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Description  |
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FIELD OF THE INVENTION
This invention relates to fiber optic technology. More specifically, the
invention relates to an apparatus for providing optical interprocessor
communication.
BACKGROUND OF THE INVENTION
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.
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.
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.
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.
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
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
Referring now to the figures wherein the like elements are numbered alike.
FIG. 1 illustrates an exemplary embodiment of the present invention.
FIG. 2 illustrates a side view of the exemplary embodiment depicted in FIG.
1.
FIG. 3 shows an example of optical communication between multichip modules
using an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
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.times.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.
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.times.12 vs. 2.times.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.
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.
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 306308. Each node 306308 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
306308 are connected to each other in "system 1" 302 through an electrical
backplane 310. In an exemplary embodiment of the present invention, the
nodes 306308 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 312314 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.
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).
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.
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.
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