Patent Publication Number: US-6709607-B2

Title: Flexible optic connector assembly

Description:
This application is divisional application of copending application Ser. No. 09/268,191, filed on Mar. 15, 1999 U.S. Pat No. 6,404,960, which is a continuation-in-part of application Ser. No. 08/775,330, filed on Dec. 31, 1996 now abandoned. 
    
    
     The U.S. Government may have rights in the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention pertains to optical connection and particularly to optical connectors making use of flexible connectors. More specifically, this invention pertains to the use of self-aligned, flexible optical connectors for coupling light between an optoelectronic device and an optical fiber, waveguide, or arrays of devices, fibers, and/or waveguides and a method of manufacture thereof. 
     A common problem is to simultaneously provide electrical connection to an optoelectronic device as well as optical coupling from the device to an optical fiber in a connected package. Previous methods of such coupling have included placing the device in a first level package, such as a TO header in an optical connector receptacle, rather than placing the device directly on the circuit board providing an electrical connection from the circuit board to the TO header by means of an electrical flex circuit. This approach involves costly and time consuming methods of manufacturing. 
     U.S. Pat. No. 5,375,184 (&#39;184 patent) by inventor Charles T. Sullivan and issued Dec. 20, 1994, hereby incorporated by reference in this specification, appears to disclose 1) self-aligning mechanical approach for lateral waveguide to lateral waveguide alignment; 2) the use of visual alignment marks for lateral waveguide to lateral waveguide alignment or the alignment of a vertical port to a waveguide with a 45 degree facet; and 3) a coupling approach having alignment, but not for vertically coupled optical ports such as photodetectors, vertical cavity surface emitting lasers (VCSELs), or surface emitting light emitting diodes (LEDs). The alignment marks of the &#39;184 patent for use with a vertically coupled optical port still require manual alignment which is not self-aligning. This patent also does not deal with the approach for coupling between a flexible optical waveguide and an optical fiber waveguide within the connector receptacle. 
     SUMMARY OF THE INVENTION 
     The present invention has self-aligning features for waveguide self-alignment to a vertically coupled optical port such as a photodetector, VCSEL, or surface emitting LED. The invention also includes self-aligning or passively aligned structures for connector receptacles and backplanes. While the optical devices or optoelectronic integrated circuits are still at the wafer level, the wafer is coated and photolithographically patterned to provide the self-aligning features. Specifically, a pillar of dielectric or polymer material on the photodetector or laser wafer is fabricated in a fashion such that it fits or snaps into a recess of the waveguide thus providing a passive alignment with an accuracy of better than 5 micrometers (μm). The added feature of this is the holding of the waveguide in place after the aligning fixtures are removed. 
     The features of the present invention include a self-aligning approach for coupling a waveguide or a plurality of waveguides to a vertically coupled device or devices, which can then be mounted directly onto a circuit board with other chips, without a first level package, and for connecting the other ends of the waveguides or connectors into optical connector receptacles of a module or a backplane. 
     The features of this approach include an optical waveguide or array of waveguides, that optically connect an optical device or devices at one end to a connector/connector receptacle at the other end. Self-aligning mechanical features provide 1 to 5 μm alignment tolerances at the optical chip, while mechanical or visual alignment marks allow accurate placement of the waveguide in the connector receptacle at the other end, allowing automatic alignment to take place with machine vision. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1 a  and  1   b  illustrate a self-alignment mechanism between a flexible coupler and a circuit board having a vertical light port. 
     FIGS. 2 a  and  2   b  show a plug and receptacle connection of flexible couplers. 
     FIGS. 3 a  and  3   b  illustrate alignment grooves and ridges of a flexible-coupler-to-board connection. 
     FIG. 4 shows an approach using visual alignment marks for positioning a flexible waveguide in an optical fiber connector receptacle. 
     FIGS. 5 a  and  5   b  reveal the structure of the flexible waveguide having two forty-five degree facet ends. 
     FIGS. 6 a ,  6   b ,  6   c ,  6   d ,  6   e  and  6   f  illustrate various connections of two flexible light waveguides/fibers. 
     FIG. 7 illustrates an alternative approach for aligning the waveguide structure at the connector receptacle end. 
     FIGS. 8 a ,  8   b ,  8   c ,  9   a ,  9   b ,  9   c ,  10   a ,  10   b ,  10   c ,  11   a ,  11   b ,  11   c ,  12   a ,  12   b ,  12   c ,  13   a ,  13   b ,  13   c ,  14   a ,  14   b ,  14   c ,  15   a ,  15   b ,  15   c ,  16   a ,  16   b ,  16   c ,  17   a ,  17   b  and  17   c  illustrate a process for fabricating a passively aligned flexible optical circuit. 
     FIGS. 18 a - 18   e  reveal process steps for fabricating a flexible coupler having self-alignment keys. 
     FIG. 19 shows the alignment of a flexible coupler secure with rigid keys. 
    
    
     DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
     A polymer waveguide structure  12  (FIG. 1 a ) is used to guide light  13  to/from optical chip  14  through flexible optical circuit  12  and connector receptacle  15  and into fiber  16  which is centered with respect to connector receptacle  15  by ferrule  30 . Flex circuit  12  containing waveguide  20  can be constructed from a variety of materials. One example is an ULTEM defined waveguide circuit  12  with BCB cladding polymer layers  31 . A two dimensional sheet of waveguide flex circuit structures can be fabricated as will be described below, and then the individual flex circuits  12  may be separated by sawing or cutting. In the embodiment shown in FIG. 1 a , a perpendicular cut  21  at one end is used where waveguide  12  couples to fiber  16  via connector receptacle  15  and ferrule  30 . At the other end of the optical flex circuit  12 , a 45-degree cut  17  is made to form a turning mirror  18  to direct light out of laser  19  and into waveguide  20 , or out of waveguide  20  and into a photodetector  22 . 
     The present invention adapts the self-aligning waveguide technology to enable an optical chip  14  on-board packaging approach for modules and backplanes. An optical chip  14  containing a light source  19  or detector  22 , can be placed directly on a board  58 , with the light coupled in or out of the port  19 ,  22 , through waveguide  20 , as noted in FIG. 1 b . Although not shown in the figure, this board  58  would also contain laser driver or receiver amplification I.C.s as discrete components. The use of a metalized 45-degree reflector  18  (in FIG. 1 a ) then allows device  12  to be covered with glob top  38  (a covering of protective dark epoxy) without affecting the optical properties. The other end of waveguide  20  is coupled into a connector receptacle  15  which accepts a ferrule  30  containing a fiber  16  which fits with precision inside receptacle  15 , and, as a result, is precisely aligned to waveguide  20 . There is a connector receptacle  15  holding onto both device  12  and fiber ferrule  30 . The same approach described here can be used to couple two waveguides together at a board to backplane interface, for instance, as shown in FIG. 2 b.    
     FIG. 2 a  shows the parallel waveguide flexible connector/interface  12  used to provide a rugged interface between a package or multichip module containing an optoelectronic die, and a connectorized optical fiber  16  which provides an interconnect path to the next module, board, or cabinet, via connector receptacle  23 , connector  24  and ferrule  30 . 
     In order to passively align optical flex circuit  12  to an optical chip  14 , locking structures  27  and  28  are defined in both optical flex circuit  12  and on top of optical chip  14 , respectively. FIGS. 1 a  and  1   b  show examples of waveguide-to-chip passive mechanical alignments. Grooves  27  are etched into optical flex circuit  12  as illustrated in FIGS. 3 a  and  3   b . A matching alignment ridge  28  is formed on the surface near laser  19  or photodetector  22  on chip  14 , by depositing and patterning an additional thick polymer layer on the surface of the chip. The polymer optical flex circuit contains both waveguide  20  for guiding the optical signal, and alignment wells  27 . End  17  of optical flex circuit  12  which is coupled to the optical chip  14  is cut or sawed at a 45-degree angle  17 . Alignment wells  27  in the optical flex circuit  12  are then snapped into place on top of ridges  28  on the laser  19  or photodetector  22  chip. This can be designed as two tracks so that the optical flex circuit  12  can be slid over chip  14  until turning mirror  18  is properly placed over optical device  19  such as a VCSEL. Ridges  28  and slots  27  provide alignment in lateral directions  34  and  35 . The top surface of chip  14  and the bottom surface of flex circuit  12  provide alignment in vertical direction  36 . 
     Alignment ridges  28  can be photolithographically defined at the wafer level during fabrication of lasers  19  and photodetectors/receivers  22 , so that the fabrication can be very cost effective. Optical flex circuit  12  is multiply fabricated in large sheets, and then sawed into individual circuits  12  (with both 90-degree  21  and 45-degree  17  facets for mirrors  18 ), respectively, much as one saws up a wafer in the art. The fabrication of optical flex circuit  12  uses integrated circuit (IC) type manufacturing processes. The surface of 45-degree turning mirror  18  on optical flex circuit  12  is coated with gold or gold-titanium so that the reflectivity of turning mirror  18  is insensitive to the refractive index of the medium placed in contact with this mirror. 
     Similar self-aligning structures  27  are fabricated in optical flex circuit  12  to match alignment structures  28  in connector receptacle  15 , so that flex circuit  12  is passively aligned for coupling from waveguides  20  to fibers  16  or other waveguides  20  in flex circuits  12  plugged into connector receptacle  15 ,  24  or  25 . This is illustrated in FIGS. 1 a ,  1   b ,  2   a  and  2   b.    
     The assembly of the optical components onto a printed circuit board  37  includes optical chips  14  being mounted onto board  37  using standard pick and place and surface mount techniques (FIG. 2 b ). For example, in current manufacturing processes, board  37  is stenciled with epoxy, chips  14  are placed on board  37 , and an anneal step fastens the chips to board  37 . The sheet of optical flex circuits  12  is cut into individual pieces, in one embodiment, each with a 90-degree facet  21  and a 45-degree facet  17  or with two 45-degree facets  17 . Part of the 45-degree facets  17  are all metalized into mirrors  18  in a single batch metallization step. Optical flex circuit  12  can then be snapped onto the optical chips  14  and slid into place so 45-degree facet  17  with mirror  18  is properly located over the top of vertical port optical devices  19  and  22  on chip  14 . The other end  21  of the optical flex circuit  12  having slots  27  may be slotted or threaded for sliding into connector receptacle  15  having ribs  28 . This provides alignment to a fiber in a mating ferrule  30 , or to another waveguide in a mating ferrule  25  in FIG. 2 b.    
     Electrical wire bonding is performed to the optical chips  14  as well as the other I.C.s on board  37 , and then an encapsulation step can be carried out such as depositing a “glob top” or dark epoxy covering  38  (in FIG. 2 b ) over all the chips  14 . Since waveguide  12  turning mirror  18  is metalized, the use of glob top  38  provides mechanical and moisture protection for chips  14 , without affecting the coupling efficiency from optical devices  19  and  22  to optical flex circuit  12  (illustrated in more detail in FIGS. 1 a ,  1   b ,  3   a  and  3   b ). 
     Features of the self-aligning interface are simple fabrication, no critical alignment steps required, no special training needed for optical component assembly because of self-aligning parts, low optical waveguide connection loss (i.e., 0.6 dB), permanent or temporary integration with an appropriate adhesive, and versatility wherein the self-aligning interface can be used to connect polymer waveguides  12  or optical fibers  16  to optoelectronic devices  19  and  22  packaged within multichip modules, hybrids, conventional packages or chip-on-board technology. 
     This description builds on the above-noted items to implement a chip-on-board connector. The invention includes a method for packaging either a serial or parallel optoelectronic module with optical chips  14 , i.e., having laser  19  and photodetector/receiver  22  devices, mounted directly on printed circuit board  37 . The method is for coupling light out of the laser  19 , through a connector receptacle  24  and plug  23  and into fiber  16 , as well as from fiber  16  into the photodetector/receiver  22 . 
     This invention extends an adaptation for passive alignment to optical chips  14 . This invention allows the optical chips  14  to be placed on the printed circuit board  37  rather than in first level packages such as TO headers. This approach provides both cost and speed advantages. Speed advantages occur because one can eliminate the leads from the package, such as those one typically finds in TO headers. These leads give rise to resonances which limit the ultimate package speed. This limit is around one gigahertz (GHz) for a TO 5.6 millimeter (mm) package. The flexible waveguide self-aligning interconnection also provides cost advantages by eliminating the first level package, allowing the optical chips  14  to be attached to board  37  at the same time as all the other I.C.s, and eliminating the need for an active optical alignment and soldering of a TO header with respect to the connector receptacle  24 . 
     FIG. 4 shows another embodiment for the connector which provides a sleeve type connection between flex circuit  12  and fiber  16 . Fiber  16  is inserted with a hole  39  which may be slightly tapered with the larger diameter at the end where the fiber is inserted so as to ease the insertion of fiber  16  into hole  39 . As fiber  16  is inserted the tapering of hole  39  this causes fiber  16  to enter into a tight fit in hole  39  as it approaches the other end of ferrule  30 . An epoxy may be applied at the end where fiber  16  enters hole  39  on ferrule  30  to secure fiber  16  to ferrule  30 . Ferrule  30  is fit into sleeve or connector receptacle  24  to where the ends of fiber  16  and ferrule  30  are at the other end of sleeve or connector receptacle  24 . The end with 45-degree mirror  18  of flex circuit  12  is attached to a piece  40  that fits on the end of sleeve  24  like a cap. The metalized 45-degree facet mirror  18  is centered on the receptacle  24  center so it is aligned with the end of fiber  16  along axis or alignment line  41 . Alignment grooves or slots  27  are not used at that end. Alignment marks  42  allow for manual or machine vision alignment of mirror  18  with the center of receptacle  24  and thus to the end of fiber  16 . 
     FIGS. 5 a  and  5   b  show plane and lateral views of flex circuit  12  for the embodiment having 45-degree facets for mirror  18  on both ends. An ULTEM waveguide is enclosed by BCB clads  31 . At both ends are visual alignment marks  42  used for aligning mirrors  18  to other optical devices  19  or  22 , waveguides  20 , mirrors  18  or fibers  16 . 
     In FIGS. 6 a  through  6   f , connections of various combinations of flexible waveguides and/or optical fibers, having optical or mechanical alignment, are illustrated. In FIGS. 6 a ,  6   e  and  6   f , a flex waveguide circuit  12  may be attached to another flex waveguide circuit  12  via receptacle  25  and sleeve  45 . In FIG. 6 a , alignment slots or keys  47  are at the ends of plug  23  and receptacle  25 . Plug  23  and receptacle  25  fit into sleeve  45  and are self-aligned with mechanical slots and keys  47 , respectively. 
     FIG. 6 b  shows a waveguide  12  in plug  20  which is optically connected to an optical fiber  16  in receptacle  25 . Waveguide  12  and fiber  16  are brought into optical alignment when plug  23  and receptacle  25  are inserted into sleeve  24  and to be proximate to each other. 
     FIG. 6 c  shows flex circuit  12  having waveguide  20  aligned mechanically to plate  40  via depressions  59  on circuit  20  which are mechanically aligned to pillars on plate  40 . Edge  61  of plate  40  is mechanically aligned with the inside diameter sleeve  45 . Plate  40  fits on sleeve  24  and ferrule  30  having fiber  16 , is slipped into sleeve  24  and the end of fiber  16  is optically aligned with mirror  18  which reflects light to or from waveguide  20  in flex circuit  12 . FIG. 6 d  shows flex circuit  12  having a lens  60 . The depressions of circuit  12  are mechanically aligned with pillars on lens  60 . Lens  60  is mechanically aligned and fitted to the inside diameter of sleeve  24 . Ferrule  30  with fiber  16  is likewise mechanically aligned and fitted to the inside diameter of sleeve  24 , thus resulting in the optical alignment of fiber  16  with waveguide  20  of the flex circuit  12 . 
     FIG. 6 e  shows flex circuit  12  aligned with and attached to plate  40  like that in FIG. 6 c . This flex circuit  12  is optically connected to another flex circuit  12  which is similarly aligned with and attached to a plate  40 . The plates are attached to the respective ends of sleeve  45  whose inside diameter is mechanically aligned to edges  61  of plates  40 , respectively, resulting in optical alignment of mirrors  18  of waveguides  20  of flex circuits  12 . FIG. 6 f  reveals a similar interconnection of flex circuits  12  except instead of mechanical alignment of pillars and depressions  59  of circuit  12  to pillars on plate  40 , there are visual alignment marks  42  which allow for either manual or machine vision alignment of flex circuit  12  to plate  40  such that when plates  40  are attached to sleeve  45 , mirrors  18  will be optically aligned with each other. 
     FIG. 7 shows an interface device  15  for connecting fiber  16  to waveguide  20  of flex circuit  12 . Fiber  16  is fit into ferrule  30 . Ferrule  30  fits into receptacle  24  which is inserted into device  15 . The end of fiber  16  is approximately flush with the surface of device  15  which mates with the 90-degree facet  21  of circuit  12 . A hole  43  in device  15  and alignment marks or cross-hairs  48  on 90-degree facet  21  permit manual or machine vision alignment of the end of fiber  16  in device  15  with waveguide  20  of circuit  12  so that light can efficiently propagate between fiber  16  and waveguide  20 . 
     The following shows an example of fabrication of the subject invention. FIG. 8 a  shows a silicon die  49  for casting an ULTEM waveguide core  20  in trench-like space  50 . Die  49  may be made of other materials, i.e., various metals rather than silicon. ULTEM is a General Electric (GE) plastic-like polyetherimide which is a liquid-type of substance used for injection molding. On each side of space  50  are smaller trenches  51 . Trenches  51  create raised fudicial marks when injected with ULTEM. FIG. 8 a  is a side view which shows 45-degree folds, slants or cuts  17  in trench  50  for waveguide  20 . 
     ULTEM material  52 , or other equivalent material, is inserted into trenches  50  and  51 , and overfilled as illustrated in FIGS. 9 a ,  9   b  and  9   b . A “syringe” technique may be used for inserting material  52 . Die  49  with material  52  applied is put in a chamber which is subjected to an environment of N 2  at a pressure between 0.05 and 0.1 atmosphere. ULTEM material  52  is cured at a temperature between 350 and 400 degrees Centigrade (C.). After the cure, the excess or footing of material  52  is removed with sandpaper, from coarse to fine. The removal of the excess material  52  may be instead removed with a diamond machine or metalinechloride vapor polishing. FIGS. 10 a ,  10   b  and  10   c  show the results after the removal of excess material  52  in that the new surface of material  52  in trenches  50  and  51  is even or matches up with the surfaces of die  49 . A mask (not shown) is formed over die  49  and material  52  to form or deposit metal on material  52  in trenches  51  and the mask for masking fudicial marks for alignment purposes or a mechanical key. The mask is then removed with just metal  53  remaining on material  52  in trenches  51 . 
     In FIGS. 12 a ,  12   b  and  12   c , a cladding layer may be formed on the “top” surfaces of die  49 , on and about metal fudicial marks  53 , and on top of ULTEM  52  of waveguide  20 . Cladding layer  54  is a Dow Chemical Inc. BCB (benzocyclobutene) which is a commercial liquid product that is spun on and thermally cured. Cladding  54  has a lower index of refraction than waveguide  20  and is used to confine light transmission to waveguide  20 . In FIGS. 13 a ,  13   b  and  13   c , ULTEM layer  55  is formed on cladding layer  54 . Layer  55  is a carrier or back layer for BCB layer  54 . Layer  55  provides ruggedness and mechanical stability and is a handle. Layer  55  is cured at 210 degrees C. 
     FIGS. 14 a ,  14   b  and  14   c  show the fabrication of alignment keys  27  with a RIA (reactive ion agent) etchant (SF 6 ). Keys  27  are etched in ULTEM layer  55  and BCB layer  54 . Key photoresist masks (not shown) are placed over layer  55  and are aligned according to metalized fudicial marks  53 . Then keys  27  are formed via etching resulting in keys  27  aligned with marks  53 . 
     Quartz carrier  56  in FIGS. 15 a ,  15   b  and  15   c , is put on ULTEM back or carrier layer  55  via an adhesion technique. Quartz carrier  56  is already fabricated and polished before it is attached to carrier  55 . FIGS. 16 a  and  16   b  are effectively FIGS. 15 a  and  15   b , flipped upside down, having silicon die  49  removed from ULTEM material  52  which formed waveguide  52  in trench  50  and material  52  formed by trenches  51  of die  49 . The assembly of FIGS. 15 a ,  15   b  and  15   c  is put in a pressure cooker to effect the release of silicon master die  49  from the remaining assembly as shown in FIGS. 16 a ,  16   b  and  16   c.    
     Metalized mirrors  18  are formed on portion of cuts or surfaces  17 . A metal such as gold or aluminum is formed on cuts or surfaces  17  to result in metalized fold mirrors  18 . A photoresist mask (not shown) is formed on the assembly of FIGS. 16 a ,  16   b  and  16   c , such that only the areas of surfaces  17  to be used for mirrors  18  are exposed for the ensuing deposition of the metal. In FIGS. 17 a ,  17   b  and  17   c , a final BCB cladding layer  57  is formed on ULTEM material  52 , waveguide  20 , mirrors  18  and ULTEM material  52 . Then quartz carrier  56  is released and removed from ULTEM back layer  55 . Thus, one has an example of a fabricated flex circuit  12 . 
     FIGS. 18 a - 18   e  show another process sequence which may be used to produce flex circuit or flexible polymer waveguide ribbon  12 . A waveguide  65  layer may be deposited and photolithographically defined on a low refractive index cladding buffer layer  82 , which in turn is deposited on a flexible polymer substrate  68 . Wide regions of waveguide core  65  are defined adjacent to the waveguide array using a single mask level  69  in FIG. 18 a . Waveguides  65  are buried using a cladding polymer  29 , the same as that of layer  82 , to yield a composite waveguide system with a core refractive index higher than that of cladding  29 , and with additional buried wide regions of waveguide  65  core material in FIG. 18 b . Next, in FIG. 18 c , a second mask level  32  is used to expose resist  33  in an area slightly wider than the wide core  65  regions. Resist  33  is exposed using a non-critical alignment step. A non-selective light etch (e.g., O 2 +CF 4  RIE) which attacks both core  65  and cladding materials  29  is used to remove the upper regions of multilayer system  12 , exposing the waveguide core  65  material in the previously defined regions where slots  66  are to be located, as illustrated in FIG. 18 d . Next a selective etch (O 2  RIE only) is used to remove the waveguide core  65  material in the same regions to form slots or grooves  66 , as shown in FIG. 18 e.  Resulting grooves  66  are of a width defined by first level mask  69 , and thus the alignment steps are automatically aligned with respect to waveguide cores  65  located at the outside edges. 
     In another embodiment, a rigid “key”  75  having mirror image protrusions to grooves or slots  66  and possibly formed with the same mask, is fabricated using a rigid substrate and defining polymer ribs (FIG.  19 ). Key  75  may be used to align the various selections of waveguide  12  providing a complete interconnect. Key  75  is moved across surfaces  80  and  81  at the interconnect location until ribs  76  register with the grooves or slots  77  and  78 . At least one groove or slot  78  may also be fabricated from the same mask as grooves or slots  77  so as to provide alignment between waveguides  65  and  82  of less than 5 micrometers (μm). Similarly, one or several grooves or slots may fabricated to facilitate alignment between waveguide  65  and waveguide  82 . Downward pressure  45  is used to align waveguide cores  65  and  82 , and insert key  75  with ribs  76  into slots  77  and  78  to interlock flex coupler  12  to board or chip  64 . One of a number of different adhesives can be used to cover and glue the interlocking parts together in a permanent manner. A second section of waveguide  65  can be aligned to a first section of waveguide  82  by sliding the two components together using key  75  to define lateral alignments  34  and  35  and vertical alignment  36 , and interlock the components in an aligned fashion. 
     Self-alignment with respect to the waveguides  65  and  82  may also be obtained by the automatic positioning of two parts (a board or chip  64  and a flexible waveguide  12 ) using previously defined steps or features. This type of automatic positioning is an important part of a backplane approach; however, the process of defining the steps or features is still costly. The present invention as to a ‘self-aligning connector” uses the term “self-aligning” to describe the process by which the alignment features  66  and  78  are made in the connector components  12  and  64  using unique selective etch features of the waveguide  65  system. There is no need for any mask registration. The two-polymer system makes this approach possible as noted above. The impact of this idea is applicable to optical backplanes at the system level. The self-aligning process allows simple fabrication of the self-aligning connector assemblies  75 ,  77  and  78 .