  42  and the light-emitting device  50 , the light-receiving device  60 , the driver  70 , and the TIA  80  are filled with a resin  43 . 
         [0029]    The optical module is assembled such that the light-emitting device  50  and light-receiving device  60 , the through holes  41 , the lenses  31 , and the mirror  23  are aligned with each other. More specifically, components of the optical module are aligned and joined together such that light emitted from the light-emitting device  50  passes through the corresponding through hole  41  and the corresponding lens  31 , is reflected by the mirror  23 , and propagates through the core  21   a  of the optical waveguide  20 ; and light propagating through the core  21   a  is reflected by the mirror  23 , passes through the corresponding lens  31  and the corresponding through hole  41 , and enters the light-receiving device  60 . 
         [0030]    The bumps  42  are formed on the first surface  40   a ; and the light-emitting device  50 , the light-receiving device  60 , the driver  70 , and the TIA  80  are positioned and mounted on the bumps  42  and are bonded together by ultrasonic flip-chip bonding. Thereafter, thermoset resin is injected into spaces between the bumps  42  and the components, and is thermally hardened. 
         [0031]    Separately from the above process, the lens ferrule  22  is attached to one end of the optical waveguide  20 , the adhesive sheet  91  is bonded to the surface  20   a  of the optical waveguide  20 , and the second surface  30   b  of the lens sheet  30  is bonded to the adhesive sheet  91 . When the lens sheet  30  is bonded to the adhesive sheet  91 , the mirror  23  and the lenses  31  are aligned with each other. 
         [0032]    Next, the adhesive sheet  92  is bonded to the first surface  30   a  of the lens sheet  30 , and the second surface  40   b  of the substrate  40  is bonded to the adhesive sheet  92 . When the substrate  40  is bonded to the adhesive sheet  92 , the lenses  31 , the through holes  41 , and the light-emitting and light-receiving devices  50  and  60  are aligned with each other. 
         [0033]    The process of mounting the light-emitting device  50  and the light-receiving device  60  on the substrate  40  is described below in more detail. 
         [0034]    To bond the light-emitting device  50  and the light-receiving device  60  to the substrate  40  by ultrasonic flip-chip bonding, a flip-chip bonding apparatus illustrated in  FIGS. 3A through 4B  is used. The flip-chip bonding apparatus includes a table  910  on which the substrate  40  is placed, a mounting head  920  for carrying the light-emitting device  50  and the light-receiving device  60 , and a double-sided camera  930  that can capture images of objects above and below the camera  930 . 
         [0035]    The camera  930  includes a first imager  931  on a first side and a second imager  932  on a second side. The imaging direction of the first imager  931  indicated by an arrow A is 180-degrees opposite the imaging direction of the second imager  932  indicated by an arrow B. Thus, the first imager  931  captures an image of the substrate  40  placed on the table  910 , and the second imager  932  captures images of the light-emitting device  50  and the light-receiving device  60  held by the mounting head  920 . 
         [0036]    First, as illustrated by  FIG. 3A , the light-emitting device  50  is held by the mounting head  920 , and is aligned with a through hole  41   a . Bumps for mounting the light-emitting device  50  are formed around the through hole  41   a.    
         [0037]    The camera  930  is moved to a position between the substrate  40  on the table  910  and the light-emitting device  50  held by the mounting head  920 . An image of the substrate  40  located in a −Z direction relative to the camera  930  is captured by the first imager  931 , and an image of the light-emitting device  50  located in a +Z direction relative to the camera  930  is captured by the second imager  932 . Based on the position of the through hole  41   a  captured by the first imager  931  and the position of a light emitter  51  of the light-emitting device  50  captured by the second imager  932 , the mounting head  920  is moved to align the light-emitting device  50  with the through hole  41   a  such that the light emitter  51  is placed in a desired position in the through hole  41   a.    
         [0038]    Next, as illustrated by  FIG. 3B , the camera  930  is moved in a +X direction (right in  FIG. 33 ) to a position outside of the space below the light-emitting device  50 , the mounting head  920  is moved in the −Z direction (downward in  FIG. 3B ) to place the light-emitting device  50  on the substrate  40 , and the light-emitting device  50  is bonded to the substrate  40  by flip-chip bonding. 
         [0039]    Next, as illustrated by  FIG. 4A , the light-receiving device  60  is held by the mounting head  920 , and is aligned with the through hole  41   b  formed in the substrate  40 . Bumps for mounting the light-receiving device  60  are formed around the through hole  41   b.    
         [0040]    The camera  930  is moved to a position between the substrate  40  and the light-receiving device  60 . An image of the first surface  40   a  located in the −Z direction is captured by the first imager  931 , and an image of the light-receiving device  60  located in the +Z direction is captured by the second imager  932 . Based on the position of the through hole  41   b  captured by the first imager  931  and the position of a light receiver  61  of the light-receiving device  60  captured by the second imager  932 , the mounting head  920  is moved to align the light-receiving device  60  with the through hole  41   b  such that the light receiver  61  is placed in a desired position in the through hole  41   b.    
         [0041]    Next, as illustrated by  FIG. 4B , the camera  930  is moved in the +X direction in  FIG. 4B , the mounting head  920  is moved in the −Z direction in  FIG. 4B  to place the light-receiving device  60  on the substrate  40 , and the light-receiving device  60  is bonded to the substrate  40  by flip-chip bonding. 
         [0042]    Through the above process, the light-emitting device  50  and the light-receiving device  60  are mounted on the substrate  40 . In  FIGS. 3A through 4B , only one light emitter  51  and only one light receiver  61  are illustrated for brevity. 
         [0043]    Each of  FIGS. 5A through 5C  illustrates the light-emitting device  50  and the light-receiving device  60  that are bonded to the substrate  40 . In  FIGS. 5A through 5C , the light-emitting device  50  includes four light emitters  51  and the light-receiving device  60  includes four light receivers  61 . In  FIGS. 5A through 5C , each dashed-dotted line connects the four light emitters  51  and the four light receivers  61 . 
         [0044]    As illustrated by  FIG. 5A , the light-emitting device  50  and the light-receiving device  60  are ideally or preferably bonded to the substrate  40  such that each of the light emitters  51  is positioned in the center of the corresponding one of four through holes  41   a , and each of the light receivers  61  is positioned in the center of the corresponding one of four through holes  41   b . With this alignment, the loss of light is small. In  FIG. 5A , the light emitters  51  and the light receivers  61  are collinear. 
         [0045]    In the above process, however, the step of aligning the through holes  41   a  with the light emitters  51  and bonding the light-emitting device  50  to the substrate  40  is performed separately from the step of aligning the through holes  41   b  with the light receivers  61  and bonding the light-receiving device  60  to the substrate  40 . For this reason, as illustrated by  FIG. 5B , there is a case where the light emitters  51  of the bonded light-emitting device  50  are misaligned with the centers of the corresponding through holes  41   a , and the light receivers  61  of the bonded light-receiving device  60  are misaligned with the centers of the corresponding through holes  41   b . In  FIG. 5B , the light emitters  51  are not collinear with the light receivers  61 . In this case, the lenses  31  of the lens sheet  30  to be bonded at a later step are also misaligned with the light emitters  51  and the light receivers  61 , which results in the loss of light. 
         [0046]      FIG. 5C  illustrates still another case. In  FIG. 5C , although the light emitters  51  are misaligned with the centers of the corresponding through holes  41   a  and the light receivers  61  are misaligned with the centers of the corresponding through holes  41   b , the light emitters  51  are collinear with the light receivers  61 . In this case, by properly positioning and bonding the lens sheet  30  at a later step, it is possible to properly align the light emitters  51  and the light receivers  61  with the corresponding lenses  31  such that the loss of light is reduced. 
       First Embodiment 
       [0047]    Next, a first embodiment is described. The first embodiment provides a device mounting apparatus and a device mounting method that can band the light-emitting device  50  and the light-receiving device  60  to the substrate  40  such that the light emitters  51  are aligned collinearly with the light receivers  61 . 
         [0048]      FIG. 6  illustrates a flip-chip bonding apparatus that is a device mounting apparatus of the first embodiment. The device mounting apparatus includes a table  110  on which the substrate  40  is to be placed, a mounting head  120  for carrying the light-emitting device  50  and the light-receiving device  60 , a double-sided first camera  130  that is capable of capturing images of objects above and below the first camera  130 , a second camera  140  embedded in the table  110 , and a controller  150  that controls the position of the mounting head  120  based on images captured by the first camera  130  and the second camera  140 . The controller  150  controls the entire device mounting apparatus. 
         [0049]    The first camera  130  includes a first imager  131  on a first side and a second imager  132  on a second side. 
         [0050]    The imaging direction of the first imager  131  (indicated by an arrow A) is 180-degrees opposite the imaging direction of the second imager  132  (indicated by an arrow B). Thus, the first imager  131  can capture an image of the substrate  40  placed on the table  110 , and the second imager  132  can capture images of the light-emitting device  50  and the light-receiving device  60  held by the mounting head  120 . 
         [0051]    The device mounting apparatus of the first embodiment includes the second camera  140  used as a third imager. The second camera  140  captures an image of the light-emitting device  50  mounted on the substrate  40 , especially the light emitters  51 . 
         [0052]    Next, a device mounting method of the first embodiment is described with reference to  FIGS. 7A through 8B . In  FIGS. 7A through 8B , only one light emitter  51  and only one light receiver  61  are illustrated for brevity. 
         [0053]    First, as illustrated by  FIG. 7A , the light-emitting device  50  held by the mounting head  120  is aligned with the through hole  41   a . The through hole  41   a  is formed in an area of the substrate  40  where the light-emitting device  50  is to be mounted, and the through hole  41   b  is formed in an area of the substrate  40  where the light-receiving device  60  is to be mounted. Bumps (not shown) for mounting the light-emitting device  50  are formed around the through hole  41   a . The first camera  130  is moved to a position between the substrate  40  and the light-emitting device  50 . An image of the first surface  40   a  of the substrate  40  located in the −Z direction is captured by the first imager  131 , and an image of the light-emitting device  50  located in the +Z direction is captured by the second imager  132 . Based on the position of the through hole  41   a  captured by the first imager  131  and the position of the light emitter  51  captured by the second imager  132 , the mounting head  120  is moved to align the light-emitting device  50  with the through hole  41   a  such that the light emitter  51  is placed in a desired position in the through hole  41   a.    
         [0054]    Next, as illustrated by  FIG. 7B , the first camera  130  is moved in the +X direction in  FIG. 7B , the mounting head  120  is moved in the −Z direction in  FIG. 7B  to place the light-emitting device  50  on the substrate  40 , and the light-emitting device  50  is bonded to the substrate  40 . In the present embodiment, before the light-emitting device  50  is bonded, an image of the light-emitting  50 , especially light emitters  51 , may be captured by the second camera  140 , and the light-emitting device  50  may be adjusted to a desired position by moving the mounting head  120  based on the captured image. 
         [0055]    Next, as illustrated by  FIG. 8A , the light-receiving device  60  held by the mounting head  120  is aligned with the through hole  41   b . Bumps for mounting the light-receiving device  60  are formed around the through hole  41   b . The first camera  130  is moved to a position between the substrate  40  and the light-receiving device  60 . An image of the first surface  40   a  located in the −Z direction is captured by the first imager  131 , and an image of the light-receiving device  60  located in the +Z direction is captured by the second imager  132 . Based on the position of the through hole  41   b  captured by the first imager  131  and the position of the light receiver  61  of the light-receiving device  60  captured by the second imager  132 , the mounting head  120  is moved to align the light-receiving device  60  with the through hole  41   b  such that the light receiver  61  is placed in a desired position in the through hole  41   b . Also, in the first embodiment, an image of the light-emitting device  50  mounted on the substrate  40 , especially an image of the light emitters  51 , is captured by the second camera  140 , and the position of the light-receiving device  60  is adjusted relative to the position of the captured light-emitting device  50  such that the light emitters  51  are aligned collinearly with the light-receivers  61 . Captured images of the light emitters  51  and light receivers  61  are therefore used as a sort of reference marks for alignment. 
         [0056]    Next, as illustrated by  FIG. 8B , the first camera  130  is moved in the +X direction in  FIG. 8B , the mounting head  120  is moved in the −Z direction in  FIG. 8B  to place the light-receiving device  60  on the substrate  40 , and the light-receiving device  60  is bonded to the substrate  40 . 
         [0057]    As described above, in the first embodiment, the position of the light-receiving device  60  to be bonded to the substrate  40  is adjusted relative to the position of the light-emitting device  50  already bonded to the substrate  40 . This method makes it possible to align the light emitters  51  collinearly with the light receivers  61  as illustrated by  FIGS. 5A and 5C . When the light emitters  51  and the light receivers  61  are collinear, by properly positioning the lens sheet  30  to be bonded at a later step, the light emitters  51  and the light receivers  61  can be properly aligned with the corresponding lenses  31  such that the loss of light is reduced. This in turn increases the production yield. 
         [0058]    In the first embodiment, the light-emitting device  50  and the light-receiving device  60  are positioned based on the positions of the light emitters  51  and the light receivers  61 . Alternatively, reference marks may be provided on the light-emitting device  50  and the light-receiving device  60 , and the light-emitting device  50  and the light-receiving device  60  may be positioned based on the reference marks. In this case, images of the reference marks are captured by cameras. 
       Second Embodiment 
       [0059]    Next, a second embodiment is described. As illustrated by  FIG. 9 , a device mounting apparatus of the second embodiment includes a table  210  that includes a table body  211  and a transparent board  212  disposed on the table body  211 . The substrate  40  is placed on an upper surface of the board  212 . The second camera  140  used as the third imager is disposed on the table body  211  under the board  212 . An image of the light-emitting device  50  mounted on the substrate  40  is captured by the second camera  140  via the board  212 . The above configuration of the second embodiment eliminates the need to embed the second camera  140  in the table  210  and the second camera  140  can be easily installed. 
         [0060]    The board  212  may be made of, for example, tempered glass. The second camera  140  may be movably disposed in a space formed in the table body  211 . In  FIG. 9 , only one light emitter  51  and only one light receiver  61  are illustrated for brevity. 
         [0061]    Other configurations of the device mounting apparatus of the second embodiment are substantially the same as those of the device mounting apparatus of the first embodiment. 
       Third Embodiment 
       [0062]    Next, a third embodiment is described. As illustrated by  FIG. 10 , a device mounting apparatus of the third embodiment includes a table  310  made of a transparent material. A mirror  311  is disposed in the table  310  and reflects light propagating in an in-plane direction through the table  310  at a right angle. A second camera  340  used as a third imager is disposed on a side surface of the table  310 . The second camera  340  captures an image of an object located in the −X direction indicated by an arrow D, and captures an image of the light emitter  51  reflected by the mirror  311 . Thus, the device mounting apparatus of the third embodiment is configured such that an image of the light emitter  51  is reflected by the mirror  311 , propagates through the table  310  as indicated by an arrow E, and capture by the second camera  340 . In  FIG. 10 , only one light emitter  51  and only one light receiver  61  are illustrated for brevity. 
         [0063]    Other configurations of the device mounting apparatus of the third embodiment are substantially the same as those of the device mounting apparatus of the first embodiment. 
       Fourth Embodiment 
       [0064]    Next, a fourth embodiment is described. As illustrated by  FIGS. 11A and 11B , a device mounting apparatus of the fourth embodiment includes a camera  440  that provides the functions of the first camera  130  and the second camera  340  of the third embodiment with a single unit. The camera  440  can capture images of objects in three directions. In  FIGS. 11A and 11B , only one light emitter  51  and only one light receiver  61  are illustrated for brevity. 
         [0065]    The camera  440  includes a first imager  441  on a lower surface, a second imager  442  on an upper surface, and a third imager  443  on a side surface. The first imager  441  captures an image of an object located in the −Z direction indicated by an arrow A, the second imager  442  captures an image of an object located in the +Z direction indicated by an arrow B, and the third imager  443  captures an image of an object located in the −X direction indicated by an arrow F. 
         [0066]    The imaging direction of the first imager  441  is 180-degrees opposite the imaging direction of the second imager  442 . The imaging direction of the third imager  443  forms an angle of 90 degrees with the imaging directions of the first imager  441  and the second imager  442 . The third imager  443  captures, from a side surface of the transparent table  310 , an image of the light-emitting device  50  reflected by the mirror  311  provided in the table  310 . 
         [0067]    In the fourth embodiment, similarly to the first embodiment, the light-emitting device  50  is bonded to the substrate  40  by using the first imager  441  and the second imager  442 . Next, as illustrated by  FIG. 11A , the camera  440  is moved to the side surface of the table  310 , and an image of the light emitter  51  reflected by the mirror  311  is captured by the third imager  443 . The captured image is sent to the controller  150  to determine the position of the light emitter  51 . 
         [0068]    Next, as illustrated by  FIG. 11B , the camera  440  is moved to a position between the substrate  40  and the light-receiving device  60  held by the mounting head  120 . An image of the substrate  40  is captured by the first imager  441 , and an image of the light-receiving device  60  is captured by the second imager  442 . Based on the position of the through hole  41   b  captured by the first imager  441  and the position of the light receiver  61  of the light-receiving device  60  captured by the second imager  442 , the mounting head  120  is moved to align the light-receiving device  60  with the through hole  41   b . In this step, the position of the light-receiving device  60  is adjusted relative to the position of the light-emitting device  50  captured by the third imager  443  such that the light emitters  51  are aligned collinearly with the light-receivers  61 . 
       Fifth Embodiment 
       [0069]    Next, a fifth embodiment is described. As illustrated by  FIG. 12 , a device mounting apparatus of the fifth embodiment includes a table  510  made of a transparent material. A first mirror  511  and a second mirror  512  are disposed in the table  510  and reflect light propagating in an in-plane direction through the table  510  at a right angle. The device mounting apparatus of the fifth embodiment does not include a second camera. 
         [0070]    A device mounting method of the fifth embodiment is described with reference to  FIGS. 12A and 12B . In  FIGS. 12A and 12B , only one light emitter  51  and only one light receiver  61  are illustrated for brevity. 
         [0071]    First, as illustrated by  FIG. 12A , the light-emitting device  50  held by the mounting head  120  is aligned with the through hole  41   a . The first camera  130  is moved to a position between the substrate  40  and the light-emitting device  50 . An image of the substrate  40  is captured by the first imager  131 , and an image of the light-emitting device  50  is captured by the second imager  132 . Based on the image of the through hole  41   a  captured by the first imager  131  and the image of the light emitter  51  captured by the second imager  132 , the mounting head  120  is moved such that the light emitter  51  is placed in a desired position in the through hole  41   a . Then, the light-emitting device  50  is placed on and bonded to the substrate  40 . 
         [0072]    Next, as illustrated by  FIG. 12B , the light-receiving device  60  held by the mounting head  120  is aligned with the through hole  41   b . The first camera  130  is moved to a position between the substrate  40  and the light-receiving device  60 . An image of the substrate  40  is captured by the first imager  131 , and an image of the light-receiving device  60  is captured by the second imager  132 . Based on the image of the through hole  41   b  captured by the first imager  131  and the image of the light receiver  61  captured by the second imager  132 , the mounting head  120  is moved such that the light receiver  61  is placed in the through hole  41   b.    
         [0073]    At this stage, as illustrated by  FIG. 12B , an image of the light emitter  51  reflected by the first and second mirrors  511  and  512  is captured by the first imager  131 . As indicated by an arrow G, the image of the light emitter  51  enters the surface of the table  510  at a substantially-right angle, is reflected by the first mirror  511 , propagates through the table  510 , is reflected by the second mirror  512 , exits from the surface of the table  510  at a substantially-right angle, passes through the through hole  41   b , and enters the first imager  131 . 
         [0074]    This configuration enables the first camera  130  to detect the position of the light emitter  51 , the through hole  41   b , and the light receiver  61  of the light-receiving device  60  held by the mounting head  120 . Based on the detected positions, the position of the light-receiving device  60  is adjusted relative to the position of the light-emitting device  50  mounted on the substrate  40  such that the light emitters  51  are aligned collinearly with the light-receivers  61 . Then, the light-receiving device  60  is placed on and bonded to the substrate  40 . 
       Sixth Embodiment 
       [0075]    Next, a sixth embodiment is described. In the sixth embodiment, a device mounting method performed by a device mounting apparatus to produce an optical module is described. In the sixth embodiment, as illustrated in  FIGS. 13A through 15 , the device mounting apparatus includes a double-sided camera  630  includes a first imager  631  on a first side and a second imager  632  on a second side that can capture images of objects above and below the camera  630 . 
         [0076]    The imaging direction of the first imager  631  (indicated by the arrow A) is 180-degrees opposite the imaging direction (indicated by the arrow B) of the second imager  632  (indicated by the arrow B). 
         [0077]    The device mounting method of the sixth embodiment is described with reference to  FIGS. 13A through 16 . 
         [0078]    At step S 102  of  FIG. 16 , the controller  150  causes the camera  630  to capture an image of the light-emitting device  50  held by the mounting head  120 , and moves the mounting head  120  to align the light-emitting device  50  with the through hole  41   a . The controller  150  moves the camera  630  to a position between the substrate  40  and the light-emitting device  50 , causes the first imager  631  to capture an image of the first surface  40   a , and causes the second imager  632  to capture an image of the light-emitting device  50 . Based on the position of the through hole  41   a  captured by the first imager  631  and the position of the light emitter  51  captured by the second imager  632 , the controller  150  moves the mounting head  120  to align the light-emitting device  50  with the through hole  41   a  such that the light emitter  51  is placed in a desired position. At this step, the controller  150  determines the outer shape of the light-emitting device  50 , the position of a reference mark on the light-emitting device  50 , and the position of the light emitter  51  based on the image of the light-emitting device  50  captured by the second imager  632 . 
         [0079]    At step S 104 , the controller  150  moves the mounting head  120  to mount the light-emitting device  50  on the substrate  40 . Specifically, as illustrated by  FIG. 13B , the controller  150  moves the camera  630  to the right in  FIG. 13B , moves the mounting head  120  downward in  FIG. 13B  to place the light-emitting device  50  on the substrate  40 , and bonds the light-emitting device  50  to the substrate  40  by flip-chip bonding. 
         [0080]    At step S 106 , the controller  150  causes the camera  630  to capture an outer image of the light-emitting device  50  mounted on the substrate  40 . As illustrated by  FIG. 14A , the controller  150  moves the camera  630  to a position above the light-emitting device  50 , and causes the first imager  631  to capture an image of the light-emitting device  50 . Based on the captured outer shape of the light-emitting device  50 , the controller  150  estimates the position of the light emitter  51  or a reference mark on the light-emitting device  50  and stores the coordinates of the estimated position in a storage provided in the controller  150 . 
         [0081]    At step S 108 , the controller  150  causes the camera  630  to capture an image of the light-receiving device  60  held by the mounting head  120 . The controller  150  moves the camera  630  to a position between the substrate  40  and the light-receiving device  60 , causes the first imager  631  to capture an image of the first surface  40   a , and causes the second imager  632  to capture an image of the light-receiving device  60 . At this step, the controller  150  determines the outer shape of the light-receiving device  60 , the position of a reference mark on the light-receiving device  60 , and the position of the light receiver  61  based on the image of the light-receiving device  60  captured by the second imager  632 . 
         [0082]    At step S 110 , the controller  150  moves the mounting head  120  to position the light-receiving device  60 . Specifically, the controller  150  moves the mounting head  120  to align the light-receiving device  60  with the through hole  41   b  based on the images of the first surface  40   a  and the light-receiving device  60  captured at step S 108 , and adjusts the position of the light receiver  61  relative to the position of the light emitter  51  or the reference mark of the light-emitting device  50  estimated at step S 106 . 
         [0083]    At step S 112 , the controller  150  moves the mounting head  120  to mount the light-receiving device  60  on the substrate  40 . As illustrated by  FIG. 15 , the controller  150  moves the camera  630  in the +X direction, moves the mounting head  120  in the −Z direction to place the light-receiving device  60  on the substrate  40 , and bonds the light-receiving device  60  to the substrate  40  by flip-chip bonding. 
         [0084]    According to the device mounting method described above, the light-emitting device  50  and the light-receiving device  60  can be mounted on the substrate  40  such that the light emitters  51  are aligned collinearly with the light receivers  61 . 
         [0085]    An aspect of this disclosure provides a device mounting apparatus and a device mounting method that can mount a light-emitting device and a light-receiving device at accurate positions on an optical waveguide. 
         [0086]    A device mounting apparatus and a device mounting method according to embodiments of the present invention are described above. However, the present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.