Abstract:
A method of integrating an optoelectronic device, for example a vertical cavity surface emitting laser, onto the electronic substrate of a parallel optical transceiver package by positioning and maintaining the exact relative alignment of the optoelectronic device relative to the electronic substrate for application of adhesive and curing. The method includes the utilization of a multi-piece fixture which clamps the elements into position and maintains position throughout the curing process. Alternatively, the fixture can comprise a unitary assembly for clamping the elements into position and maintaining their position throughout the curing process.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method for the integration of an optoelectronic device, with a parallel optical transceiver package, and more particularly to integrating a vertical cavity surface emitting laser with the electronic interface substrate of the parallel optical transceiver package. The invention further relates to an assembly tool for the above-described method utilizing a fixture which aligns the VCSEL with the electronic interface substrate and maintains the relative position of the components for adhesive application and curing. 
   BACKGROUND OF THE INVENTION 
   There is a need for a high-speed cost effective optical transmitters which can operate as parallel communications data links. The primary function of the optical transmitter is to translate electrical signals into optical signals which are carried over a fiber optic path. The optical transmitter includes an optoelectronic device, such as a vertical cavity surface emitting laser (VCSEL). 
   Connection of the optoelectronic device with the electronic interface substrate of the package is complicated due to geometric constraints. The VCSEL emits light in a generally perpendicular direction to the plane of the optical fibers and substrate, therefore making stacking of such components difficult. To solve the packaging problem, the VCSEL is either mounted parallel to the substrate and the output photons directed 90° through mirrors or the VCSEL is mounted perpendicular to the substrate and the electric interface connectors are rotated 90°. The optical bending solution is less than optimal due to the difficult optical design and mirror alignment required. Conversely, the bending of electrical conductors is well known in the art through the implementation of flexible circuits. Therefore, flexible electrical circuits capable of achieving the necessary 90° bend are generally the accepted solution. 
   There is a need then for a manufacturing method which allows for efficient positioning of the flexible circuit on the electronic interface substrate and maintaining that position during the curing of the adhesive. The problem of alignment and bending of the flexible circuit are exacerbated as data rates of optoelectronic devices increase. Closer connections must be established in order to retain electrical performance levels. The placement and bending of the flexible surface on the substrate is typically performed manually by a skilled technician just prior to application of a bonding adhesive. Unfortunately, the existing techniques employed in connection with this process are time consuming, expensive and prone to failure due to misalignment. If the placement of the flexible circuit fails to align with the substrate connectors, the entire component may need to be scrapped. While manual bending and aligning techniques exist for mounting an optoelectronic device to the substrate, it would be desirable to improve the efficiency and reduce the cost of coupling. 
   SUMMARY OF THE INVENTION 
   The present invention is a method and integration tool for the alignment and placement of an optoelectronic device onto the electronic interface substrate of a parallel optical transceiver package. The tool is comprised of a clamp assembly and a VCSEL spring. The clamp assembly positions the VCSEL with the flexible circuit disposed on the electronic interface substrate. Once positioned, the VCSEL is held in place by means of a spring pushing against the blind side of the VCSEL. The clamp assembly is preferably constructed of a metal such as aluminum to facilitate the curing process, which may require elevated temperatures. 
   The present invention provides a cost efficient method for aligning and connecting an optoelectronic device to the electronic interface circuitry of the package. It is essential that the exact alignment of the VCSEL position be maintained throughout the assembly process. The present invention enables repeatable and consistent placement of the flexible circuit onto the electronic substrate. The integration method maintains the integrity of the VCSEL flex circuit connection while reducing the time intensive manual component of assembly. Furthermore, the mechanical aspect of the process provides a reliable means of duplicating successful placement, thus increasing the output of properly aligned assemblies. 
   In a preferred embodiment, the optoelectronic devices are VCSEL arrays to which a flexible circuit is attached. The flexible circuit contains electrical traces on one side which provide current pathways to the VCSEL from the integrated circuits of the package. The flexible circuit extends from the VCSEL and is attached to a spacer block so that the electrical traces may be attached to the parallel optical transceiver package. 
   In operation, the VCSEL is placed within a recess in the spring clamp frame of the clamp assembly with the flexible circuit extending toward the electronic substrate. The clamp base and spring clamp frame are then loosely fastened over the parallel optical transceiver package. Relative position is maintained by a support element of the clamp base which is inserted into the optical connector port of the parallel optical transceiver package. Lateral motion is further restricted by a projecting flange of the clamp base which caps the spring clamp frame. The completed fixture, when properly positioned, is fixed by a threaded fastener. The VCSEL is held in place by a flat spring pinning the VCSEL to a support element of the clamp base. Adhesive is applied and the entire system allowed to cure. 
   In an alternative embodiment, the clamp assembly comprises a unitary assembly in which the clamp base and spring clamp frame are hingedly connected to rotatably interface with the VCSEL and the parallel optical transceiver package. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a perspective view of a laser package integration tool with a parallel optical package in place. 
       FIG. 2  is an isometric view of the clamp base block of the present invention 
       FIG. 3  is a side view of the clamp base block of the present invention. 
       FIG. 4  is an isometric view of the spring clamp block of the present invention. 
       FIG. 5  is a sectional side view of the spring clamp block of the present invention. 
       FIG. 6  is a side view of the spring clamp block of the present invention. 
       FIG. 7  is a perspective view of an embodiment of a laser package integration tool with a parallel optical package in place. 
       FIG. 8  is a perspective view of an embodiment of a laser package integration tool with a parallel optical package in place. 
       FIG. 9  is a perspective view of an embodiment of a laser package integration tool with a parallel optical package in place. 
       FIG. 10  is a side view of an embodiment of a clamp base of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as to not to unnecessarily obscure aspects of the present invention. 
   The present invention is a tool and method for the integration of an optoelectronic device to a parallel optical transceiver package. Fiber optic transmitter and receiver electrical elements are implemented on two separate substantially parallel boards. The boards are disposed substantially perpendicular to the base of the optoelectronic device. A flexible circuit is bent 90° in order to join the optoelectronic device to the electrical circuitry of the parallel optical package. 
   In the preferred embodiment, the present invention is used for the integration of a vertical cavity surface emitting laser (VCSEL) within a parallel optical transceiver package. In this geometric configuration, light emitted from the surface of the VCSEL laser is oriented nominally along a plane parallel to the substrate. This is the preferred direction for the optical portion of the package because the optical cable can then extend parallel to the substrate thus allowing multiple packages to be stacked. 
   A flexible circuit, bent at a substantially right angle, is used to electrically connect pads on the substrate to pads on the optoelectronic dies, which are oriented perpendicular to the substrate. The flexible circuit has leads defined in one single layer protected by a sheet of insulating material. At a first end of the flexible circuit, bonds are used for the connection to the optoelectronic dies. At the opposing end an array of large pads provides landing sites. After the flexible circuit is secured in its aligned position in the cavity of the substrate, each of its leads are electrically connected to corresponding pads on the substrate by a series of wire bonds. 
   Accurately aligning the bonding sites on the substrate to the flexible circuit is a challenging step. Too great an offset between bonding sites can effect wire bond yields and process time and create a high inductance electrical subsystem, due to the longer wires and higher wire loops required to accommodate a large lateral offset. It should be noted that placement errors can impact the amount of noise introduced into the system, possibly making the system not functional. However, these placement errors can be kept to a minimum through the careful design and assembly of component parts using the present invention. 
   An optoelectronic device integration tool  10 , in accordance with the present invention, is constructed as shown in  FIGS. 1–6 .  FIG. 1  is an exploded perspective view of the optoelectronic device integration tool  10 , which provides for alignment of the optoelectronic device, and a parallel optical transceiver package  12 . In a first embodiment, the optoelectronic device  10  is comprised of a vertical cavity surface emitting laser (VCSEL)  14 , the active face of which is mounted on a flexible circuit  16 . 
   As illustrated by  FIG. 1 , a single optoelectronic device integration tool  10  integrates a single VCSEL  14  with the parallel optical transceiver package  12 . It is possible for a pair of tools  10  to be used simultaneously. The present invention requires that integration of VCSEL  14  occurs prior to the alignment and connection of the fiber optic assembly  18 . Note that  FIG. 1  includes an illustration of a completed system in the first port with an integrated VCSEL  14  and connected fiber optic assembly  18  and the second port contains a VCSEL  14  in the process of integration by tool  10 . 
   The optoelectronic device integration tool  10  is substantially a clamp comprised of two separate blocks, which are preferably machined from aluminum or another metal suitable for said bonding and alignment. The optoelectronic device integration tool is designed to position and immobilize the VCSEL  14 , with flexible circuit  16 , and electrical spacer block  13  relative to the parallel optical transceiver package  12 . The frame for the spring clamping element  28  is the clamp base  22  and the spring clamp frame  25 . The clamp base  22  and spring clamp frame  25  have the same uniform width. The optoelectronic device integration tool  10  straddles the parallel optical transceiver package  12  so as to position spring clamp  28  for contact with VCSEL  14 . 
   The clamp base  22 , as illustrated in  FIG. 2 , includes on the proximal side  23  a package engaging face  30 , a spring block mating face  32 , and a spring block flange  34 . Opposing distal face  24  is a vertical wall with a single opening for insertion of fastener screw  40  into fastener screw hole  42 . The rectangular foot  27  of clamp base  22  sits outside parallel optical transceiver package  12  during operation. 
   On proximal face  23 , package engaging face  30  mates with the exterior face of package frame  17 . Protruding from package engaging face  30  is VCSEL support  36 , which is a rectangular block projection, sized for insertion into the MT connector port  15  of the parallel optical transceiver package  12 . It is envisioned that VCSEL support  36  is dimensioned to replicate the connector portion of fiber optic assembly  18 . The VCSEL support  36  extends proximally toward spring clamp frame  25  and stabilized clamp base  22  due to the close dimensioning of VCSEL support  36  and MT connector port  15 . Additional stability is provided by package cap  38 , that projects over package frame  17  perpendicular to package engaging face  30 . 
   As illustrated in  FIG. 3 , spring block mating face  32  extends vertically from the distal end of package flange  38 . Fastener screw hole  42  is set within face  32 . Spring block mating face  32  abuts the complimentary face on spring clamp frame  25  when fastener  40  joins the blocks. Spring block flange  34  extends perpendicular to spring block-mating face  32  to form a cap over spring clamp frame  25 . Flange face  44  rests on spring clamp frame  25  to prevent rotational movement and for alignment on the electronic interface substrate  26 . 
   As illustrated in  FIGS. 4–6 , spring clamp frame  25  includes three operative faces; distally disposed spring clip face  50 , spring block base  52 , and spring block mating face  54 . Spring clamp frame  25  has generally a horseshoe shape with bottom center open for access to the flexible circuit  16  and electronic substrate  26 . Spring clip face  50  includes a pair of horizontal clip holes  51   a ,  51   b  at the upper end for attaching flat spring  60 . Flat spring channel  56 , which is inclined proximally toward spring block mating face  54 , is centered between clip holes  51   a  and  51   b  and extends toward electronic substrate  26 . Threaded clamp fastener hole  58  extends horizontally from spring block mating face  54  through the lower portion of flat spring channel  56 . 
   As illustrated in  FIG. 1 , flat spring  60  is generally “T” shaped with fastener holes  62   a  and  62   b  aligned horizontally at the top. Threaded fasteners  64   a ,  64   b  extend through holes  62   a ,  62   b  into horizontal clip holes  51   a ,  51   b . The lower end of flat spring  60  contains a proximally projecting curl  66  for contact with the blind face of VCSEL  14 . The width of spring curl  64  is slightly less than the width of spring channel  56 . The slope of channel  56 , combined with the radius of projecting curl  66 , provide the required force to maintain VCSEL  16  position.  FIG. 6  is a sectional view of  FIG. 4  taken along line  6 — 6 . As illustrated in  FIG. 6 , in a first embodiment, the slope of channel  56  is 8° from vertical. 
   Spring block base  52  includes two “L” shaped legs  68   a ,  68   b  disposed below spring block mating face  54  and extending distally. The footprint of the present invention is intentionally minimized so as not to interfere with components of the electronic substrate  26 . Moreover, spring block base  52  must provide access for attaching the flexible circuit  16 . 
     FIG. 5  is a sectional view from  FIG. 4  taken along line  5 — 5 . As illustrated in  FIG. 5 , spring block mating face  54  is comprised of an upper vertical surface  70  and lower vertical surface  72 . Upper vertical surface  70  includes clamp fastener hole  58  and spring holes  51   a  and  51   b . Lower vertical surface  72  is set back from upper vertical surface  70  to allow for placement of the package frame  17  and a recess for VCSEL  14 . 
   In operation, the optoelectronic device integration tool  10  is applied as follows. Parallel optical transceiver package  12  is disposed on a work surface (not shown). The VCSEL  14  is inserted onto lower vertical surface  72  of spring clamp frame  25 . Clamp base  22  and spring clamp frame  25  are then loosely mated by inserting VCSEL support  36  through MT connector port  15  while maintaining the position of flexible circuit  16  on substrate  26 . Fastener screw  40  is then inserted through screw hole  42  of clamp base  22  and threadably engages clamp fastener hole  58  of spring clamp frame  25 . VCSEL  14  is now clamped between the projecting curl  66  of flat spring  60  and VCSEL support  36 . Adhesive is applied to the interface between electronic substrate  26  and flexible circuit  16 . In the alternative, adhesive may be applied as the first step. Note that at this point the VCSEL  14  is not attached to MT connector  18 . 
   As illustrated in  FIGS. 7 ,  8  and  9 , an alternative embodiment of a single optoelectronic device integration tool  80  can take the form of a unitary assembly. As depicted, tool  80  comprises a clamp base  82  and a spring clamp frame  85 . Clamp base  82  and spring clamp frame  85  are preferably machined from aluminum or another metal suitable for use during bonding and alignment of the fiber optic assembly  18 . Clamp base  82  preferably includes many of the features previously described and illustrated in  FIGS. 2 and 3  with reference to clamp base  22  with the additional inclusion of a pair of recessed surfaces  84   a ,  84   b  defining a mounting wall  87 . Mounting wall  87  includes a throughbore  86  as shown in  FIG. 10  connecting recessed surfaces  84   a ,  84   b . Spring clamp frame  85  preferably includes many of the features previously described and illustrated in  FIGS. 4 ,  5 , and  6  with reference to spring clamp frame  25  with the additional inclusion of a pair of projecting arms  90   a ,  90   b , each arm including a bore  92 . Preferably, spring clamp frame  85  has a width equal to clamp base  82  while projecting arms  90   a ,  90   b  are spaced apart such that mounting wall  87  is accommodated within projecting arms  90   a ,  90   b  with the bores  92  in alignment with throughbore  86 . A hinge pin  94  can then be inserted through bores  92  and throughbore  86  allowing for rotational interaction between clamp base  82  and spring clamp frame  85  around hinge pin  94 . In an alternative embodiment of the single optoelectronic device integration tool  80 , throughbore  86  can be replaced with a pair of partial bores, one on each side of mounting wall  87 , while hinge pin  94  can be replaced with a pair of hinge pins. 
   In operation, the single optoelectronic device integration tool  80  performs a similar function as previously described with respect to the single optoelectronic device integration tool  10 . In general, the parallel optical transceiver package  12  is disposed on a work surface (not shown). The VCSEL  14  is inserted into the lower vertical surface  72  of spring clamp frame  84 . VCSEL support  36  on clamp base  82  is inserted into MT connector port  15  while maintaining the position of flexible circuit  16  on substrate  26 . Spring clamp frame  85  is rotated downwardly around hinge pin  94  such that projecting curl  66  of flat spring  60  clamps VCSEL  14  against VCSEL support  36 . Fastener screw  40  is then inserted through screw hole  42  of clamp base  82  and threadably engages clamp fastener hole  58  of spring clamp frame  85 . Adhesive is then applied to the interface between electronic substrate  26  and flexible circuit  16 . In the alternative, adhesive may be applied as the first step. 
   It is to be understood that the embodiments described herein are only illustrative and modifications of the various dimensions and materials can be made still within the spirit and scope of this invention.