Abstract:
In one aspect of the invention, an assembly includes an optical-electronic die having electrically conductive pads and a submount with first and second opposing sides and a third side essentially perpendicular to the first submount side. The first and third submount sides have an adjoining edge, with electrically conductive pads on the first submount side bonded to the die pads, second electrically conductive pads on the third side of the submount, and electrically conductive traces interconnecting the first and second submount pads. The conductive traces are formed on the first and third sides and adjoining edge of the submount by a process that uses a shadow mask.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to an optical-electronic array module for connecting to a fiber optic cable and to electronic devices on an electronic circuit board, and more particularly to an arrangement of the module that facilitates assembly of the module and alignment and connection of the module to the fiber optic cable electronic circuit board.  
           [0003]    2. Related Art  
           [0004]    Optical-electronic transmitters and receivers on an optical-electronics module are conventionally coupled to optical signals via a fiber-optic cable. The optical fibers of the cable are typically aligned to optics of the optical-electronic transmitters and receivers with a required precision of about 5 microns.  
           [0005]    Is also conventional that the optical-electronics module with the transmitters and receivers are supplied to a customer having electrical circuitry on the customer&#39;s circuit board to be connected to the optical-electronic transmitters and receivers. Generally the plane of the optical paths in the optical coupler is parallel to the plane of the customer&#39;s electronic circuitry, and the plane of the electronic inputs and outputs (“I/O”) of the optical receivers and transmitters is orthogonal to the plane of the I/O of the customer&#39;s electronic circuitry. Therefore, in order to connect the I/O of the electronic circuitry on the customer&#39;s board to the electrical I/O of the optical-electronic transmitters and receivers it is necessary to turn the electrical path between the respective sets of I/O.  
           [0006]    Referring to FIG. 1, the alignment of optical paths and the turning of the electrical path between I/O sets is illustrated in a prior art module  100 , which includes a carrier  110  mounted on a heat sink  180 , an optical-electronic die  120  mounted on the carrier  110 , a coupler  140 , a signal conditioning die  190 , a flexible cable  130 , a first circuit board  170 , and C 4  solder balls  175 . The circuit board  170  has die  190  mounted thereon. The die  190  has signal conditioning circuitry that interconnects to and operates with the optical-electronic circuitry of the die  120  by means of the flexible cable  130 . The die  190  also interconnects to a customer&#39;s circuit board, second circuit board  172 , via conductors (not shown) and C 4  solder balls  175 .  
           [0007]    The carrier  110  is for structural purposes and for conducting thermal energy away from the die  120 . The carrier  110  does not have embedded conductors, but the carrier  110  itself is conductive, and it electrically connects a cathode on the laser die  120  to ground. The prior art apparatus uses two carriers, side-by-side. Only one of the carriers  110  is shown in FIG. 1. On one of the carriers  110 , the die  120  is a laser die. On the other carrier  110 , the die  120  is a photo detector die. (The term “optical-electronic die” will be used herein to refer to either a laser die or a photo detector die.) In FIG. 1, the die  120  is bonded to the carrier  110 , such as with a die attach epoxy, on the same side of the carrier  110  as an optical coupler  140 . The carrier  110  has alignment holes for receiving pins  142  from the coupler  140 . The coupler  140  attaches to the carrier  110  with a retainer (not shown) and alignment pins  142 .  
           [0008]    A fiber-optic cable  160  having a number of embedded fibers  162  mates to the optical coupler  140 . A connector  150  of the fiber-optic cable  160  has alignment holes for receiving alignment pins  152  from the coupler  140 . The coupler  140  attaches to the connector  150  with a retainer (not shown) and alignment pins  152 .  
           [0009]    The flexible cable  130  is a composition of gold-coated, copper conductors etched in a polyimid and covered with an insulating jacket. The flexible cable  130  is attached at attachment  137  to the first circuit board  170  at one end and at attachment  134  to carrier  110  for the optical-electronic die  120  at the other end. The flex cable  130  is electrically connected at  132  to the die  120  by wire bonds  136 . Likewise, the flex cable  130  is electrically connected at  139  to die  190  with wire bonds  138 .  
           [0010]    The flex cable  130  provides a 90 degree turn between the I/O plane of the optical-electronic die  120  and the customer&#39;s board, second circuit board  172 , however, it is problematic to use the flex cable to provide this 90 degree bend because of its cost and because of the relatively large number of interconnections at  132 ,  134 ,  137 , etc. Also, with conventional arrangements such as that of FIG. 1 it is problematic to achieve required alignment precision since it requires expensive and time consuming “active” alignment, according to which the optical-electronic die is powered and its output monitored, then secured with adhesive once alignment is optimized. There is therefore a need for an improved optical-electronics module.  
         SUMMARY OF THE INVENTION  
         [0011]    The foregoing need is addressed in an optical-electronic module having a submount. The submount forms an aperture which extends all the way through the submount. An optical-electronic die is mounted on a first side of the submount. The module also has an optical coupler, with a fiber-optic path in the coupler, for coupling optical signals from or to a fiber-optic cable on a first end of the coupler and for coupling the optical signals from or to the die at a second end of the coupler. The second end of the coupler has a feature matching the submount aperture and inserted into the submount aperture. An optical input or output of the die faces the second end of the coupler and is aligned to the coupler fiber-optic path and optically coupled to the fiber-optic path through the aperture.  
           [0012]    In another aspect, pads for electronic inputs or outputs on the optical-electronic die face, align with, and are electrically coupled to first electrical pads on the submount first side.  
           [0013]    In another aspect, the aperture is tapered, narrowing toward the submount first side, and the coupler feature matching the submount aperture comprises a tapered nose narrowing toward the coupler second end.  
           [0014]    In another aspect, the coupler end proximate to the die (the coupler second end) is sub-flush to the submount first side. From the coupler side which is proximate the die, the coupler extends through the submount aperture and beyond the submount second side.  
           [0015]    In still another aspect, the submount first side is in a first plane, and the submount has a third side in a plane oblique or perpendicular to the first plane. The third side has second electrical pads, for connecting to electrical pads on a circuit board. The second electrical pads are connected by conductors of the submount to respective ones of the first electrical pads, so that electrical paths from the electronic inputs or outputs of the optical-electronic die turn by at least an acute angle from the first to the second submount electrical pads.  
           [0016]    In a still further aspect, the coupler has mechanical pads for coupling to the circuit board. In an alternative, the coupler mechanical pads are on a bottom side of the coupler and the coupler bottom side is in the same plane as the submount third side.  
           [0017]    In a method form of the invention, a method for fabricating an optical-electronic array module includes a providing a submount having first and second opposing sides and a third side essentially perpendicular to the first submount side. The first and third submount sides have an adjoining edge, and the submount forms an aperture extending through the submount from the first to the second sides. Conductive traces are formed on the first and third sides and adjoining edge of the submount using a shadow mask. The traces interconnect electrically conductive pads on the first submount side and second electrically conductive pads on the submount third side. An optical coupler is inserted into the submount aperture and secured therein. The coupler has a fiber-optic path therein for coupling optical signals from or to a fiber-optic cable on a first end of the coupler and for coupling, at a second end of the coupler, the optical signals from or to a die mounted on the submount first side. The second end of the coupler has a feature matching the submount aperture. An optical input or output of the die is aligned to the coupler fiber-optic path facing the second side of the coupler concurrently with aligning pads on the die for electrical inputs or outputs to the electrical pads on the submount first side.  
           [0018]    In a further aspect, the method includes mounting the die on the submount, wherein the optical inputs or outputs of the die are aligned to the coupler fiber-optic path, and the electronic input or output pads on the die are aligned to first electrical pads on the submount first side prior to the die being mounted on the submount.  
           [0019]    In a still further aspect, the optical inputs or outputs of the die are aligned to the coupler fiber-optic path, and the electronic input or output pads on the die are aligned to first electrical pads on the submount first side after the coupler is secured to the submount.  
           [0020]    In yet another aspect, the optical inputs or outputs of the die are aligned to the coupler fiber-optic path, and the electronic input or output pads on the die are aligned to first electrical pads on the submount first side with the die deenergized.  
           [0021]    It is an object of the invention to bend an electrical path between I/O of a circuit board and I/O of an optical-electronic die without using a flexible cable, thus reducing cost, shortening the electrical path, and improving electrical properties of the interconnections.  
           [0022]    It is another object of the invention to facilitate precise alignment between fiber-optic paths and optics of devices on the optical-electronic die.  
           [0023]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 shows a prior art optical-electronic array module connected to a fiber-optic cable and a customer&#39;s circuit board.  
         [0025]    [0025]FIG. 2 shows an isometric view of an optical-electronics array module, according to an embodiment of the present invention.  
         [0026]    [0026]FIG. 3A shows an orthographic view of the bottom of the module of FIG. 2.  
         [0027]    [0027]FIG. 3B shows an enlarged view of a portion of the module of FIG. 2.  
         [0028]    [0028]FIG. 3C shows a side view of the detector die of FIG. 3B.  
         [0029]    [0029]FIG. 3D shows a portion of the sides of the submount and coupler of FIG. 3B.  
         [0030]    [0030]FIG. 4 shows a cross-section of the submount, detector die, and a portion of the coupler of FIG. 3A, along with a portion of a customer&#39;s circuit board, according to an embodiment of the present invention.  
         [0031]    [0031]FIG. 5 shows details of how I shadow mask is used to fabricate conductors on the submount of FIGS. 3B and 3D, according to an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    The claims at the end of this application set out novel features which applicants believe are characteristic of the invention. The invention, a preferred mode of use, further objectives and advantages, will best be understood by reference to the following detailed description of an illustrative embodiment read in conjunction with the accompanying drawings.  
         [0033]    Referring now to FIG. 2, an optical-electronic array module  200  is shown. The large trapezoidal shaped object  240  is an optical coupler for coupling optical signals between optical fibers in a fiber-optic cable (not shown) and laser die  220  and photo detector die  225 . The fiber-optic cable secures to the end of the coupler  240  having pins  242 . The coupler  240  pins  242  are for aligning the coupler  240  with a connector (not shown) on the end of the fiber-optic cable. The other end of the coupler  240  has a tapered nose  244  which precisely fits a silicon submount  210 . The optical-electronic laser die  220  and photo detector die  225  are flip chip mounted onto the submount  210  on the outboard side of the submount  210 , that is, the side opposite the one that engages the optical coupler  240 . Flip chip mounting is characterized by the chip (that is, the die) being mounted with its I/O facing the carrier on which it is mounted, which for dies  220  and  225  is the submount  210 . Electrical conductors  360  provide electrical interconnection between laser die and detector die to bottom of submount  210  where pads (not shown) connect to a circuit board  270  that has signal conditioning circuitry (not shown), for connecting to a customer circuit board (not shown).  
         [0034]    The coupler  240  has two sheets of silicon (not shown), with V-shaped grooves (not shown) in at least one of the sheets, and optical fibers (not shown) in the grooves. The two sheets are laminated together so as to embed the fibers. The optical fibers inside the coupler  240  run in a single plane from the end with the pins  242  to the end fitted up to the submount  210 . The ends (not shown) of the coupler  240  are highly polished, so that light easily transmits through the ends and the fibers. The optical fibers are typically  50  microns in diameter, or a little larger.  
         [0035]    Referring now to FIG. 3A, an orthographic view of the bottom of the coupler  240  and submount  210  is shown. The embedded fibers  310  traverse the length of the coupler  240 , as previously stated. The silicon submount  210  is shown positioned on the nose  244  of the coupler  240 . (In an embodiment, this submount  210  is attached to the coupler  240  by a ultraviolet light cured epoxy.) An aperture  330  through the submount  210  is visible as hidden lines in the submount  210 . Since the coupler nose  244  is fitted into the aperture  330 , even though the laser die  220  and detector die  225  are mounted on the outboard side of the silicon submount  210 , there is nevertheless an optical path through the aperture  330  of the submount  210 , so that the optics of the laser die  220  and detector die  225  may be coupled to optical signals via the fibers  310  in the coupler  240  and the fiber optic cable, which is coupled to the coupler  240  at the pin  242  end.  
         [0036]    In one aspect, the nose  244  and submount  210  aperture  330  shapes serve an alignment purpose. Note that the aperture  330  of the submount  210  is tapered on all four sides, as is the nose  244  of the coupler  240 . The aperture  330  is formed in the submount  210  by etching. It is well known that silicon has a natural tendency to etch precisely at a certain well-controlled angle. The silicon submount  210  is therefore ground to a tapered shape at an angle matching that of the aperture  330 , so that the nose  244  of the coupler  240  and the submount  210  fit together precisely. With the coupler secured in the submount  210  aperture  330  there is a 50 to 75 micron gap between the nose  244  of the coupler and the inboard side of the laser  220  and detector  225  dies facing the nose  244  of the coupler  240 , which permits collecting divergent light yet provides a space. The bottom of the optical coupler  240  has a number of sets of pads  351  through  354  for mounting the coupler  240  on circuit board  270  (not shown). Likewise, the bottom of the submount  210  (which in the present embodiment is coincident with the bottom of the optical coupler  240 ) has two sets of pads  355  and  356  for attaching the submount  210  to the circuit board  270  as well. The submount  210  and dies  220  and  225  have other pads as well, which will be shown more clearly in enlarged views described herein below.  
         [0037]    Once the silicon submount  210  is attached to the coupler, the laser  220  and detector  225  dies are placed on the submount  210  by an precision alignment die placement machine (not shown) and flip chip bonded to the submount  210 . Applying flip chip mounting of the dies  220  and  225  to the submount  210 , which itself has been securely and precisely fit to the coupler  240 , further enables precise alignment of the optical fibers  310  in the coupler  240  to the optical outputs of the laser die  220  and optical inputs of the detector die  225 . As previously stated, for flip chip mounting, the chip (that is, the die) is mounted with its I/O, including optical and electronic I/O, facing the carrier (that is, submount  210  in this case). Thus the precision alignment die placement machine can align the optics of the dies  220  and  225  with the fibers  310  at the same time that it ensures the electronic inputs of the laser die  220  and electronic outputs of the detector die  225  are sufficiently aligned with the corresponding pads of the submount  210 . This increases precision of alignment of the optical fibers  310  in the coupler  240  to the optics of the dies  220  and  225 , because it allows compensation by the machinery for some misalignment between the submount  210  and coupler  240 . In one embodiment, the fibers  310  themselves are used by the machine for alignment, at least in part. Alternatively, the coupler  240  grooves are used as alignment fiducial for the machine vision system.  
         [0038]    Referring now to FIG. 3B, further details are shown of the flip chip mounting aspect of the embodiment in an enlarged view. (In the view of FIG. 3B only one of the dies  225  is visible, but it should be understood that similar details apply to both die  225  and die  220 .) In the embodiment of the present invention, the sides of the die  225  and the submount  210  that face each other have respective gold pads  357  and  358 , which are bonded together by heating. In FIG. 3B, the combination of the bonded submount pads  358  and die pads  375  are referred to as respective bonds  370 . In addition to the bonds  370  which connect conductors  360  on the submount  210  to electronic I/O of the die  225  (via pads  357  of the die  225 ), FIG. 3B also shows a bond  371  , including pad  372  on the die  225  and pad  373  on the submount  210 , and bond  374 , including pad  375  on the die  225  and pad  376  on the submount  210 , which are solely for mechanical attachment of the die  225  to the submount  210 . Two other such mechanical bonds exist between the die  225  and the submount  210 , but are not visible in this view.  
         [0039]    Eutectic gold is used instead of solder for bonding the respective pads of the submount  210  and dies  220  and  225  to one another, which includes pads  358  and  357 , pads  372  and  373 , pads  374  and  375 , etc. Eutectic gold is more stable than solder, that is, does not shift as much. Also, eutectic gold wets at a higher temperature, provides a tighter and more precise fit, and uses less material. These factors further contribute to more precise aligning of the optical fibers  310  to the optics of the dies  220  and  225 .  
         [0040]    The submount  210  also has pads  359  on its bottom side, which is the side adjacent to the outboard side, and has conductors  360  that interconnect respective pads  358  and  359  on these two adjacent sides of the submount  210 . These conductors  360  will be further explained herein below.  
         [0041]    Referring now to FIG. 3C, the side of the detector die  225  that faces the submount  210  is shown. In the embodiment, the detector die  225  has four detectors, and therefore four optical inputs  381  are visible in this view. (Herein optical inputs or outputs may be referred to as “optics.”) Also shown in this view are the pads  357  for electrical outputs from the photo detectors of the detector die  225 , and four pads  372 ,  375 ,  377  and  379  for mechanical attachment to the submount  210 .  
         [0042]    Referring now to FIG. 3D, portions of the sides of the submount  210  and coupler  240  that face the die  225  are shown. A polished end of the coupler  240  is visible in this view. Four of the fibers  310  may be seen through the end. Also visible in this view are the pads  359  and  358  on the bottom and side, respectively, of the submount  210 , as well as the conductors  360  that interconnect respective ones of the pads  359  and  358 . The pads  358  on the side of the submount  210  face the die  225  and connect to the pads  357  (FIG. 3B) for the electrical outputs of the detector die  225  by means of the eutectic gold bonding previously described. Also shown are four pads  373 ,  376 ,  378  and  380  for mechanical attachment to the die  225  pads  372 ,  375 ,  377  and  379 .  
         [0043]    Referring once again to FIG. 3A, a section line  4 - 4  is shown indicating orientation of a view in FIG. 4. In the view of FIG. 4 a cross-section of the submount  210 , die  220  and a portion of the coupler  240  are shown along with a portion of the circuit board  270 , according to an embodiment, to illustrate certain aspects of the invention in greater detail.  
         [0044]    Visible in the view of FIG. 4 is another one of the bonds  374  between the submount  210  and the die  225  that is solely for mechanical and not electrical purposes. In the embodiment, the detector die  225  and four mechanical bonds, bonds  371  and  374  of which are visible in this view, create a sort of four legged table, so that the mechanical bonds secure the die  225  in a precise position with respect to the submount  210 .  
         [0045]    The bottom of the coupler  240  is connected to the circuit board  270  by C4 solder balls, which together with pads  358  are components of mechanical bonds  430 . The submount  210  is also connected to the circuit board  270  by C4 solder balls. Also visible in FIG. 4 are bonds  420  between the bottom of the submount  210  and circuit board  270 . These bonds  420  include the C4 solder balls and pads  359  on the submount  210  that were previously shown in detail and described in connection with FIG. 3B. The bonds  420  are electrically connected to conductors  450  on the circuit board  270  which connect to signal conditioning circuitry (not shown) on the circuit board  270 , which in turn connect to circuitry on a customer&#39;s circuit board (not shown). In this manner electronic devices on the customer&#39;s board are electrically coupled, via signal conditioning circuitry, to the electronic I/O of the detector die  225 . In similar fashion the electronic I/O of the detector die  225  are also electrically coupled to electronic devices on the customer&#39;s board.  
         [0046]    [0046]FIG. 5 illustrates how shadow masking is used to etch conductors onto the submount  210 , including around a corner  520  of the submount  210 . (The term “submount” that is used herein encompasses a carrier which provides a 90 degree bend, or at least an acute angle bend, for an electrical connection.) A photosensitive material  570  is deposited on the submount  210 . Then a mask  560  is held nearby the outboard side  510  and the bottom  530  of the submount  210 , at an angle somewhere between parallel to the bottom  530  and parallel to the side  510  such that a light  550  shined on the side of the mask  560  opposite that of the submount  210  casts a shadow  540  on the submount  210  which delineates areas on the bottom  530 , corner  520  and outboard side  510  of the submount, including at least a portion of the pads  359  on the bottom  530  and pads  358  on the outboard side  510 , where the photosensitive material  570  is to be etched. Then the light  550  is extinguished, the mask  560  is removed, and the exposed areas are etched away and metallized, such as by sputtering, thereby creating conductors  360  (FIG. 3B) on the bottom  530 , around the corner  520  and on the outboard side  510  of the submount  210 , where the conductors are in electrical contact with respective ones of the pads  359  on the bottom  530  and pads  358  on the outboard side  510 .  
         [0047]    Having achieved the metalizing on the bottom  530 , around the corner  520  and on the side  510  of the submount  210 , including the pads  358  and  359 , and having electrically bonded the pads  358  on the side  510  of the submount  210  to the die  220  (FIG. 3B), the pads  359  on the bottom  530  of the submount  210  are connected to circuitry on the circuit board  270  (FIG. 4), thereby securing the submount  210  and electrically connecting circuitry on the circuit board  270  to the die  220 . Moreover, this is done without a flexible cable.  
         [0048]    The description of the present embodiment has been presented for purposes of illustration, but is not intended to be exhaustive or to limit the invention to the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. For example, in the embodiment described above the optical-electronic array assembly  300  has 4 lasers and 4 detectors. In alternative configurations, there are 12 lasers and 12 detectors in an array. This configuration uses a single 12 fiber cable for the lasers, and another 12 fiber cable for the detectors. In another configuration, there are four lasers and for detectors in an array. This configuration uses a single 12 fiber cable, with four of the fibers near one edge of the cable dedicated to the lasers, for of the fibers near the other edge of the cable dedicated to the photo detectors. The four fibers in between are not used. A small form factor array has just one laser and one detector. Numerous other alternative embodiments exist.  
         [0049]    Other means exist for providing the conductors between the pads of the inboard and bottom sides of the submount, other than that described in connection with FIG. 5. According to one alternative, gold conductors are plated instead of being deposited by sputtering. According to another alternative, the submount has layers and vias for internal conductors between the pads of the inboard and bottom sides of the submount.  
         [0050]    To reiterate, the embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, and to enable others of ordinary skill in the art to understand the invention. Various other embodiments having various modifications may be suited to a particular use contemplated, but may be within the scope of the present invention.