Abstract:
An optoelectronic apparatus is described herein, including a transmitter, a receiver, and an optical waveguide, all of which are embedded in a PCB. The transmitter includes a laser generator and other circuits for generating electrical and optical signals, which are transmitted through the waveguide to the receiver. The receiver includes circuits and detectors for detecting and converting the optical signals to electrical signals. The circuit and optical components of the transmitter and receiver are integrated in 3D hybrid chip sets where the chip components are stacked in a 3D structure. Because all of the circuit and optical components are embedded in the PCB, the apparatus is made very compact and suitable for implementation in portable products.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application No. 61/202,348, filed Feb. 20, 2009, which is incorporated by reference in its entirety. 
     
    
       [0002]    TECHNICAL FIELD 
         [0003]    The present invention relates in general to integrated circuits (ICs) for optoelectronic interconnects and in particular to embedded chip-to-chip optical interconnect in a 3D system in package (SiP) format. 
       BACKGROUND OF THE INVENTION 
       [0004]    The advanced fabrication techniques are driving the electronic products to an ever smaller scale, making them more compact and complex. On the other hand, as the integrated circuit (IC) technology advances, the requirement on transmission speed between circuit components increases tremendously. Optoelectronic interconnects are an attractive solution to the bandwidth limitation of data transmissions for board level interconnections. A typical integrated optical-electrical circuit includes a laser source in the transmitter, a photo detector in the receiver, and an optical waveguide connecting the transmitter and receiver. The optical waveguide is a physical structure including transmission medium, such as optical fibers, that transmits and guides light signals from the transmitter to the receiver. 
         [0005]    The speed, date rate, power, and size of optoelectronic devices generally scale with the wavelength of the light signals. Nowadays, the complexity of two-dimensional (2D) photonic modules is limited by their substrate sizes and the difficulty in connecting large numbers of electrical connections. Also, the high-profile of these packages prevents them from usage in many applications, e.g., portable electronics products. The 2D system on chip (SoC) technique can reduce the size and increase the performance, but limits the flexibility and posts challenges on fabrication process. For example, during the fabrication process, different materials, e.g., Si and GaAs, must be combined on the same chip. 
         [0006]    On the other hand, three-dimensional (3D) IC techniques allow a better integration of devices with different materials and achieves a dramatic cost and labor reduction. A 3D integrated circuit includes two or more layers of active electronic components, integrated both vertically and horizontally into a single circuit. Novel combinations of materials/geometries allow for devices far superior than their 2D in-plane counterparts. Additional flexibility can be achieved in package design, interconnect routing, and package placement through 3D bare-chip integrations. 
         [0007]    In “SMT-compatible large-tolerance ‘OptoBump’ interface for interchip optical interconnections,” IEEE Transactions on Advanced Packaging, Vol. 26, No. 2, May, 2003, Ishii et al. lay open an “OptoBump” interface for inter-chip optical interconnections having an embedded optical polymer waveguide. However, the optical chips are packaged in a 2D thick module, and thus large holes on the PCB are required. Because the optical and electrical ICs are packaged in a module at a large distance from the waveguide, the system requires collimating or focusing lenses on the electrical modules and on the PCB, resulting in poor electrical and optical performance and higher costs. In addition, the high-profile design restricts its implementation in many portable products. 
         [0008]    In U.S. Patent Application Publication No. 2005/0089264, Johnson et al. lay open an optoelectronic card and PCB including a passive alignment between the Vertical Cavity Surface Emitting Laser (VCSEL) devices and waveguides. In there, a self-alignment characteristic or flip chip is utilized during the solder reflow to assemble the VCSELs to the waveguide on the surface of a PCB. In Johnson&#39;s system, both the VCSEL and optical waveguide are sitting on top of the PCB, and thus the integration is poor. 
         [0009]    In “Terabus: tebrabit/second-class card-level optical interconnect technologies,” IEEE Journal of Selected Topic in Quantum Electronics, Vol. 12, No. 5, September/October 2006, Schares et al. lay open an optoelectronic device having a VCSEL and its driver chip (or a photo diode and its receiver chip) supported by a silicon carrier. In there, the optical polymer waveguide and all the circuit components are not embedded, and thus the package is too thick to be useful for most portable products. In addition, because silicon carriers with holes are used to support the chips and optical lenses are required, the material and production costs are inevitably high. 
         [0010]    In “300-Gb/s 24-channel bidirectional Si carrier transceiver optochip for board-level interconnects,” Proceedings of 58 th  ECTC, Electronic Components and Technology Conference 2008, pp. 238-243, May, 2008, Doany et al. lay open a bidirectional transceiver optochip supported by a silicon carrier, which is attached on a standard plastic ball grid array package. However, in Doany&#39;s system, the polymer optical waveguide is not embedded, but on top of the PCB. The optical and electrical ICs are packaged in a carrier with a very long distance to the optical polymer waveguide, thereby requiring collimating or focusing lenses in the package and on the PCB. In addition, the IC components are not integrated in a 3D stack structure, thereby occupying large footprint and rendering the device unsuitable for portable products. 
         [0011]    U.S. Pat. No. 7,373,033 to Lu et al. describes a chip-to-chip optical interconnect including a substrate, an optoelectronic die, and a waveguide structure. The substrate includes an optical via passing through the substrate. The optoelectronic die is disposed on the substrate and aligned to optically communicate through the optical via. A waveguide structure is positioned proximate to the substrate and aligned with the optical via to communicate optical signals with the optoelectronic die through the optical via. In there, not all of the electrical and optical chips are embedded and a carrier is used to support the chip, thereby rendering the package too thick to be useful for most portable products. In addition, the electrical and optical chips are not stacked in a 3D IC structure, causing the package to occupy a large footprint as well as compromising the performance. 
         [0012]    U.S. Pat. No. 6,243,509 to Chen describes a board level optoelectronic interconnects having an embedded optical polymer waveguide and the optical chips. However, the semiconductor devices in the system are not embedded. The electrical and optical chips are not integrated in a 3D stack structure, thereby causing the package too large to be applied to most portable products and comprising the performance of the device. 
         [0013]    As discussed above, in existing 3D optoelectronic devices, the electrical and optical chips are not integrated in the 3D circuit structure, which can be embedded with the optical waveguide. As a result, the footprints of the circuit components are not optimized and the systems do not achieve the optimal performance. As the trend towards miniature and faster electronic products continues, innovative structures and fabrication process for improving optoelectronic circuit package and lowering material and fabrication costs are highly desired. 
       BRIEF SUMMARY OF THE INVENTION 
       [0014]    Described herein is an apparatus and system having three-dimensional (3D) IC components embedded in a printed circuit board (PCB) or an organic laminated substrate. The optoelectronic components include a transmitter and receiver with a bare chip design. A bare chip IC is defined as an IC that has been cut out from the wafer and is ready for packaging. 
         [0015]    In some embodiments, the system includes a rigid PCB (or a substrate) having an embedded optical polymer waveguide, an embedded transmitter, and an embedded receiver. The transmitter further includes an embedded vertical cavity surface emitted laser (VCSEL), an embedded driver chip, and an embedded serializer. The receiver further includes an embedded photo-diode detector, an embedded tans-impedance amplifier (TIA), and an embedded deserializer. 
         [0016]    In a further embodiment, the VCSEL, on the transmitter side, the driver chip, and the serializer chip are 3D stacked, embedded, and then aligned with the mirror on one end of the embedded optical polymer waveguide in the PCB. Similarly, on the receiver side, the photo-diode detector, the TIA chip, and the deserializer chip are 3D stacked, embedded, and then aligned with the mirror on the other end of the embedded optical polymer waveguide in PCB. 
         [0017]    In a further embodiment, the system includes embedded heat slugs and a heat spreader for dissipating heat from the transmitter and receiver. The back-side of the driver or serializer and the TIA or deserializer chips is attached to the heat slug and the heat spreader if necessary. 
         [0018]    In some alternative embodiments, an optoelectronic apparatus is provided, including a circuit board having a cavity, a waveguide structure embedded in the circuit board for transmitting optical signals, and a laser transmitter for generating the optical signals. The laser transmitter is disposed within the cavity in proximity to the waveguide and the waveguide is aligned with the laser transmitter for receiving the optical signals from the laser transmitter. 
         [0019]    In still some alternative embodiments, an optoelectronic apparatus is provided, including a circuit board including a cavity, a waveguide structure embedded in the circuit board for transmitting optical signals, and a laser receiver disposed within the cavity in proximity to the waveguide. The laser receiver is aligned with the waveguide structure for receiving the optical signals through the waveguide structure. 
         [0020]    The novel structural design offers optimal solutions for low-cost and high-performance semiconductor circuits with optical devices in wide-bandwidth and low-profile optoelectronic packaging for chip-to-chip optical interconnect applications. Because of the embedded structure, the apparatus and system provide small form factor, low cost and high (optical, electrical, thermal, and mechanical) performances. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0021]      FIG. 1  depicts an optoelectronic apparatus having embedded transmitter and receiver with heat slugs and a heat spreader; 
           [0022]      FIGS. 2A-F  depict various embodiments of the 3D chip structure for the laser transmitter and receiver; 
           [0023]      FIG. 3  depicts an optoelectronic apparatus having embedded transmitter and receiver with heat slugs attached to the back of the transmitter and receiver; 
           [0024]      FIG. 4  depicts an optoelectronic apparatus having embedded transmitter and receiver without the heat slugs or the heat spreader; 
           [0025]      FIG. 5  depicts an optoelectronic apparatus having embedded transmitter and receiver attached on different surfaces of the waveguide; and 
           [0026]      FIG. 6  depicts an optoelectronic apparatus having transmitter and receiver integrated in a transceiver attached to each end of the waveguide. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Now turning to the drawings and referring to  FIG. 1 , optoelectronic apparatus  100  for providing chip-to-chip optical interconnects is depicted therein. The apparatus  100  includes transmitter  101 , receiver  103 , and waveguide structure  108 , all of which are embedded in printed circuit board (PCB) or suitable substrate  105 . The transmitter  101  receives input signals, such as digital or analog signals, and converts the input signals to optical signals, which are transmitted through the waveguide structure  108  to the receiver  103 . At the receiver side, the optical signals are converted to output signals for further processing. 
         [0028]    Although  FIG. 1  depicts the transmitter  101  and the receiver  103  on the same PCB board  105 , it is readily understood by one skilled in the art that the transmitter  101  and the receiver  103  can reside on different PCB boards separated at a long distance. In such embodiments, the transmitter  101  and the receiver  103  are interconnected through a long distant waveguide structure  108 , and optical amplifiers may be required to ensure reliable transmission of the optical signals. 
         [0029]    As shown in  FIG. 1 , the PCB  105  includes one or more layer of conductive material  104  and one or more layers of insulating material  106  laminated together. The insulating layers  106  can include FR4 material or a BT material. One or more insulating layers  106  forms cavities suitable for enclosing the transmitter  101  and receiver  103 , so that the optoelectronic chips can be fully embedded in the PCB  105 . 
         [0030]    With respect to the transmitter  101 , it includes laser generator chip  118 , driver chip  116 , and serializer chip  114 , all integrated in a 3D stack structure. The laser chip  118  can take the form of any semiconductor laser generator such as the Vertical Cavity Surface-Emitting Laser (VCSEL) chips or the Quantum cascade laser chips, that are well known in the art. The driver chip  116  and serializer chip  114  includes standard circuits for processing the incoming signals and driving the laser chip  118  to generate optical signals corresponding to the incoming signals. 
         [0031]    In order to reduce the footprint and the dimensions of the transmitter, the laser chip  118  is attached to the driver chip  116  using the flip-chip method as well known in the art. As shown in  FIG. 1 , when the laser chip  118  has laser emitter on the back side of the chip, the front side of the laser chip  118  is attached to the driver chip  116  so that the emitter faces the mirror  136 . In order to provide electronic connections to the laser emitter, one or two through silicon vias (TSVs) are disposed in the laser chip  118 . 
         [0032]    Alternatively, when the laser chip  118  has laser emitter on the front side (i.e., the active surface) of the chip, the back side of the laser chip  118  is attached to the driver chip  116 . In this embodiment, the TSVs in the laser chip  118  is optional. 
         [0033]    Similarly, the driver chip  116  is attached to the serializer  114  using the flip-chip method. Accordingly, a plurality of through silicon vias (TSVs)  120  and  110  are disposed in the driver chip  116  in order to provide electronic connections, and a jointing material or underfills, such as a transparent polymer, are then used to secure the jointing interfaces among the chips. 
         [0034]    The TSVs in the chips can be filled with various metals including cooper, gold, silver, etc. Detailed teaching of through silicon via is found in Khan et al. “3D packaging with through silicon via (TSV) for electrical and fluidic interconnections,” Proceedings of Electronic Components and Technology Conference 2009, pages 1153-1158, May-June, 2009, and Zhang et al. “Development of through silicon via (TSV) interposer technology for large die (21×21 mm) fine-pitch Cu/low-k FCBGA package,” Proceedings of Electronic Components and Technology Conference, pages 305-312, May-June, 2009, which are hereby incorporated by reference in their entireties and for everything they include. 
         [0035]    With respect to the receiver  103 , it includes photo diode detector  130 , amplifier chip  128 , and deserializer  126 , which are stacked in a 3D IC structure as shown in  FIG. 1 . In particular, the detector  130  and the amplifier chip  128  include TSVs  132  and  134  similar to  120  and  110 , for providing electronic connections. The detector  130  is then soldered to the amplifier chip  126  using the flip-chip method and the amplifier  126  is then similarly soldered to the deserializer  128 . The jointing materials or underfills are then used to secure the connections in the 3D chip structure. The photo diode detector  130 , the amplifier chip  132 , and the deserializer  126  also include standard processing circuits for receiving the optical signals form the waveguide structure  108 , and convert them into electronic signals for further processing. In particular, the amplifier chip  128  can take the foam of a trans-impedance amplifier. 
         [0036]    The stacked chip sets in the transmitter  101  and receiver  103  are encapsulated by special underfills, e.g., transparent polymer. 
         [0037]    As further shown in  FIG. 1 , in order facilitate heat dissipation from the transmitter  101  and receiver  103 , the apparatus  100  includes heat slugs  112  attached to the back of the serializer  114  and deserializer  126 . On the transmitter side, the PCB  105  includes an opening aligned with the transmitter embedded in the cavity. The opening connects the cavity to the external environment through the outer surface of the PCB  105 . A similar opening is disposed within the PCB  105  on the receiver side and aligned with the cavity enclosing the transmitter. The heat slugs  112  are disposed within the opening, abutting the back surfaces of the serializer  114  and deserializer  126 . Thermal interface material (TIM) is applied between the chips and the heat slugs  112 . 
         [0038]    In order to provide additional cooling, the apparatus  100  further includes a heat spreader  102  attached to the outer surface of the PCB  105  and abutting the heat slugs  112 . The heat spreader draws heat from the heat slugs  112  and dissipating it into the external environment, thereby providing strengthen cooling effects. The heat slugs  112  and heat spread  102  can be made of any materials that are suitable for conducting heats and are well known in the art. 
         [0039]    The waveguide structure  108  includes polymer-based materials that are suitable for optical interconnect due to its low cost, suitability for mass production, high thermal stability and low optical loss. The optical waveguide  108  is embedded in the PCB  105  and supported by an optical layer support film  110  and provides the benefits of low propagation loss, high thermal stability, low fabrication costs, etc. In particular, the typical loss for the waveguide used in optical PCBs, depending on the fabrication process, materials and its cross sectional area, ranges from 0.6 dB/cm to 0.05 dB/cm. Depending on the fabrication processes and materials, polymer-based waveguides have a thermal stability of more than 260° C., which is adequate for lead-free assembly process. In addition, the optical polymer waveguide  108  is usually designed in multi-mode and has a larger aperture for reasons of low cost and compatibility with PCB process. 
         [0040]    With respect to the laser source  118 , the VCSEL laser chip can provide better laser quality. Unlike edge emitting diodes that emit IR radiation in the plane of the semiconductor, the VCSEL produces a nearly circular beam of symmetrical laser light with low threshold current and small output beam divergence angle that is perpendicular to the plane of the semiconductor. These devices may be constructed on the surface of a fabricated wafer and tested for their optical and electrical properties on the wafer, and have the advantage of combining large 2D emitter arrays and active devices such as CMOS drivers with conventional technology. 
         [0041]    Due to the surface emitting characteristic, an out-of-plane 45° reflector (i.e., mirror  136 ) is provided in the waveguide  108  to reflect the incoming optical signals along the propagation direction. Similar mirror  137  is also provided in the waveguide aligned with the photo detector  130 , for receiving the optical signals from the waveguide structure  108 . The back sides of the mirrors  136  and  137  are coated with metal such as gold, silver, etc to form the reflective surfaces. 
         [0042]    The waveguide structure  108  including the mirrors  136  and  137  and its fabrication is further described in Chang, et al. “Fabrication of fully embedded board-level optical interconnects and optoelectronic printed circuit boards,” Proceedings of Electronics Packaging Technology Conference 2009, pages 973-976, December 2009, Lim, et al. “Optimization and characterization of flexible polymeric optical waveguide fabrication process for fully embedded board-level optical interconnects,” Proceedings of Electronic Packaging Technology Conference 2008, pages 1114-1120, December, 2008, and Calvin, et al. “Fabrication and optimization of the 45° micro-mirrors for 3-D optical interconnections,” Proceedings of Electronic Packaging Technology Conference 2008, pages 1121-1125, December, 2008, which are hereby incorporated by reference in their entireties and for everything they include. 
         [0043]    As further depicted in  FIG. 1 , metal pads  124  are disposed on the surface of the waveguide structure  108  and the transmitter  101  and the receiver  103  are directly attached (soldered) onto the metal pads  124  through solder bumps  122 . In some embodiments, the gap between the laser emitter and the waveguide surface and that between the photo detector and the waveguide surface are less than 0.4 millimeter. Because the laser source  118  and the detector  130  are so close to the mirrors  136  and  137  of the optical polymer waveguide  108 , the optical lens for converging the optical signals in the conventional systems becomes optional and is not necessary. 
         [0044]    According to some alternative embodiments, the 3D chip sets in the transmitter  101  and the receiver  103  can be constructed using various connection methods as shown in  FIGS. 2A-F . Specifically, in the transmitter  101 , the laser chip  118  can be either flip-chip bumped or wire bonded to the driver chip  116 , which can also be either flip-chip bumped or wire bonded to the serializer  114 . Similarly, in the receiver  103 , the photo detector chip  130  can either flip-chip bumped or wire bonded to the amplifier chip  128 , which can be either flip-chip bumped or wire bonded to the deserializer  126 . 
         [0045]    As depicted in  FIG. 2A , when the laser emitter is on the back side of the laser chip  118 , the laser chip  118  can be attached to the driver chip  116  using a flip-chip method so that the emitter faces the mirror  136 . The driver chip  116  can also be attached to the serializer  116  using the flip-chip method. Here, TSVs are disposed in the driver chip  116  and laser chip  118  for providing electronic connections. 
         [0046]    In an another embodiment, shown in  FIG. 2B , the laser emitter is on the back side of the laser chip  118 , which is flip-chip soldered to the driver chip  116 , and the driver chip  116  is wire bonded to the serializer chip  114 . In this embodiment, no TSV is needed in the driver chip  116 , while one or two TSVs are disposed in the laser chip  118 . 
         [0047]    In still another embodiment as shown in  FIG. 2C , when the laser emitter is on the front side of the laser chip  118 , the laser chip  118  can be wire bonded to the driver chip  116 , which can then be either flip-chip soldered or wire bonded to the serializer chip  114 . In this embodiment, no TSV is needed in either the driver chip  116  or the laser chip  118 . 
         [0048]    In still another embodiments, as further shown in  FIG. 2D , the driver chip  116  is attached to the serializer  114  via flip-chip connections and include a plurality of TSVs. For the laser chip  118 , because the active surface (laser emitter) is on the front side of the chip, no TSV is needed when it is flip-chip bumped to the driver chip  116 . 
         [0049]    As shown in  FIG. 2E , no TSV is need in either the driver chip  116  or the laser chip  118  because the driver chip  116  is wire bonded to the serializer  114  and the laser chip  118  has its emitter on the front of the chip. 
         [0050]    As further shown in  FIG. 2F , the transmitter  101  can be attached to the metal pads on the waveguide structure  108  through driver chip  116 , rather than the serializer chip  114 . In this embodiment, the solder bumps  112  are applied between the driver chip  116  and the metal pads on the waveguide surface. 
         [0051]    As depicted in  FIGS. 2A-F , one skilled in the art will appreciate that similar configurations can also be used to construct the receiver  103 . 
         [0052]    As shown in  FIG. 3 , when the chip sets in the transmitter  101  and receiver  103  are not very large and heat generation is not significant, the heat spreader  102  may not be necessary. Heat is drawn by the heat slugs from the embedded optoelectronic components and dissipated into the external environment. 
         [0053]    As shown in  FIG. 4 , if the heat generation of the embedded chip sets is very small, then the heat slugs  112  and the thermal conductive materials are not even needed. Because the cooling components are unnecessary, the entire system becomes even more compact. 
         [0054]    As further shown in  FIG. 5 , the transmitter  101  and receivers  103  can be attached to different surfaces of the waveguide structure  108 . In this embodiment, the mirrors  136  and  137  are substantially parallel with each other so that the optical signals are correctly transmitted from the transmitter  101  to the receiver  103 . 
         [0055]    According to still some alternative embodiments, in many applications the serializer chip  114  and deserializer chip  126  may not be needed or can be placed outside the PCB  105 . In these embodiments, the embedded transmitter  101  includes only the laser chip  118  and the driver chip  116 , and the receiver  103  includes only the detector chip  130  and the amplifier  128 , stacked together and enclosed in the PCB cavities. In these embodiments, if cooling is needed, the heat slugs  112  can be attached to the driver chip  116  and the amplifier chip  128 . 
         [0056]    According to still anther embodiment as shown in  FIG. 6 , the transmitter  101  and receiver  103  are integrated into transceiver  606  and  608  for providing duplex (bidirectional) communications. In particular, two waveguide structures similarly to waveguide  108  are embedded in the PCB  105 . The transceiver  606  and  608  are also embedded into the PCB  105  and aligned with the mirrors (not shown) within the waveguides. In operation, the electronic signals are received through the input ports  602  and converted into the optical signals, which can be transmitted between the transceiver  606  and  608  in both directions. At the receiving ends, the optical signals are converted back to the electronic signals and output through the output ports  604 . 
         [0057]    During the fabrication process, the driver chip  116 , with the laser chip  118  attached thereon, is die-attach onto the serializer chip  114  while it is in the wafer format. After the 3D chip set is diced, it is placed on the PCB substrate on top of the polymer waveguide  108 . Special encapsulants such as the transparent polymer may be needed to protect the chip set. 
         [0058]    The fabrication of the optoelectronic apparatus  400  depicted in  FIG. 4  is further described with respect to  FIG. 7 . One skilled in the art would appreciate that the fabrication of other embodiments depicted in  FIGS. 1-3  and  5 - 6  are similar. 
         [0059]    In particular, at step  702 , the serializer  114  and deserializer  126  are fabricated and remain in wafer form. Solder Balls are attached to the serializer and deserializer wafers. At step  704 , the driver wafer  116  and the amplifier wafer  128  are fabricated in wafer form. The TSVs are disposed in the driver and amplifier wafers. Solder bumps are then attached to these wafers. At step  706 , the laser chip  118  and detector chip  130  are fabricated and the TSVs are disposed in these chips. (When the laser chip  118  uses transparent VCSEL, the TSVs are not needed on the chip.) Similarly, solder bumps are attached to these chips. 
         [0060]    At step  708 , the laser chip  118  and detector chip  130  are diced and soldered to the wafers of the driver chip  116  and amplifier chip  128 , respectively. 
         [0061]    At step  710 , the driver chip  116  and amplifier chip  128  are diced and attached to wafers of the serializer chip  114  and deserializer chip  126 . The serializer and deserializer are then diced to obtain the individual chip set assembly. 
         [0062]    At step  712 , the waveguide  108  is disposed and attached to the supporting layer on a bottom portion of the PCB substrate. Metal pads are disposed onto the waveguide  108 . Alignment marks are made on the waveguide surface for aligning the transmitter  101  and receiver  103  with the mirrors  136  and  137 , respectively. 
         [0063]    At step  714 , the chip set assemblies (transmitter  101  and receiver  103 ) obtained at step  710  are attached and soldered onto the metal pad through the solder balls and aligned with the mirrors. Suitable underfills materials are applied to enclose the chip set assemblies. 
         [0064]    At step  716 , the top portion of the PCB substrate is attached and laminated with the bottom portion. The top portion of the PCT includes cavities which enclose the transmitter  101  and receiver  103 . 
         [0065]    If desired, the top portion of the PCB may include openings for accommodating the heat slugs  112  and the heat spreader  102  can be attached to the top portion of the PCB as depicted in  FIG. 1 . 
         [0066]    After reading this specification, one skilled in the art would appreciate that in the various embodiments depicted in  FIGS. 1-6 , all of the electrical and optical IC circuits are in bare chip form, thereby reducing package and module costs, reducing system weights and system profile and dimensions, enhancing electrical performance. In addition, because all of the 3D chip sets and the optical waveguide are embedded in the PCB, the distances between the chip sets to the waveguide are minimized, thereby maximizing optoelectrical performance of the system. As note above, lenses are unnecessary between the chip sets and the waveguide. Furthermore, if needed, the system can include heat slugs and heat spreaders to dissipate heat from the back-side of the chip sets. The design of the system allows the stacked chip set to be assembled separately prior to attachment to the waveguide in the PCB. 
         [0067]    One skilled in the art would also appreciate that optical (the VCSEL and photodiode) and electrical (the laser driver, serializer, amplifier, and deserializer) IC chips integrated in the 3D chip sets can be fabricated using chip-to-wafer assembly and wafer-level redistribution methods. Although only PCB is described in this application, one skilled in the art would appreciate that the PCB can be substitute with organic laminated substrate for the same purpose.