Abstract:
An exemplary embodiment of the present invention is an apparatus for providing optical interprocessor communication. The apparatus comprises a multichip module and an optical module. The multichip module includes a substrate, an integrated circuit electrically connected to the substrate and a hermetically sealed cover. The hermetically sealed cover encloses a sealed portion of the substrate and the integrated circuit is inside of the sealed cover. The optical module includes an optical transceiver located on the substrate outside of the sealed portion and the optical transceiver is electrically connected to the integrated circuit through the substrate.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to fiber optic technology. More specifically, the invention relates to an apparatus for providing optical interprocessor communication.  
         BACKGROUND OF THE INVENTION  
         [0002]    A multichip module (MCM) is an electronic package structure consisting of two or more “bare” or unpackaged integrated circuits (ICs) interconnected on a common substrate (e.g., a ceramic substrate). The interconnects are usually multiple layers, separated by insulating material, and interconnected by conductive vias. MCMs are known to provide significant performance enhancements over single chip packaging approaches. Advantages of MCMs include a significant reduction in the overall size and weight of the package, which directly translates into reduced system size. Thus, first level advantages include: higher silicon packaging density, short chip-to-chip interconnections and low dielectric constant materials. These advantages lead to the following secondary benefits: increased system speed, increased reliability, reduced weight and volume, reduce power consumption and reduced heat dissipated for the same level of performance.  
           [0003]    The ICs can be attached to the common substrate using a flip chip attachment method in which all the input/output (I/O) bumps on an IC are first terminated with a solder material such as a lead/tin high melting temperature alloy. The IC is then flipped over and the solder bumps are aligned and reflowed in a reflow furnace to effect all the I/O connections with the bonding pads on the substrates. A related interconnect technology is C4 (controlled collapse chip connection) which is a method of using a lead-rich lead/tin alloy to mount chips directly to high temperature ceramic substrates. C4 flip chip structures can be built directly over exposed aluminum vias located at the top surface of a wafer.  
           [0004]    Computer systems built with multiple MCMs or multiple nodes require the ability for MCMs within the computer system to communicate back and forth. One way to provide high-speed communication of MCM to MCM data is to send the signals electronically. However, the electronic approach can suffer from a lack of scalability in speed due to losses and signal distortion within the printed circuit board that the MCM is attached to, due to electrical connectors, and due to backplane boards that may connect multiple boards containing MCMs. The electrical signal distortion is particularly acute when the boards containing MCMs are on different backplanes. Optical fiber technology has been used as an I/O data interface between computer systems. As processor speeds and densities increase, electrical signaling may not scale with the processor speeds and optical technologies may be required to play a role in: board-to-board (inter-frame) interconnection, card-to-card (intra-frame) communication, module-to-module interconnection, and any combination of these.  
           [0005]    One approach to providing high speed optical communication between components (e.g., MCMs) within a computer is to place an optical transceiver on the support printed circuit (PC) board that mounts either single chip modules or a MCM. This approach may not provide speed and scalability because the electrical signal still needs to exit the MCM through the PC card which can limit the speed due to factors such as pin inductance, signal loss in the card, and distortion. It can also consume more power because of the required module drivers and will take up extra space on the PC board.  
           [0006]    Another approach to providing high speed optical communication between MCMs is to place the optical transceivers on the MCM within the hermetic seal portion of the MCM. The hermetic seal design should be sufficient to protect the ICs and assist in ensuring chip reliability. The seal typically includes a metal or ceramic casing or cover which encapsulates and seals the MCM to protect against both stray electrical fields and to protect against environmental factors such as water vapor and gases. Placing the optical transceivers on the MCM within the hermetic seal solves the electrical problems associated with the first approach, but requires the development of a new method of exiting fiber optics through the seal or including an optical connector within the seal. This could present a difficult technical and manufacturing problem and may compromise the integrity of the seal.  
         SUMMARY OF THE INVENTION  
         [0007]    An exemplary embodiment of the present invention is an apparatus for providing optical interprocessor communication. The apparatus comprises a multichip module and an optical module. The multichip module includes a substrate, an integrated circuit electrically connected to the substrate and a hermetically sealed cover. The hermetically sealed cover encloses a sealed portion of the substrate and the integrated circuit is inside of the sealed cover. The optical module includes an optical transceiver located on the substrate outside of the sealed portion and the optical transceiver is electrically connected to the integrated circuit through the substrate. An additional embodiment includes a system for providing interprocessor communication. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0008]    Referring now to the figures wherein the like elements are numbered alike.  
         [0009]    [0009]FIG. 1 illustrates an exemplary embodiment of the present invention.  
         [0010]    [0010]FIG. 2 illustrates a side view of the exemplary embodiment depicted in FIG. 1.  
         [0011]    [0011]FIG. 3 shows an example of optical communication between multichip modules using an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    An exemplary embodiment of the present invention utilizes fiber optic technology to enable high speed serial communication between MCMs within the same computer system or on different computer systems. FIG. 1 illustrates an exemplary embodiment of the present invention. As depicted in FIG. 1, a MCM  100  includes a MCM substrate  102  (e.g., a ceramic substrate, an organic material substrate or an Si_substrate) that extends beyond the hermetically sealed MCM cover  104 . In an exemplary embodiment the substrate  102  extends half an inch beyond the MCM cover  104  but other lengths are possible depending on the amount of space required by the optical module. The exemplary optical modules shown in FIG. 1 include a transceiver  106 , fiber  108  and a connector  110 . The optical tranceiver  106  converts electrical signals from the MCM  100  into optical signals to be sent to the fiber  108  and it converts optical signals from the fiber  108  into electrical signals to be sent to the MCM  100 . A plurality of optical transceivers  106  with pigtail fiber optic cables  108  are attached to the extended MCM substrate  102  outside of the MCM cover  104 . Other optical embodiments can include an optical connector  110  integrated in the optical transceiver  106 , separate transmitter and receiver modules, and a separate interface chip which may be within the seal area. The electrical signals can be brought out beyond the seal area via internal metal signal lines and surface on the top of the substrate  102  with high density electrical interconnections (e.g., C4 pads). Internal wires are added to the MCM  100  to allow the connection from a C4 on an optical transceiver  106  to ICs that are under the MCM cover  104 . The wires extend from the C4 in the optical transceiver  106  IC to the other ICs on the substrate  102 . This allows the optical transceiver  106  to be placed close to the appropriate driver chips (e.g., a few millimeters). An alternate embodiment includes a single optical transceiver  106  attached to the extended MCM substrate  102 . In the exemplary embodiment depicted in FIG. 1, the optical transceivers  106  include a multiplicity of fibers (e.g., twenty four, 2×12) for a full duplex link. Some fibers contain outbound light (from lasers) and some fibers contain incoming light fibers (to photodiodes). As depicted in FIG. 1, the pigtail fiber optic cables  108  are terminated in industry standard optical connectors  110 .  
         [0013]    Any kind of fiber  108  and any kind of optical module known in the art (e.g., 50/125 micrometer multimode fiber, parallel optical module) may be utilized in an alternate embodiment of the present invention. In another alternate exemplary embodiment, the optical transceivers  106  include high density fiber optic connectors. The use of high density optical connectors enable more optical signals in the same space as a lower density connector (e.g., 6×12 vs. 2×12). In another alternate embodiment of the present invention, the optical transceiver  106  has been fabricated on a ceramic carrier, or similar material, in order to minimize the thermal expansion mismatch with the MCM  100  and therefore allowing for the use of fine pitch solder joins (e.g., C4). In another exemplary embodiment, the optical transceiver  106  module includes VCSELs (vertical cavity surface emitting laser), photodiodes, necessary support electronics for mux/demux, code/decode, and is optically interconnected using multimode or singlemode optical fiber. The integration of the functions into the optical module reduces cost and size. Also, the inclusion of code/decode and singlemode fiber enables the link to go longer distances. The embodiment depicted in FIG. 1 shows a 2D implementation, however an embodiment of the present invention could be used to support a 3D implementation.  
         [0014]    [0014]FIG. 2 illustrates a side view of the exemplary embodiment depicted in FIG. 1. The MCM  100  includes a substrate  102 , MCM cover  104  and optical transceiver  106 . The exemplary MCM  100  includes a heat sink  204  on the top of the MCM cover  104  to reduce the temperature of the MCM  100  and a plurality of pins  206  or other connecting means for power, signals and ground on the bottom of the substrate  102 . Good thermal contact is made to the heat sink/cover combination by placing a thermal grease or other heat conveying means between the chips and the cover. In an exemplary embodiment, the apparatus is powered electrically. Under the MCM cover  104  are the silicon chips  202  or ICs  202 . Similar to FIG.  1 , the optical transceiver  106  is connected to fiber  108  which is connected to an optical connector  110 , alternatively, the optical connector  110  could be integrated into the optical transceiver  106 . The electronics or ICs  202  are placed the same way on the MCM  100  that they would have been placed in the absence of the optical transceiver  106 . In addition, the heat sink  204  and pins  206  do not need to be reconfigured to allow for the optical transceiver  106 . The exemplary embodiment depicted in FIG. 2 also contains a separate heat sink  208 . Having an independent heat sink ensures that the heat generated by the chips under the MCM cover  104  does not directly heat the optical transducer (also referred to as the optical module). The heat sink and optical transducer may be combined into one unit. Alternately, the optical module heat sink  208  may be connected to and/or common with, heat sink  204 .  
         [0015]    [0015]FIG. 3 shows an example of optical communication between MCMs using an embodiment of the present invention. A computer system can include one or more nodes and each node can include one or more MCMs  100 . Each node can function as a self-contained computer and includes elements to implement the MCM  100 , memory, and I/O functions. Additional nodes are added as additional processing power is needed and the resulting nodes need a way to communicate back and forth. FIG. 3 depicts “system 1”  302  that includes several nodes  306   308 . Each node  306   308  includes an MCM  100  with optical transceivers  106  as depicted in FIG. 1 along with the corresponding fiber  108  and optical connectors  110 . In another exemplary embodiment, the optical connector may be integrated with the transceiver and an additional connector placed at the end of the node (e.g., tailstock). The nodes  306   308  are connected to each other in “system 1”  302  through an electrical backplane  310 . In an exemplary embodiment of the present invention, the nodes  306   308  communicate internal data through optical connectors  110  as depicted in FIG. 3. In addition, FIG. 3 illustrates a second system “system 2”  304  that includes a plurality of nodes  312   314  similar to “system 1”  302 . An MCM  100  in a node  306  in “system 1”  302  can communicate to an MCM  100  in a node  312  in “system 2”  304  through corresponding optical connectors  110 . The amount of distance allowed between “system 1”  302  and “system 2”  304  is dependent on the particular optical module hardware and fiber type being utilized. Though not shown in FIG. 3, an alternate embodiment of the present invention includes multiple MCMs  100  within the same node communicating data using an optical module. In general, all nodes must communicate with all other nodes in the total system (e.g., System  1  and System  2  in FIG. 3). This can be done via a direct connection, or point to point topology, from any node to all other nodes. An alternate exemplary embodiment is shown in FIG. 3 where node  306 , node  308 , node  312 , and node  314  are connected in a ring topology. In addition, an exemplary embodiment may utilize a switched topology to interconnect all the nodes.  
         [0016]    The present invention allows for the implementation of fiber technology to enable high speed (e.g., ten to over one-hundred gigabyte per second) serial communication between nodes. By placing the optic transceivers  106  directly on the MCM and outside of the hermetically sealed MCM cover  104 , the speed of the fiber link becomes the limiting criteria. Keeping the lines short and using the MCM substrate  102  characteristics allows the highest possible electrical data rate to and from the optical transceivers  106 . The speed of the fiber optics allows the multiplexing of several electrical signals onto one fiber. For example, a five byte wide electrical interface (running at two gigabytes per second) can be multiplexed into a single byte wide optical interface running at ten gigabytes per second. In addition, electrical interconnect at these speeds requires differential signaling (two wire per signal). The reduction of electrical wires as compared to fibers can be up to ten times (five for multiplexing, two for a single fiber as compared to a wire pair).  
         [0017]    The present invention allows for using optics for data communication without making changes to the placement of the ICs, the heat sink, and the pins that are already on the MCM. In an exemplary embodiment of the present invention, both electrical and optical I/O signals may be utilized. An advantage of keeping the optic transceiver  106  outside of the hermetically sealed MCM cover  104  is that the expense of moving data optically into and out of the hermetic seal is avoided. In addition, if the optic module fails it can be pulled off and repaired or replaced without affecting the ICs under the MCM cover  104 . The present invention can yield the high speed advantages described above without compromising the seal and can be easily implemented using existing MCMs.  
         [0018]    While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.