Optoelectronic substrates--will it happen? The economic model for the optoelectronic interconnect favors high data rate transmission over moderate to long distances, limiting applications to high-end telecommunication systems.

Optoelectronic (OE) interconnect is an alternative to copper that can provide increased bandwidth and other advantages for special applications. For example, OE interconnect does not have the problem of "noise" that a copper interconnect can (a significant issue for high-speed communications). OE technology is also appealing for use in aircraft because it eliminates the weight of thousands of copper cables.

Several companies and laboratories are currently working on new waveguide technology and, therefore, OE substrate technology is continually changing. However, despite all the good research that has been accomplished, this technology has not moved forward. In North America, fiber cable is at the curb, but it has not entered the home, office or technical institutions. In Japan, fiber to the home is growing at a rapid pace.

Part of the reason for lack of movement of this technology is that continued improvements in (less-expensive) copper technology have kept pace with circuit bandwidth needs. Good design practice has also helped. However, as the thirst for faster signal processing continues and home electronics or distribution systems become the workhorse of every household, there may be applications that are willing to pay for the initial additional cost.

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Optoelectronic substrates with embedded waveguides are still years from being in production, but they continue to be discussed and compared with traditional substrates. Many of the manufacturing dilemmas associated with optoelectronic substrates have to do with the cost and the reliability of the optical fiber polymer interface. OE substrates will not become prevalent until the cost-performance benefits are proven (FIGURE 1).

This article discusses the current state of optoelectronic substrate technology and highlights some of the key issues surrounding its implementation, based on information from the 2007 iNEMI Roadmap.

Current State of the Art

The advent of increased data rates to support growing bandwidth requirements is certain to continue, and electrical transmission of signals will, presumably, soon run up against its limits.

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Telecommunications systems appear to be the primary driver for optoelectronic interconnect technology. There are currently optical wide area and local area networks using fiber-based OE technology for infrastructure and hybrid fiber/organic substrates in the supporting backplanes. Today's systems are generally operating at 10 Gbps, with 40 Gbps coming soon, and 100 Gbps already being discussed.

Signal conditioning technology has achieved bit rates on copper of 10 Gbps over high-performance and FR-4 boards. While this allows for electrical 10G line-speed on the backplane, it also adds cost and complexity. Furthermore, power dissipation is increasing, and edge density is limited. So optical interconnect solutions are still promising, but cost and technology reliability will determine the breakpoint.

For future generations of data and telecommunication, there is a growing demand for higher data rates and increasing performance. For PCBs in telecommunication systems, there is a growing need for better base materials and circuit board technologies for transmitting high-speed signals. It is clear that further advances in speed and bandwidth can only be achieved by taking advantage of new optical technologies for board-toboard and chip-to-chip interconnection on board.

Current Situation

Increased data rates to support growing bandwidth requirements are certain to continue. The timing of which applications will convert to optical technology remains unclear, but is not expected to happen within the 10-year period covered by the 2007 iNEMI Roadmap. The optoelectronic substrate technologies that will support the applications are also very unclear. Some experts believe the waveguide needs to be embedded or laminated between conventional base materials; other experts are developing waveguide technology external to the PCB.

Optical interconnect is expected to compete with copper interconnect technology for backplane and daughter card applications where data rates are 10-15 Gbps and higher. Both electrical and optical technologies suffer from signal attenuation and degradation problems that can be improved through circuit design and materials.

Optical Substrate Interconnect

Optical interconnects are being used on some boards today. Optoelectronics are currently used as the backplane interconnect if there is some architectural reason the signal needs to remain optical as it goes board to board, or if the system is distributed and a distance (typically > 1-10 meters at 5 > 5 Gbps) exists between connections.

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Optical interconnections used in backplanes are currently fiber-based and exist as separate physical layers from the electrical backplane. The mechanical connection of the optical interconnect off the line card is done through a cutout in the electrical back panel with an adapter placed in the cut-out. Optical jumpers or circuits are then plugged into the adapters to create the fiber connections in the backplane (the connections between cards). Issues with this type of interconnect include difficulties with cleaning and inspection, difficult fiber routing/handling and high cost.

Several different types of optical interfaces between OE components and circuit boards are being developed.

The optical path on the PCB or backplane. In current backplane technology, the optical path is generally provided through the use of optical fiber loops linking components to connectors or other optical or optoelectronics packages. The main issues with this approach are that it is impossible to perform any signal manipulation, and it is difficult to achieve high interconnect densities due to the limited bend radius of the optical fiber. Also, the difficulties in manufacturing and handling make this a costly and often low-yield approach. Because of the limitations of radius bending, this technology is limited to large boards (backplanes).

Optoelectronic module (component) connection to optical board (PCB) with integrated waveguides. Two coupling methods are being considered. The first is "free space" (without waveguide) interconnection using microlenses and special connectors; and the second is "direct butt coupling." The direct butt coupling technique takes advantage of in- and out-coupling without any additional micro-optical elements, such as lenses and mirrors. The VCSEL-arrays/PIN-diode-arrays have to be positioned directly in front of the waveguide end. On the other hand, there are thermal and alignment problems, and the modules cannot be assembled using surface-mount technology processes (FIGURE 2).

Guided wave, 90[degrees] beam deflection. Out-of-plane light deflection (Z-direction) can be accomplished using gratings (incoupling) or mirrors. A number of publications have demonstrated mirror fabrication by cutting the end of the waveguide with a dicing saw, wet chemical etch or laser to create a 45[degrees] facet. The facet can be metallized to improve the reflection properties of the mirror. Mirrors have the advantage of being wavelength independent, but can cause high losses due to surface roughness of the mirror surface. Aligning the mirror to the waveguide and active device poses significant problems, and will have to meet similar tolerances as required for the transmitters and receivers (FIGURE 3).

Materials and Processes

Cost of the materials and the associated processes need to be considered when developing materials to meet optoelectronic performance specifications. There are a number of inorganic materials that can be used for embedded optical interconnects, including Si/Si[O.sub.2] silica and glass sheets. Of these, only glass sheets have been applied to embedded waveguide technology for PCBs (FIGURE 4).

Si[O.sub.2] and silica materials are used in device applications and are processed directly on silicon wafers. Waveguides are formed through a combination of lithographic printing, etching (wet chemical or laser) or ion implantation.

The material of choice, whether it is inorganic or organic in composition, needs to be compatible with the board manufacturing environment and equipment. The material will be exposed to various chemicals and temperatures, handled most of the time outside a cleanroom environment and cannot be sealed hermetically. It also should be compatible with standard processing (lamination, etching, drilling, soldering, etc.) techniques.

Any new material introduced as a waveguide solution for PWB interconnects needs to undergo extensive reliability testing, including failure analysis. Similar requirements extend to the electro-optic hybrid laminates and assembled boards (FIGURE 5).

Some of the techniques used for polymer waveguide structuring include hot embossing, photolithography and laser writing. In particular, for medium-sized boards and if large amounts of waveguide foils for mass-production are desired, hot embossing seems to be the most promising technology. However, for the up to 1 meter waveguides needed in backplanes, lithographic or laser writing techniques appear to be the most appropriate. Recently, a working EOCB (electrical-optical circuit board) with hot embossed planar polymer waveguides was demonstrated.

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Optical PCB Manufacturing