Abstract:
An optical interconnect is provided which may allow flexible high-bandwidth interconnection between chips, eliminate the need for optical alignment between the optoelectrical (OE) die and waveguide during assembly because the OE die is at least partially embedded inside the waveguide (lower cladding layer, upper cladding layer, and core layer), eliminate the need for handling the optical interconnect at OEM, and not impact current substrate and motherboard technology.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to an optical interconnect. In particular, the present invention relates to an optical interconnect which provides a high-speed connection between electrical circuits on different substrates 
     BACKGROUND OF THE INVENTION 
     As input/output (I/O) speeds continue to increase, the electrical interconnects currently used to connect circuit elements will run out of bandwidth. Optical interconnects have been proposed as a potential solution to this problem since optical interconnects are capable of providing very high total bandwidth. However, current optical interconnect solutions are inadequate for many reasons. 
       FIG. 1  shows an example of an optical interconnect. In  FIG. 1  a control chip  110  is electrically connected to an optoelectrical (OE) die  120 . An OE die is an interface between electrical signals and optical signals OE die  120  either converts electrical signals from the control chip to optical signals or converts optical signal from the waveguide to electrical signals. The optical signals are transmitted from the OE die&#39;s optical source (such as a laser) through planar waveguide  130  which is partially buried in substrate  140 . The optical waveguide carries the optical signals to connector  150 . Although the optical interconnect of  FIG. 1  is operable, there are many problems with this approach:
         1. The fabrication and assembly of planar waveguide  130  and connector  150  are not High Volume Manufacturing (HVM) compatible;   2. A trench must be made in the substrate so that the planar waveguide can fit under the OE die; and   3. Alignment between the OE die and planar waveguide is currently done by semi-active alignment       

     Other problems with current optical interconnect methodologies include:
         1. Optical alignment between the OE die and the waveguide is a big challenge;   2. Handling of the optical interconnect is considered a major concern from Original Equipment Manufacturers (OEM).       

     There has been some research on embedding an OE die inside the core of a waveguide where the waveguide is embedded inside a motherboard or a substrate. For example, embedding a thin edge emitting laser into an organic substrate has been demonstrated before. However this type of approach has too large of an impact on current motherboard and substrate technology and is too costly. 
     Thus, there is a need for an optical interconnect which, for example:
         1. Provides flexible high-bandwidth interconnect between chips (such as a CPU and a Memory Controller Hub (MCH));   2. Eliminates the need for optical alignment between the OE die and waveguide during assembly because the OE die is at least partially embedded inside the waveguide (lower cladding layer, upper cladding layer, and core layer);   3. Eliminates the need for handling the optical interconnect at OEM; and   4. Does not impact current substrate and motherboard technology.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be understood and appreciated more fully from the following detailed description in conjunction with the drawings in which like reference numerals indicate corresponding, analogous or similar elements, and in which: 
         FIG. 1  shows an example of an optical interconnect; 
         FIGS. 2-7  show a series of operations in fabricating an embodiment of the present invention employing thin film edge emitting lasers; 
         FIG. 8  shows an embodiment of the present invention in which an optical waveguide is connected to a controller chip by a solder joint connected to the controller chip by a high-speed transmission line; 
         FIG. 9  shows an embodiment of the present invention in which an optical waveguide is separably connected to a controller chip by a connector connected to the controller chip by a high-speed transmission line; 
         FIG. 10  shows an embodiment of the present invention in which an optical waveguide is directly connected to a controller chip by a solder joint; 
         FIG. 11  shows an embodiment of an optical waveguide with embedded thin lasers and detectors being used as a high speed link between two packages; 
         FIGS. 12-15  show a series of operations in fabricating an embodiment of the present invention employing vertical-cavity surface-emitting lasers; and 
         FIG. 16  shows an embodiment of optical waveguides having a heat spreader 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following description, various aspects of the present invention will be described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without the specific details presented herein. Furthermore, well-known features may be omitted or simplified in order not to obscure the present invention Various examples are given throughout this description. These are merely descriptions of specific embodiments of the invention. The scope of the invention is not limited to the examples given. 
     In an embodiment of the present invention, thin optoelectronics dies may be embedded inside a core of a free-standing optical waveguide The OE dies may be for example an edge emitting laser, a Vertical-Cavity Surface-Emitting Laser (VCSEL), and the like or a detector for a laser. The waveguide with the embedded OE dies may then be used as for example a separable high speed link capable of providing a high speed connection between any two chips or packages. In an embodiment of the present invention the OE dies are at least partially embedded in the waveguide. A waveguide may be defined by an upper cladding layer, a lower cladding layer and a core layer disposed in between the two cladding layers, for example. 
     An optical waveguide according to one embodiment may include a lower cladding layer, an upper cladding layer, and a core layer disposed between the lower and upper layer, where the core has a higher index of refraction than the lower layer and the upper layer. The waveguide may include a first optical die having an optical source to transmit an optical signal into the core layer, where the first optical die is as at least partially embedded in one end of the waveguide. The waveguide may further include a second optical die having a detector for the optical signal, where the second optical die is at least partially embedded in waveguide. 
     In an embodiment of the present invention using thin film edge emitting lasers and thin film edge receiving detectors, fabrication of the waveguide with embedded optoelectronics dies may include the following operations, but may include other operations. As is apparent to those skilled in the art, some of the following operations may not be necessary for the fabrication of an operable waveguide and may instead be included since a better waveguide results Additionally, although separated into different, ordered operations some of the operations may be performed out of order, condensed into a single operation, or split into several operations. 
     An embodiment of operation  1  is illustrated in  FIG. 2 . A lower cladding layer  210  may be coated on to a releasable support substrate  220 . The releasable support substrate may be for example a Teflon™ non-stick surface coated substrate or the like. The releasable support substrate may alternatively be a tape or film cladding layer. Thin OE dies are precisely placed on the lower cladding layer. The OE dies may be less than 50 μm tall. Alternately, the OE dies may be between 8 μm and 100 μm tall. The cross-section of the finished waveguide may thus be between 8 μm×8 μm to 100 μm×100 μm. On one side of the lower cladding layer thin film edge emitting lasers  310  are placed and on the opposite side of the lower cladding layer thin film edge receiving detectors  320  are placed. As shown in  FIG. 3 , the thin film edge emitting lasers are precisely placed opposite and distal to the thin film edge receiving detectors such that one faces the other. Other dimensions may be used 
     An embodiment of operation  2  is illustrated in  FIG. 4 . The lower cladding layer is coated with a photo-definable core layer  510 . The photo-definable core layer has a higher refractive index than the cladding layer. As shown in  FIG. 5 , the core layer is then photo-defined to strip away the excess layer and create separate cores for each OE die pair. Other methods to create separate cores are possible other than photo-defining. 
     An embodiment of operation  3  is illustrated in  FIG. 6 . The lower cladding layer and photo-definable core layer are coated with an upper cladding layer  710 . The upper cladding layer may have the same index of refraction as the lower cladding layer. The upper cladding layer may be photo definable or non-photo-definable. 
     An embodiment of operation  4  is illustrated in  FIG. 7 . Form vias  810  in the upper cladding layer to expose the metal pads on the embedded OE dies If the upper cladding layer is photo-definable, the vias may be formed by photo-defining. Alternately, laser drilling may be used. Fill the vias with copper  910  thereby creating an accessible electrical connection to the metal pads on the OE dies. Alternatively, other conductive materials may be used other than copper In an alternate embodiment, when the upper cladding layer is coated on top of the lower cladding later and the core layer, it is not coated on top of the OE dies. In such an embodiment is may not be necessary to form vias. In such an embodiment, electrical contact may be made directly to the metal contacts on the OE die. 
     In an embodiment of operation  5 , the lower cladding layer, core layer, and upper cladding layer are diced into discrete optical interconnects. The discrete optical interconnect may contain one or more waveguides each having a pair of OE dies embedded therein. Optical interconnects may also be referred to as waveguide arrays The waveguide arrays may then be released from the support substrate. 
     Embodiments of the current invention may be used in a variety of ways. 
       FIG. 8  illustrates an embodiment of the present invention in which a control chip  1110  may have an electrical connection  1120  to a solder joint  1130  on a substrate  1140 . A waveguide array  1150  may be soldered to the solder joint on to the substrate thereby providing the control chip with a high-bandwidth optical interconnect. In such an embodiment the waveguide array is soldered to the substrate for a non-separable connection 
       FIG. 9  illustrates an embodiment of the present invention in which a control chip  1210  may have an electrical connection  1220  (for example, using a high-speed transmission line) to a solder joint  1230  on a substrate  1240 . A connector  1250  may be soldered to the solder joint on to the substrate. A waveguide array  1260  may be inserted into the connector thereby providing the control chip with a high-bandwidth optical interconnect. In such an embodiment the waveguide array is inserted into the connector for a separable connection. 
       FIG. 10  illustrates an embodiment of the present invention in which a control chip  1310  may have several of its I/Os  1370  soldered directly to various electrical connections  1380  on a substrate  1320 . At least one of the I/Os  1330  may instead be soldered using solder paste  1360  directly to the via of a waveguide array  1340  which is connected to the substrate by an adhesive layer  1350 . In an alternative embodiment I/O  1330  may instead be soldered using solder paste directly to the electrical contact of the OE die in waveguide array  1340 . In the embodiment shown in  FIG. 10 , the optical interconnect provides the highest speed because of the extremely short connection between the control chip and the OE die. 
       FIG. 11  shows an embodiment of a waveguide  1410  with embedded thin lasers  1420  and detectors  1430  being used as a high speed link between two packages  1440 . 
     In an embodiment of the present invention using top emitting VCSEL lasers and top receiving detectors, fabrication of the waveguide with embedded optoelectronics dies may include the following operations, but may include other operations. As is apparent to those skilled in the art, some of the following operations may not be necessary for the fabrication of an operable waveguide and may instead be included since a better waveguide results. Additionally, although separated into different, ordered operations some of the operations may be performed out of order, condensed into a single operation, or split into several operations. 
     An embodiment of operation  1  is illustrated in  FIG. 12 . Thin OE dies are precisely placed on a releasable support substrate  1510 . The releasable support substrate may be a Teflon™ non-stick surface coated substrate or the like. The OE dies may be less than 50 μm tall. Alternately, the OE dies may be between 8 μm and 100 μm tall. The cross-section of the finished waveguide may thus be between 8 μm×8 μm to 100 μm×100 μm. On one side of the support substrate, VCSEL lasers  1520  are placed and on the opposite side of the support substrate top receiving detectors  1530  are placed. The VCSEL lasers are precisely placed opposite and distal to the top receiving detectors such that one faces the other. Other dimensions may be used. 
     An embodiment of operation  2  is illustrated in  FIG. 1.3 . A lower cladding layer  1610  is coated on to the releasable support substrate. The releasable support substrate may alternatively be a tape or film cladding layer. 
     An embodiment of operation  3  is illustrated in  FIG. 14 . The lower cladding layer is coated with a photo-definable core layer  1710 . The photo-definable core layer has a higher refractive index than the cladding layer The core layer is then photo-defined to strip away the excess layer and create separate cores for each OE die pair. Other methods to create separate cores are possible other than photo-defining. The lower cladding layer and photo-definable core layer are coated with an upper cladding layer  1720 . The upper cladding layer may have the same index of refraction as the lower cladding layer. The upper cladding layer may be photo definable or non-photo-definable. 
     An embodiment of operation  4  is illustrated in  FIG. 15 . The edges above the VCSEL laser and top receiving detector are formed into 45 degree mirrors  1810  to guide the optical signal. The 45 degree mirrors are formed by laser ablation. Alternately, mechanical sawing or photo defining may be used. 
     In an embodiment of operation  6 , the lower cladding layer, core layer, and upper cladding layer are diced into discrete optical interconnects. The discrete optical interconnect may contain one or more waveguides each having a pair of OE dies embedded therein. Optical interconnects may also be referred to as waveguide arrays. The waveguide arrays may then be released from the support substrate. 
       FIG. 16  illustrated an embodiment of the present invention in which a heat spreader  1910  may be attached to the OE die to dissipate heat The heat spreader may be added to the waveguide in one of the first operations of the waveguide fabrication before the lower cladding layer is coated. Alternately, the heat spreader may be added to the waveguide in one of the final operations of the waveguide fabrication by removing the waveguide from the substrate, removing a portion of the lower cladding layer, and adding the heat spreader. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims, which follow: